AXL Pathways Research Articles

Abstract PO2-24-10: Anti-Tumorigenic Immuno-Microenvironment by AB-329, a Potent and Selective AXL inhibitor, combined with paclitaxel in Triple-Negative Breast Cancer Preclinical Model

Abstract BACKGROUND The Gas6/AXL signaling pathway plays a substantial role in cancer cell survival, proliferation, immunosuppression, and metastasis. Elevated AXL expression is associated with advanced cancer stages and decreased overall survival rates, underscoring its potential as a therapeutic target for metastatic cancers. We investigated this possibility by utilizing AB-329, a selective small molecule AXL kinase inhibitor, to treat triple-negative breast cancer (TNBC). TNBC, typified by the lack of ER, PR, and HER2 expression, frequently displays mesenchymal characteristics and carries a significant risk of metastasis and an immunosuppressive tumor microenvironment. METHODS We assessed AXL protein expression in 28 human and 11 mouse breast cancer cell lines using Western blotting. The Microarray database was utilized to examine AXL gene expression in 40 human breast cancer cells. The Sulforhodamine B proliferation and soft agar assays measured the antiproliferative effect of AB-329 in TNBC cell lines. The in vitro anti-metastatic impact of AB-329 was evaluated through migration and invasion assays. We appraised the anti-tumor effects of AB-329 in combination with paclitaxel using 4T1 and E0771-LMB mouse TNBC xenograft models. We used FACS and CIBERSORTx to perform immune cell type profiling in E0771-LMB xenograft tumor tissues. To understand AB-329’s immunomodulatory effects in tumors, single-cell RNA sequencing was conducted in a humanized mouse model with SUM149 human TNBC xenografts. RESULTS Microarray data revealed higher AXL expression in TNBC cells compared to non-TNBC cells. Western blotting supported this, showing higher AXL protein expression, particularly in basal and mesenchymal subtypes of TNBC cells, compared to luminal breast cancer cells. AB-329 presented moderate antiproliferative effects as a monotherapy in the TNBC cell lines tested, with half-maximal inhibitory concentrations exceeding 5 µM. Nevertheless, combining AB-329 with paclitaxel resulted in significant growth inhibition in all tested TNBC cell lines (p < 0.05). Importantly, AB-329 alone could inhibit TNBC migration and invasion at a concentration of 1 µM (p < 0.01) and showed enhanced inhibition rates when paired with paclitaxel (p < 0.05). The combination of AB-329 and paclitaxel considerably reduced tumor growth in the 4T1 and E0771-LMB xenograft models compared to paclitaxel or AB-329. Total RNA sequencing and CIBERSORTx analysis indicated that the AB-329 and paclitaxel combination increased Natural Killer (NK) cells’ infiltration into E0771-LMB mouse TNBC xenograft tumors compared to single agent-treated tumors. A nearly 2.5-fold increase in NKT cell infiltration was also detected with AB-329 treatment compared to Control in SUM149 human TNBC xenografts. CONCLUSIONS Our preclinical data suggest that AB-329, an AXL inhibitor, could synergize effectively with paclitaxel in TNBC treatment, notably inhibiting tumor cell proliferation and migration while enhancing NK cell infiltration. Intriguingly, we observed an increased infiltration of NKT cells into tumors following AB-329 treatment, which may promote an anti-tumorigenic immuno-microenvironment. These results underscore the necessity for further investigation into the role of AB-329 in modulating the tumor immuno-microenvironment through AXL pathway inhibition and accentuate the potential for combining AB-329 with immunotherapy. Citation Format: Dileep Reddy Rampa, Jon Fuson, Huey Liu, Max Pan, Naoto Ueno, Jangsoon Lee. Anti-Tumorigenic Immuno-Microenvironment by AB-329, a Potent and Selective AXL inhibitor, combined with paclitaxel in Triple-Negative Breast Cancer Preclinical Model [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-24-10.

Molecular and functional characterization of reversible-sunitinib-tolerance state in human renal cell carcinoma.

Therapy failure with the tyrosine kinase inhibitor (TKI) sunitinib remains a great challenge in metastatic renal cell carcinoma (mRCC). Growing evidence indicates that the tumour subpopulation can enter a transient, non-mutagenic drug-tolerant state to endure the treatment underlying the minimal residual disease and tumour relapse. Drug tolerance to sunitinib remains largely unexplored in RCC. Here, we show that sunitinib-tolerant 786-O/S and Caki-2/S cells are induced by prolonged drug treatment showing reduced drug sensitivity, enhanced clonogenicity, and DNA synthesis. Sunitinib-tolerance developed via dynamic processes, including (i) engagement of c-MET and AXL pathways, (ii) alteration of stress-induced p38 kinase and pro-survival BCL-2 signalling, (iii) extensive actin remodelling, which was correlated with activation of focal adhesion proteins. Remarkably, the acute drug response in both sensitive and sunitinib-tolerant cell lines led to dramatic fine-tuning of the actin-cytoskeleton and boosted cellular migration and invasion, indicating that the drug-response might depend on cell state transition rather than pre-existing mutations. The drug-tolerant state was transiently acquired, as the cells resumed initial drug sensitivity after >10 passages under drug withdrawal, reinforcing the concept of dynamic regulation and phenotypic heterogeneity. Our study described molecular events contributing to the reversible switch into sunitinib-tolerance, providing possible novel therapeutic opportunities in RCC.

RNASE4 promotes malignant progression and chemoresistance in hypoxic glioblastoma via activation of AXL/AKT and NF-κB/cIAPs signaling pathways.

Glioblastoma (GBM) is the most malignant brain tumor frequently characterized by a hypoxic microenvironment. In this investigation, we unveiled unprecedented role of Ribonuclease 4 (RNASE4) in GBM pathogenesis through integrative methodologies. Leveraging The Cancer Genome Atlas (TCGA) dataset and clinical specimens from normal brain tissues, low- and high-grade gliomas, alongside rigorous in vitro and in vivo functional analyses, we identified a consistent upregulation of RNASE4 correlating with advanced GBM pathological stages and poor clinical survival outcomes. Functional assays corroborated the pivotal influences of RNASE4 on key tumorigenic processes such as cell proliferation, migration, invasion, stemness properties and temozolomide (TMZ) resistance. Further, Gene Set Enrichment Analysis (GSEA) illuminated the involvement of RNASE4 in modulating epithelial-mesenchymal transition (EMT) via activation of AXL, AKT and NF-κB signaling pathways. Furthermore, recombinant human RNASE4 (hRNASE4)-mediated NF-κB activation through IκBα phosphorylation and degradation could result in the upregulation of inhibitors of apoptosis proteins (IAPs), such as cIAP1, cIAP2, and SURVIVIN. Notably, treating RNASE4-induced TMZ-resistant cells with the SURVIVIN inhibitor YM-155 significantly restored cellular sensitivity to TMZ therapy. Herein, this study positions RNASE4 as a potent prognostic biomarker and therapeutic target, offering new insights into molecular pathogenesis of GBM and new avenues for future therapeutic interventions.

1889P Phase Ib/II study of batiraxcept (BT, AVB-S6-500, an AXL pathway inhibitor) in combination with cabozantinib (cabo) in patients with advanced clear cell renal cell carcinoma (ccRCC) who failed first-line (1L) therapy

1889P Phase Ib/II study of batiraxcept (BT, AVB-S6-500, an AXL pathway inhibitor) in combination with cabozantinib (cabo) in patients with advanced clear cell renal cell carcinoma (ccRCC) who failed first-line (1L) therapy

Single-cell RNA sequencing of a poorly metastatic melanoma cell line and its subclones with high lung and brain metastasis potential reveals gene expression signature of metastasis with prognostic implication.

The molecular mechanisms underlying melanoma metastasis remain poorly understood. In this study, we aimed to delineate the mechanisms underlying gene expression alterations during metastatic potential acquisition and characterize the metastatic subclones within primary cell lines. We performed single-cell RNA sequencing of a poorly metastatic melanoma cell line (WM239A) and its subclones with high metastatic potential to the lung (113/6-4L) and the brain (131/4-5B1 and 131/4-5B2). Unsupervised clustering of 8173 melanoma cells identified three distinct clusters according to cell type ('Primary', 'Lung' and 'Brain' clusters) with differential expression of MITF and AXL pathways and putative cancer and cell cycle drivers, with the lung cluster expressing intermediate but distinct gene profiles between primary and brain clusters. Principal component (PC) analysis revealed that PC2 (the second PC), which was positively associated with MITF expression and negatively with AXL pathways, primarily segregated cell types, in addition to PC1 of the cell cycle pathway. Pseudotime trajectory and RNA velocity analyses suggested the existence of cellular subsets with metastatic potential in the Primary cluster and an association between PC2 signature alteration and metastasis potential acquisition. Analysis of The Cancer Genome Atlas melanoma samples by clustering into PC2-high and -low clusters by quartiles of PC2 signature expression revealed that the PC2-high cluster was an independent significant factor for poor prognosis (p-value = 0.003) with distinct genomic and transcriptomic characteristics, compared to the PC2-low cluster. In conclusion, we identified signatures of melanoma metastasis with prognostic significance and putative pro-metastatic subclones within a primary cell line.

AXL pathway as a target to overcome resistance to PARP inhibitors in high-grade serous ovarian (HGSOC) and triple-negative breast cancer (TNBC) cell lines.

e17502 Background: PARP inhibitors (PARPi) as olaparib (OLA) have become the new standard in maintenance treatment in high grade serous ovarian carcinoma (HGSOC) and triple negative breast cancer (TNBC). Thus there is an important need to identify the mechanisms for PARPi resistance and the strategies to overcome it. Our aim is to intervene in vitro AXL, a receptor tyrosine kinase, linking its function to the OLA-resistance mechanism and assess its suitability as a therapeutical target to overcome PARPi-resistance. Methods: Three cell lines, 2 from HGSOC (PEO1-S and Kuramochi-S) and 1 from TNBC (MDA-MB-231-S) were selected, isogenic resistant pairs generated in vitro by pulsed exposure to OLA (PEO1-R) or by exposure to growing concentrations of OLA (Kuramochi-R and MDA-MB-231-R) were obtained. Resistance was confirm by the comparison of IC50 of OLA in -S vs -R lines. AXL expression was assessed by western blot (WB). Cells were treated with OLA before and after the downregulation of AXL expression by a specific siRNA. Cell viability was assessed by MTT assay and DNA damage by the evaluation of markers by western blot (WB). Results: AXL expression was maintained upon OLA treatment in all 3 resistant cell lines. However, AXL expression was reduced upon OLA exposure in 3 sensible parental cell lines. AXL silencing (Si) lead to a significantly decrease of OLA IC50 in all 3 sensitive and resistant cell lines. For sensitive cell lines, IC50 of OLA was in 1) PEO1S with control (scramble) was 5.79 mM vs 2.24 mM after silencing with siRNA, p < 0.0001; 2) Kuramochi-S scramble OLA IC50 was 481.8 mM vs 199.7 mM with siRNA, p < 0.0001 and 3) MDA-MB-231-S with control of scramble the OLA IC50 was 61.62 mM vs 26.21 mM with siRNA, p = 0.001. For resistant cell lines, 4) PEO1R control with scramble 91.49 mM vs siRNA 45.72 mM, p < 0.0001; 5) Kuramochi-R scramble OLA IC50 579.8 mM vs 219.8 mM with siRNA, p = 0.0008 and 6) 231-R scramble OLA IC50 68.22 mM vs 9.247 mM with siRNA, p < 0.0001. These results suggest that AXL downregulation increases sensitivity to OLA in both sensitive and resistant cell lines. Histone γ-H2AX was analysed pre and post silencing and both increased when treating with OLA, especially when AXL was downregulated in alll three resistant cell lines. Conclusions: This in vitro study suggest a potential role of AXL in OLA resistance and AXL downregulation migh represent a potential mechanism to overcome resistance to OLA in ovarian and TNBC cell lines.

Abstract 3635: TBK1 inhibition potentiates the efficacy of AXL-targeted therapy by modulating tumor microenvironment in aggressive breast cancers

Abstract Introduction: Triple-negative and inflammatory breast cancer (TNBC and IBC) are the most aggressive breast cancer subtypes. Novel actionable targets and complementary therapies are critically needed. AXL, a receptor tyrosine kinase, drives pleiotropic phenotypes of TNBC and IBC aggressiveness. Targeting AXL reduces IBC tumor growth in vivo and inhibits polarization of M2 macrophages. To identify novel tumor microenvironment (TME) targets to enhance the efficacy of AXL-targeted therapy, we conducted a synthetic lethal kinome siRNA screening. We identified TBK1 as a candidate to synergize with AXL growth inhibition. TBK1 is a serine/threonine protein kinase that regulates innate, adaptive responses and antitumor immune responses. We hypothesized that inhibition of TBK1 enhances the antitumor effect of AXL-targeted therapy in aggressive breast cancers. Methods: We used knockdown, knockout (KO) and inhibitors to suppress AXL and TBK1 pathways and tested the synergistic effect of targeting AXL and TBK1 on the growth of human SUM149 and BCX010 IBC cells, and HS578T TNBC cells in vitro. To determine the synergistic effect of targeting both pathways in vivo, we inoculated control or TBK1 KO murine TNBC 4T1.2 cells into BALB/c mice. We assessed the activity of AXL inhibitor TP-0903 in reducing tumor growth. Using multicolor flow cytometry, we studied the effects of targeting AXL and TBK1 on the TME. We tested the activity of TP-0903 combined with a TBK1 inhibitor in another TNBC mouse model, E0771. We used RNA-sequencing and real-time PCR/Western blot to determine the molecular mechanisms of how TBK1 inhibition synergizes AXL-targeted therapy in these cancers. Results: Compared with AXL or TBK1 suppression alone, genomic or pharmacologic suppression of AXL and TBK1 significantly reduced the growth of SUM149, BCX010, and HST578T cells in vitro. In both 4T1.2 and E0771 syngeneic mouse models, TP-0903 was more active in reducing tumor growth in TBK1-inhibited tumors than in control tumors. Tumors with TBK1 inhibition and treated with TP-0903 had a significantly higher population of cytotoxic T cells than control tumors. Depletion of CD8+ T cells blocked the synergistic effect of targeting AXL and TBK1 pathways on reducing tumor growth, suggesting that cytotoxic T cells contributed to the anti-tumor synergy of targeting AXL and TBK1. Mechanistically, TBK1 induced M2 macrophage migration via IRF3-regulated CCL5 secretion in SUM149 and HS578T cells, and AXL KO attenuated the polarization and migration of M2 macrophages by inhibiting the CCR5/CCL5 axis. Conclusions: Targeting TBK1 enhances the efficacy of AXL-targeted therapy in aggressive breast cancer by suppressing the paracrine effect of CCR5/CCL5 axis. This combination represents a novel and effective therapy modulating the TME of aggressive breast cancer, which warrants further investigation in the clinical setting. Citation Format: Lan Phi, Takashi Semba, Ngu V. Trinh, Fang Zou, Jason M. Foulks, Steven L. Warner, Savitri Krishnamurthy, James P. Long, James M. Reuben, Debu Tripathy, Naoto T. Ueno, Xiaoping Wang. TBK1 inhibition potentiates the efficacy of AXL-targeted therapy by modulating tumor microenvironment in aggressive breast cancers. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3635.

Phase Ib study of avelumab and novel AXL inhibitor avb-S6-500 in patients with metastatic urothelial carcinoma (mUC).

4579 Background: AXL is a member of the mammalian Tyro3/AXL/Mer (TAM) receptor tyrosine kinase family, which upon binding to its ligand Gas6 promotes immunosuppression in the tumor microenvironment. AVB-S6-500 (AVB) is a novel AXL pathway inhibitor which binds to circulating Gas6 and inhibits ligand-dependent AXL signaling. In this phase Ib investigator-initiated trial (NCT04004442), we evaluated the safety, tolerability, pharmacodynamics, and preliminary antitumor efficacy of AVB with the anti-PD-L1 antibody avelumab in mUC. Methods: Patients with PD-1/L1 inhibitor naïve locally advanced unresectable or mUC with measurable disease were eligible if they were either in-eligible for, refractory to, or declined platinum-based chemotherapy. Employing a 3+3 dose escalation design, patients were treated with AVB 5mg/kg weekly (dose level; DL1), or 10 (DL2), 15 (DL3), or 20 (DL4) mg/kg every two weeks along with avelumab 800 mg every 2 weeks administered intravenously. The safety and tolerability of the combination as measured by the incidence of dose limiting toxicities (DLTs) was the primary endpoint while objective response rate (ORR) per RECIST 1.1 was key secondary endpoint. Pharmacodynamic studies evaluated baseline and on therapy (C2D1) Gas6 levels. Results: To date, a total of 15 patients have been enrolled (DL1: n = 3, DL2: n = 7, DL3: n = 3 and DL4: n = 2). Among the 13 evaluable patients, DLT was observed in 1 patient (grade 3 fatigue; DL2). Grade ≥3 adverse events irrespective of attribution were seen in 6 patients which included UTI (n = 3), hyponatremia (n = 1), elevated creatinine (n = 1), and anemia, thrombocytopenia, hematuria, anorexia and sepsis (n = 1 each; all in same patient). No treatment related deaths were noted. While enrollment on DL4 is ongoing, the maximum tolerated dose for AVB has not been reached. Treatment with AVB effectively suppressed circulating Gas6 levels from baseline (median: 28.4 ng/mL; range: 20.4-54.0 ng/mL) to below threshold of detection in 85% (n = 11/13) of patients. Median follow up was 10 months (95% CI: 3.9, 18.55). Confirmed ORR was 38% (n = 5/13) while 1 patient had stable disease as best response. Durable responses lasting > 6 months were seen in 3 patients (7, 19 and 21 mos) who continue to be on treatment without disease progression. Additional correlative studies investigating AXL, Gas6, PD-L1 expression and gene expression signatures are currently underway. Conclusions: Concurrent avelumab and AVB was safe and AVB effectively neutralized circulating Gas6 across DLs. Albeit small sample size, the efficacy of the combination was encouraging and ORR compares favorably to that previously reported with PD-1/L1 inhibitor monotherapy. Updated safety, efficacy and biomarker data will be presented. Clinical trial information: NCT04004442.

Targeting the Axl and mTOR Pathway Synergizes Immunotherapy and Chemotherapy to Butylidenephthalide in a Recurrent GBM.

Background The role of inherent tumor heterogeneity and an immunosuppressive microenvironment in therapeutic resistance has been determined to be of importance for the better management of glioblastoma multiforme (GBM). Some studies have suggested that combined drugs with divergent mechanisms may be promising in treating recurrent GBM. Methods Intracranial sustained (Z)-n-butylidenephthalide [(Z)-BP] delivery through Cerebraca Wafers (CWs) to eliminate unresectable brain tumors was combined with the administration of temozolomide (TMZ), pembrolizumab, and cytokine-induced killer (CIK) cells for treating a patient with recurrent glioblastoma. Neurological adverse events and wound healing delay were monitored for estimating tolerance and efficacy. Response Assessment in Neuro-Oncology criteria were applied to evaluate progression-free survival (PFS); further, the molecular characteristics of GBM tissues were analyzed, and the underlying mechanism was investigated using primary culture. Results Intracerebral (Z)-BP in residual tumors could not only inhibit cancer stem cells but also increase interferon gamma levels in serum, which then led to the regression of GBM and an immune-responsive microenvironment. Targeting receptor tyrosine kinases, including Axl and epidermal growth factor receptor (EGFR), and inhibiting the mechanistic target of rapamycin (mTOR) through (Z)-BP were determined to synergize CIK cells in the presence of pembrolizumab and TMZ in recurrent GBM. Therefore, this well-tolerated regimen could simultaneously block multiple cancer pathways, which allowed extended PFS and improved quality of life for 22 months. Conclusion Given the several unique functions of (Z)-BP, greater sensitivity of chemotherapy and the synergism of pembrolizumab and CIK cells could have affected the excellent prognosis seen in this patient with recurrent GBM.

Abstract P5-08-10: TBK1 inhibition potentiates the efficacy of AXL-targeted therapy in aggressive breast cancer preclinical models

Abstract Background: Novel mechanism-oriented targeted therapies are needed for aggressive breast cancers such as triple-negative breast cancer (TNBC) and inflammatory breast cancer (IBC). We previously showed the oncogenic receptor tyrosine kinase AXL crucially contributes to the aggressiveness of TNBC and TN-IBC, which suggests that AXL is a potential therapeutic target. However, AXL-targeted therapy is insufficient to control the progress of aggressive breast cancer in preclinical models. Therefore, to further enhance the efficacy of AXL-targeted therapy in aggressive breast cancer, we conducted a synthetic lethality screen of pooled kinome siRNA and identified TANK-binding kinase 1 (TBK1) as a potential target. TBK1 is a necessary serine/threonine protein kinase that regulates innate and adaptive immune responses, T cell migration, and anti-tumor immune responses. In the present work, we validated that AXL and TBK1 inhibition have a synergistic anti-tumor effect by modulating the tumor microenvironment (TME). Methods: To determine the synergistic effect of AXL and TBK1 inhibition on cell proliferation and colony formation in vitro, we first used siRNA-mediated knockdown (KD) or CRISPR/Cas9 knockout (KO) to independently silence AXL and TBK1 expression. We also used pharmacological inhibitors to inactivate AXL and TBK1 signaling and conducted similar studies. To determine the effect of targeting TBK1 and AXL pathways in vivo, we injected control or TBK1 KO murine 4T1.2 TNBC cells into BALB/c mice and assessed the efficacy of the AXL inhibitor TP-0903 in reducing the tumor growth. We used multicolor flow cytometry and multiplexed immunostaining to examine the population of TME components in control and TBK1 KO 4T1.2 tumors treated with vehicle or TP-0903. We also used an in vitro migration assay to test the effect of targeting AXL and TBK1 on the recruitment of CD8+ T cells. We performed a mouse chemokine array to identify intratumoral chemokines involved in regulating infiltrating immune cells and used RT-PCR to validate these findings. Results: Compared with AXL or TBK1 suppression alone, the genomic or pharmacological suppression of both AXL and TBK1 resulted in a significantly greater reduction of TNBC and TN-IBC cell growth and colony-forming ability (P < 0.01). In our syngeneic 4T1.2 TNBC mouse tumor model, TP-0903 was more effective at controlling tumor growth from TBK1-KO cells than from control cells (P < 0.0005). TBK1-KO 4T1.2 tumors treated with TP-0903 had a significantly higher population of functional cytotoxic T cells (GranzymeB+CD3+CD8+) than control KO tumors treated with vehicle or TP-0903. Furthermore, our migration assay showed that TBK1-KO/AXL-KD SUM149 cell-conditioned media attracted more CD8+ T cells than control cell-conditioned media, suggesting the inhibition of AXL/TBK1 signaling enhances the recruitment of CD8+ T cells through a paracrine effect. Our chemokine array revealed increased CXCL16 levels in TP-0903-treated TBK1-KO tumors. Conclusion: Targeting TBK1 could enhance the sensitivity of aggressive breast cancer cells to AXL-targeted therapy in vitro and in vivo. Our preliminary TME data suggest that combined AXL and TBK1 inhibition enhances the anti-tumor immune environment by upregulating secretion of CXCL16 by tumor cells, resulting in the increased recruitment of cytotoxic CD8+ T cells. In future studies, we will further elucidate the role that CXCL16 upregulation plays in this novel synergy. Significance of this findings: This study will enable us to develop hypothesis-driven novel combination therapies for patients with aggressive breast cancer that can be rapidly tested as a clinical trial, leading to an improvement of treatment and patients’ outcome. Citation Format: Lan Phi, Takashi Semba, Jason Foulks, Steven Warner, David Bearss, Savitri Krishnamurthy, James Long, James Reuben, Debu Tripathy, Naoto Ueno, Xiaoping Wang. TBK1 inhibition potentiates the efficacy of AXL-targeted therapy in aggressive breast cancer preclinical models [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-08-10.

Epstein-Barr virus miR-BART4-3p regulates cell proliferation, apoptosis, and migration by targeting AXL in gastric carcinoma.

To investigate the role of miR-BART4-3p in EBV-associated gastric cancer (EBVaGC) and its regulation of cell proliferation, apoptosis, and migration by targeting AXL in GC. Quantitative real-time PCR and western blot were used to detect the expression of AXL. The methylation status of AXL gene promoter region was determined by bisulfite sequencing PCR. Luciferase reporter assay was used to detect whether miR-BART4-3p targets AXL. The key molecules of EMT and PI3K/AKT pathway were used to examine by western blot. CCK8, Transwell, and flow cytometry were used to detect the phenotypic gastric cancer cells after interference with AXL and miR-BART4-3p. EBV infection inhibited the expression of AXL in GC cells and the inhibition was not caused by the change of promoter methylation status. MiR-BART4-3p directly targeted AXL. Moreover, both inhibition of miR-BART4-3p and AXL inhibited cell proliferation and migration and promoted cell apoptosis. In addition, E-cadherin, Vimentin, ZEB1, and p-AKT were found to be the downstream molecules of the miR-BART4-3p/AXL pathway. The change of promoter methylation status was not the reason for the downregulation of AXL expression in EBV-positive cells. MiR-BART4-3p may inhibit the proliferation and migration and promote apoptosis of GC cells by directly targeting AXL.

Tumor perivascular cell-derived extracellular vesicles promote angiogenesis via the Gas6/Axl pathway.

Aberrant angiogenesis is a hallmark of cancer and is critically associated with tumor progression. Perivascular cells are essential components of blood vessels, and the role of tumor perivascular cell-derived extracellular vesicles (TPC-EVs) in angiogenesis remains elusive. In the present study, using genetic mouse models and pharmacological inhibitors, we found that ablation of perivascular cells inhibited angiogenesis in allografted colorectal cancer tumors. Further studies demonstrated that TPC-EVs promoted the proliferation, migration, invasion, viability, and tube formation of HUVECs. They also facilitated vessel spouting in rat aortic rings and induced neovascularization in chick chorioallantoic membranes (CAMs). Silencing of Gas6 or blockade of the Axl pathway suppressed TPC-EV-induced angiogenesis in vitro and ex vivo. Moreover, inhibition of the Gas6/Axl signaling pathway impaired TPC-EV-mediated angiogenesis in vivo. Our findings present a deeper insight into the biological functions of TPCs and TPC-EVs in tumor angiogenesis and demonstrate that TPC-EV-derived Gas6 could be an attractive and innovative regulator of tumor angiogenesis.

Immunological Aspects of AXL/GAS-6 in the Context of Human Liver Regeneration.

AXL and its corresponding ligand growth arrest–specific 6 (GAS‐6) are critically involved in hepatic immunomodulation and regenerative processes. Pleiotropic inhibitory effects on innate inflammatory responses might essentially involve the shift of macrophage phenotype from a pro‐inflammatory M1 to an anti‐inflammatory M2. We aimed to assess the relevance of the AXL/GAS‐6‐pathway in human liver regeneration and, consequently, its association with clinical outcome after hepatic resection. Soluble AXL (sAXL) and GAS‐6 levels were analyzed at preoperative and postoperative stages in 154 patients undergoing partial hepatectomy and correlated with clinical outcome. Perioperative dynamics of interleukin (IL)‐6, soluble tyrosine‐protein kinase MER (sMerTK), soluble CD163 (sCD163), and cytokeratin (CK) 18 were assessed to reflect pathophysiological processes. Preoperatively elevated sAXL and GAS‐6 levels predicted postoperative liver dysfunction (area under the curve = 0.721 and 0.722; P < 0.005) and worse clinical outcome. These patients failed to respond with an immediate increase of sAXL and GAS‐6 upon induction of liver regeneration. Abolished AXL pathway response resulted in a restricted increase of sCD163, suggesting a disrupted phenotypical switch to regeneratory M2 macrophages. No association with sMerTK was observed. Concomitantly, a distinct association of IL‐6 levels with an absent increase of AXL/GAS‐6 signaling indicated pronounced postoperative inflammation. This was further supported by increased intrahepatic secondary necrosis as reflected by CK18M65. sAXL and GAS‐6 represent not only potent and easily accessible preoperative biomarkers for the postoperative outcome but also AXL/GAS‐6 signaling might be of critical relevance in human liver regeneration. Refractory AXL/GAS‐6 signaling, due to chronic overactivation/stimulation in the context of underlying liver disease, appears to abolish their immediate release following induction of liver regeneration, causing overwhelming immune activation, presumably via intrahepatic immune regulation.

Overcoming PLK1 inhibitor resistance by targeting mevalonate pathway to impair AXL-TWIST axis in colorectal cancer

New therapeutic targets are revolutionizing colorectal cancer clinical management, opening new horizons in metastatic patients’ outcome. Polo Like Kinase1 (PLK1) inhibitors have high potential as antitumoral agents, however, the emergence of drug resistance is a major challenge for their use in clinical practice. Overcoming this challenge represents a hot topic in current drug discovery research.BI2536-resistant colorectal cancer cell lines HT29R, RKOR, SW837R and HCT116R, were generated in vitro and validated by IG50 assays and xenografts models by the T/C ratio. Exons 1 and 2 of PLK1 gene were sequenced by Sanger method. AXL pathway, Epithelial-to-Mesenchymal transition (EMT) and Multidrug Resistance (MDR1) were studied by qPCR and western blot in resistant cells. Simvastatin as a re-sensitizer drug was tested in vitro and the drug combination strategies were validated in vitro and in vivo.PLK1 gene mutation R136G was found for RKOR. AXL pathway trough TWIST1 transcription factor was identified as one of the mechanisms involved in HT29R, SW837R and HCT116R lines, inducing EMT and upregulation of MDR1. Simvastatin was able to impair the mechanisms activated by adaptive resistance and its combination with BI2536 re-sensitized resistant cells in vitro and in vivo.Targeting the mevalonate pathway contributes to re-sensitizing BI2536-resistant cells in vitro and in vivo, raising as a new strategy for the clinical management of PLK1 inhibitors.

AXL Receptor in Cancer Metastasis and Drug Resistance: When Normal Functions Go Askew.

Simple SummaryAXL is a member of the TAM (TYRO3, AXL, MER) family of receptor tyrosine kinases. In normal physiological conditions, AXL is involved in removing dead cells and their remains, and limiting the duration of immune responses. Both functions are utilized by cancers in the course of tumour progression. Cancer cells use the AXL pathway to detect toxic environments and to activate molecular mechanisms, thereby ensuring their survival or escape from the toxic zone. AXL is instrumental in controlling genetic programs of epithelial-mesenchymal and mesenchymal-epithelial transitions, enabling cancer cells to metastasize. Additionally, AXL signaling suppresses immune responses in tumour microenvironment and thereby helps cancer cells to evade immune surveillance. The broad role of AXL in tumour biology is the reason why its inhibition sensitizes tumours to a broad spectrum of anti-cancer drugs. In this review, we outline molecular mechanisms underlying AXL function in normal tissues, and discuss how these mechanisms are adopted by cancers to become metastatic and drug-resistant.The TAM proteins TYRO3, AXL, and MER are receptor tyrosine kinases implicated in the clearance of apoptotic debris and negative regulation of innate immune responses. AXL contributes to immunosuppression by terminating the Toll-like receptor signaling in dendritic cells, and suppressing natural killer cell activity. In recent years, AXL has been intensively studied in the context of cancer. Both molecules, the receptor, and its ligand GAS6, are commonly expressed in cancer cells, as well as stromal and infiltrating immune cells. In cancer cells, the activation of AXL signaling stimulates cell survival and increases migratory and invasive potential. In cells of the tumour microenvironment, AXL pathway potentiates immune evasion. AXL has been broadly implicated in the epithelial-mesenchymal plasticity of cancer cells, a key factor in drug resistance and metastasis. Several antibody-based and small molecule AXL inhibitors have been developed and used in preclinical studies. AXL inhibition in various mouse cancer models reduced metastatic spread and improved the survival of the animals. AXL inhibitors are currently being tested in several clinical trials as monotherapy or in combination with other drugs. Here, we give a brief overview of AXL structure and regulation and discuss the normal physiological functions of TAM receptors, focusing on AXL. We present a theory of how epithelial cancers exploit AXL signaling to resist cytotoxic insults, in order to disseminate and relapse.

The FAPα-activated prodrug Z-GP-DAVLBH inhibits the growth and pulmonary metastasis of osteosarcoma cells by suppressing the AXL pathway

Osteosarcoma is a kind of bone tumor with highly proliferative and invasive properties, a high incidence of pulmonary metastasis and a poor prognosis. Chemotherapy is the mainstay of treatment for osteosarcoma. Currently, there are no molecular targeted drugs approved for osteosarcoma treatment, particularly effective drugs for osteosarcoma with pulmonary metastases. It has been reported that fibroblast activation protein alpha (FAPα) is upregulated in osteosarcoma and critically associated with osteosarcoma progression and metastasis, demonstrating that FAPα-targeted agents might be a promising therapeutic strategy for osteosarcoma. In the present study, we reported that the FAPα-activated vinblastine prodrug Z-GP-DAVLBH exhibited potent antitumor activities against FAPα-positive osteosarcoma cells in vitro and in vivo. Z-GP-DAVLBH inhibited the growth and induced the apoptosis of osteosarcoma cells. Importantly, it also decreased the migration and invasion capacities and reversed epithelial–mesenchymal transition (EMT) of osteosarcoma cells in vitro and suppressed pulmonary metastasis of osteosarcoma xenografts in vivo. Mechanistically, Z-GP-DAVLBH suppressed the AXL/AKT/GSK-3β/β-catenin pathway, leading to inhibition of the growth and metastatic spread of osteosarcoma cells. These findings demonstrate that Z-GP-DAVLBH is a promising agent for the treatment of FAPα-positive osteosarcoma, particularly osteosarcoma with pulmonary metastases.

Abstract 1041: Targeted inhibition of c-MET, Axl and DDR signaling pathways by merestinib enhances gemcitabine/nab-paclitaxel standard chemotherapy activity in pancreatic cancer models

Abstract Background: Pancreatic ductal adenocarcinoma (PDAC) is among the most lethal malignancies in Western countries. Gemcitabine (Gem) plus nab-paclitaxel (NPT) is the standard chemotherapy treatment for PDAC leading to an average survival of 8.5 months. Activated signaling of c-MET, Axl and DDR1/2 pathways have been implicated in tumorigenic growth in many cancers including PDAC. Merestinib (Mer) is a potent, multikinase inhibitor that blocks c-MET, Axl and DDR1/2 pathways with IC50 values in the low nanomolar range. In this study, we investigated the antitumor potency of merestinib to enhance standard chemotherapy response in preclinical PDAC models. Methods: In vitro cell proliferation assays of PDAC-associated cells were evaluated by colorimetric WST-1 reagent. The mechanism of action was determined by Immunoblot analysis. Tumor growth studies were conducted in 6-week-old female NOD/SCID mice in subcutaneous xenografts. Results: In human PDAC AsPC-1 cell-derived subcutaneous xenografts, gemcitabine plus nab-paclitaxel and merestinib inhibited tumor growth while their combination had an additive effect. The net increase in tumor size was 357 mm3 in controls, 102 mm3 after Gem+NPT, 177 mm3 after Mer and 66 mm3 after Gem+NPT+Mer. Gemcitabine plus nab-paclitaxel and merestinib suppressed in vitro cell proliferation of PDAC epithelial cells and stromal cells. Importantly, the combination treatment demonstrated additive inhibitory effects. Inhibition in cell proliferation by Gem+NPT, Mer and Gem+NPT+Mer treatments was 54%, 14% and 82% (AsPC-1 cells, at medium dose levels), 54%, 4% and 73% (PANC-1 cells, at the highest dose level), 55%, 58% and 92% (PDAC-associated fibroblasts, at the medium dose level). Immunoblot analysis revealed that merestinib caused a decrease in the expression of phospho-c-MET, phospho-Axl, phospho-IGF-1R, phospho-AKT and an increase in apoptosis-related protein cleaved caspase-3 in PDAC cells either alone or in combination with gemcitabine plus nab-paclitaxel. Conclusion: Simultaneous inhibition of c-MET, Axl and DDR1/2 pathways by merestinib has antitumor efficacy in PDAC and it can significantly improve the standard chemotherapy response. This therapeutic approach might lead to a clinically relevant combination to increase PDAC patients' survival. Citation Format: Eda Shi, Margaret A. Schwarz, Md Sazzad Hassan, Urs von Holzen, Roderich E. Schwarz, Niranjan Awasthi. Targeted inhibition of c-MET, Axl and DDR signaling pathways by merestinib enhances gemcitabine/nab-paclitaxel standard chemotherapy activity in pancreatic cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1041.

Perivascular cell-derived extracellular vesicles stimulate colorectal cancer revascularization after withdrawal of antiangiogenic drugs.

Antiangiogenic tyrosine kinase inhibitors (AA‐TKIs) have become a promising therapeutic strategy for colorectal cancer (CRC). In clinical practice, a significant proportion of cancer patients temporarily discontinue AA‐TKI treatment due to recurrent toxicities, economic burden or acquired resistance. However, AA‐TKI therapy withdrawal‐induced tumour revascularization frequently occurs, hampering the clinical application of AA‐TKIs. Here, this study demonstrates that tumour perivascular cells mediate tumour revascularization after withdrawal of AA‐TKI therapy. Pharmacological inhibition and genetic ablation of perivascular cells largely attenuate the rebound effect of CRC vascularization in the AA‐TKI cessation experimental settings. Mechanistically, tumour perivascular cell‐derived extracellular vehicles (TPC‐EVs) contain Gas6 that instigates the recruitment of endothelial progenitor cells (EPCs) for tumour revascularization via activating the Axl pathway. Gas6 silence and an Axl inhibitor markedly inhibit tumour revascularization by impairing EPC recruitment. Consequently, combination therapy of regorafenib with the Axl inhibitor improves overall survival in mice metastatic CRC model by inhibiting tumour growth. Together, these data shed new mechanistic insights into perivascular cells in off‐AA‐TKI‐induced tumour revascularization and indicate that blocking the Axl signalling may provide an attractive anticancer approach for sustaining long‐lasting angiostatic effects to improve the therapeutic outcomes of antiangiogenic drugs in CRC.

Inhibition of the Axl pathway impairs breast and prostate cancer metastasis to the bones and bone remodeling

Approximately 90% of cancer-related deaths result from cancer metastasis. In prostate and breast cancers, bone is the most common site of cancer cell dissemination. Key steps in the metastatic cascade are promoted through upregulation of critical cell signaling pathways in neoplastic cells. The present study assessed the role of the receptor tyrosine kinase Axl in prostate and breast cancer cell metastasis to bones using (i) Axl knockdown neoplastic cells and osteoclast progenitor cells in vitro, (ii) intracardiac injection of Axl knockdown tumor cells in vivo, and (iii) selective Axl inhibitor BGB324. Axl inhibition in neoplastic cells significantly decreased their metastatic potential, and suppression of Axl signaling in osteoclast precursor cells also reduced the formation of mature osteoclasts. In vivo, Axl knockdown in prostate and breast cancer cells significantly suppressed the formation and progression of bone metastases. Hence, therapeutic targeting of Axl may impair tumor metastasis to the bones through neoplastic and host cell signaling axes.

Targeting the AXL Receptor in Combating Smoking-related Pulmonary Fibrosis.

Tobacco smoking is a well-known risk factor for both fibrogenesis and fibrotic progression; however, the mechanisms behind these processes remain enigmatic. RTKs (receptor tyrosine kinases) have recently been reported to drive profibrotic phenotypes in fibroblasts during pulmonary fibrosis (PF). Using a phospho-RTK array screen, we identified the RTK AXL as a top upregulated RTK in response to smoke. Both expression and signaling activity of AXL were indeed elevated in lung fibroblasts exposed to tobacco smoke, whereas no significant change to the levels of a canonical AXL ligand, Gas6 (growth arrest-specific 6), was seen upon smoke treatment. Notably, we found that smoke-exposed human lung fibroblasts exhibited highly proliferative and invasive activities and were capable of inducing fibrotic lung lesions in mice. Conversely, genetic suppression of AXL in smoke-exposed fibroblasts cells led to suppression of AXL downstream pathways and aggressive phenotypes. We further demonstrated that AXL interacted with MARCKS (myristoylated alanine-rich C kinase substrate) and cooperated with MARCKS in regulating downstream signaling activity and fibroblast invasiveness. Pharmacological inhibition of AXL with AXL-specific inhibitor R428 showed selectivity for smoke-exposed fibroblasts. In all, our data suggest that AXL is a potential marker for smoke-associated PF and that targeting of the AXL pathway is a potential therapeutic strategy in treating tobacco smoking-related PF.