Presence Of MEK Inhibitor Research Articles

Temporospatial inhibition of Erk signaling is required for lymphatic valve formation

Intraluminal lymphatic valves (LVs) and lymphovenous valves (LVVs) are critical to ensure the unidirectional flow of lymphatic fluid. Morphological abnormalities in these valves always cause lymph or blood reflux, and result in lymphedema. However, the underlying molecular mechanism of valve development remains poorly understood. We here report the implication of Efnb2-Ephb4-Rasa1 regulated Erk signaling axis in lymphatic valve development with identification of two new valve structures. Dynamic monitoring of phospho-Erk activity indicated that Erk signaling is spatiotemporally inhibited in some lymphatic endothelial cells (LECs) during the valve cell specification. Inhibition of Erk signaling via simultaneous depletion of zygotic erk1 and erk2 or treatment with MEK inhibitor selumetinib causes lymphatic vessel hypoplasia and lymphatic valve hyperplasia, suggesting opposite roles of Erk signaling during these two processes. ephb4b mutants, efnb2a;efnb2b or rasa1a;rasa1b double mutants all have defective LVs and LVVs and exhibit blood reflux into lymphatic vessels with an edema phenotype. Importantly, the valve defects in ephb4b or rasa1a;rasa1b mutants are mitigated with high-level gata2 expression in the presence of MEK inhibitors. Therefore, Efnb2-Ephb4 signaling acts to suppress Erk activation in valve-forming cells to promote valve specification upstream of Rasa1. Not only do our findings reveal a molecular mechanism of lymphatic valve formation, but also provide a basis for the treatment of lymphatic disorders.

Abstract A146: Stromal neuregulin-1 modulates the response to MEK inhibitors in WT BRAF/WT NRAS (WT/WT) melanomas

Abstract MEK-ERK1/2 signaling is elevated in the majority of melanomas. While MEK inhibitors (MEKi) are FDA-approved for mutant BRAF melanoma, and have some activity against NRAS mutant tumors, they have shown little activity in patients with melanomas that are wild type for BRAF and NRAS (WT/WT). Since the tumor microenvironment (TME) often regulates drug resistance, we tested the effects of growth factors on WT/WT melanoma growth in the presence of MEKi. Neuregulin-1 (NRG1) protected patient-derived WT/WT human melanoma cell lines from growth inhibition mediated by trametinib (MEKi). Phospho-proteomic analysis and clinical grade neutralizing antibodies implicated adaptive activation of ErbB3/ErbB2 complexes in the protective effects of NRG1. NRG1 was detected in fibroblasts and cancer-associated fibroblasts (CAF), and CAF-conditioned medium activated ErbB3/ErbB2 signaling in MEK inhibited WT/WT melanoma cells. ErbB3 and ErbB2 neutralizing antibodies blocked the protective effects of fibroblast- and CAF-derived NRG1 in vitro and cooperated with MEKi to delay xenograft growth in vivo. Together our results provide a rationale for the treatment of WT/WT melanomas with the combination of MEKi and anti-ErbB3/ErbB2 antibodies. Citation Format: Claudia Capparelli, Timothy J. Purwin, Shea Heilman, Inna Chervoneva, Michael A. Davies, Jeffrey E. Gershenwald, Andrew E. Aplin. Stromal neuregulin-1 modulates the response to MEK inhibitors in WT BRAF/WT NRAS (WT/WT) melanomas [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A146.

Targeting regulatory T cells in chronic inflammatory disease. (MPF2P.749)

Abstract Human regulatory T cells (Tregs) are functionally characterized by their ability to suppress effector T cells in addition to having an essential role in maintaining immunological tolerance. Activated Tregs are upregulated in chronic inflammation and studies in human tuberculosis (TB) patients indicate that patients with active disease have increased frequency of Tregs in peripheral blood compared to patients with latent TB. Tregs also play a role in chronic viral infections. In HIV, Tregs control viral replication in early stages of the disease but limit immune activation and pathogen specific immune responses in later stages. The MAPK/ERK signaling pathway has been shown to be involved in the regulation of the FoxP3 transcription factor, the major regulator of Treg suppressive function. We aimed to target activation of disease specific Tregs by inhibition of the MEK signaling pathway based on the hypothesis that this would improve anti-HIV and anti-TB immunity. Upon stimulation of patient PBMCs with TB- and HIV-specific antigens in presence of MEK inhibitors, we observed significantly downregulated levels of FoxP3 in resting Tregs (CD45RA+FoxP3+) and a reduced number of activated Tregs (CD45RA-FoxP3++). Differential effects on production of IFNγ TNFα and IL-2 could be observed. In conclusion, MEK inhibitors modulate disease antigen-specific Treg activation which may have potential application in new treatment approaches where limiting Treg activity would be favourable.

Abstract A31: Activity-based protein profiling reveals adaptive response to pharmacological MEK inhibition in KRAS mutant non-small cell lung cancer

Abstract KRAS mutations account for development of approximately 30% of lung adenocarcinomas. While RAF-MEK-ERK signaling cascade is a key regulator of RAS-mediated proliferation and survival, the clinical efficacy of MEK inhibitors is still underwhelming in RAS-driven cancers. We treated KRAS mutant non-small cell lung cancer (NSCLC) cell lines with two MEK inhibitors (AZD6244 and MEK162) currently in clinical development. In all cell lines tested (N=11), we failed to observe significant effects on cell viability or induction of apoptosis. Because increasing evidence indicates that cancer cells undergo adaptive proteomic rewiring in response to kinase inhibitors, we performed activity-based protein profiling (ABPP) after treating 11 KRAS mutant NSCLC cell lines with MEK inhibitors in order to identify global enzymatic changes potentially involved in these adaptive processes. Our ABPP approach employs desthiobiotin-ATP probe, which enables selective labeling of active sites in ATP-binding proteins including kinases, chaperones and metabolic enzymes. Labeled proteins were subsequently purified and analyzed by mass spectrometry-based proteomics technique. Our ABPP results revealed that MEK inhibitors increased levels of multiple receptor tyrosine kinases (RTK) including EGFR and MET. MEK inhibitors induced MET phosphorylation and EGF-mediated AKT phosphorylation was markedly enhanced in the presence of MEK inhibitors, indicating increased RTK activity in adaptive response to MEK inhibition. We also found that MEK inhibition led to increased multiple heat shock proteins including HSP90 and HSP70, suggesting the possibility of chaperone-mediated adaptive resistance. Finally, MEK inhibitors globally increased enzymes involved in glucose utilization pathways implicating metabolic alterations in adaptive response to MEK inhibition. Collectively, our systematic activity-based proteomic profiling approach provides novel insights into adaptive resistance mechanisms to MEK inhibitors, suggesting rational combination treatment strategies for KRAS mutant NSCLC. Updated works will be presented. Citation Format: Jae-Young Kim, Bin Fang, Eric A. Welsh, Fumi Kinose, Jiannong Li, Steven A. Eschrich, John M. Koomen, Eric B. Haura. H. Activity-based protein profiling reveals adaptive response to pharmacological MEK inhibition in KRAS mutant non-small cell lung cancer. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr A31. doi: 10.1158/1557-3125.RASONC14-A31

The MEK-ERK Pathway Is Necessary for Serine Phosphorylation of Mitochondrial STAT3 and Ras-Mediated Transformation

Activating mutations in the RasGTPases are the most common oncogenic lesions in human cancer. Similarly, elevated STAT3 expression and/or phosphorylation are observed in the majority of human cancers. We recently found that activated Ras requires a mitochondrial rather than a nuclear activity of STAT3 to support cellular transformation. This mitochondrial activity of STAT3 was supported by phosphorylation on serine 727 (S727) in the carboxyl-terminus of STAT3. In this study we show that the H-Ras oncoprotein engages the MEK-ERK pathway to drive phosphorylation of STAT3 on S727, while phosphoinositide 3-kinase (PI3K) and mTOR activity were superfluous. Moreover, pharmacological inhibition of MEK reduced transformation by H-, K- or N-Ras. However, cells expressing a mitochondrially restricted STAT3 with a phospho-mimetic mutation at S727 were partially resistant to inhibition of the ERK pathway, exhibiting a partial rescue of anchorage-independent cell growth in the presence of MEK inhibitor. This study shows that the MEK-ERK pathway is required for activated Ras-induced phosphorylation of STAT3 on S727, that inhibition of STAT3 S727 phosphorylation contributes to the anti-oncogenic potential of MEK inhibitors, and that mitochondrial STAT3 is one of the critical substrates of the Ras-MEK-ERK- axis during cellular transformation.

Suppression of apoptosis by BRAF inhibitors through off-target inhibition of JNK signaling.

8537 Background: The advent of targeted therapy has revolutionized the treatment of cancer. The mutant BRAFV600E protein is found in over 50% of melanomas and thyroid carcinomas, resulting in elevated kinase activity, increased mitogen-activated protein kinase (MAPK) pathway signaling, and cell proliferation. Vemurafenib and PLX4720 were designed to selectively inhibit the BRAF kinase, and clinical trials of vemurafenib in metastatic melanoma have demonstrated a response rate of over 50% and an overall survival advantage over standard dacarbazine therapy. Approximately 20-30% of individuals treated with vemurafenib develop cutaneous squamous cell carcinoma (cSCC) highlighting the importance of understanding toxicities associated with this drug. Paradoxical ERK activation in BRAF wild-type, RAS-mutant cells is thought to be the major mechanism by which this occurs, as evidenced by the presence of RAS mutations in 60% of such lesions. Methods: Using a combination of BRAF-wild-type cSCC cell lines, primary human keratinocytes, as well as a UV mouse model of cSCC and human cSCC samples, we identified novel effects of BRAFi on apoptosis. Results: Here we show an unexpected and novel effect of vemurafenib and PLX4720 in suppressing apoptosis through the inhibition of multiple off-target kinases. JNK signaling and apoptosis are suppressed in cSCC lesions arising in vemurafenib-treated patients as well as in irradiated mouse skin. This occurs independently of paradoxical ERK signaling and in the presence of MEK inhibitor. Treatment with PLX4720 greatly accelerates the development of UV-induced cSCC in mice without Ras mutations. Kinome screening identified ZAK and MKK4 (MEK4 / MAP2K4) kinases as inhibited by vemurafenib, leading to suppression of MKK4 and MKK7 (MAP2K7) phosphorylation. Knockdown of inhibited off-target kinases recapitulates these anti-apoptotic effects of vemurafenib. Conclusions: Our results implicate suppression of JNK signaling, independent of ERK activation, as an additional, complementary mechanism of adverse effects of vemurafenib. This has broad implications for combination therapies with other modalities that induce apoptosis and for the long-term use of vemurafenib in the adjuvant setting.

Maturation and long-term hypoxia-induced acclimatization responses in PKC-mediated signaling pathways in ovine cerebral arterial contractility

In the developing fetus, cerebral arteries (CA) show striking differences in signal transduction mechanisms compared with the adult, and these differences are magnified in response to high-altitude long-term hypoxia (LTH). In addition, in the mature organism, cerebrovascular acclimatization to LTH may be associated with several clinical problems, the mechanisms of which are unknown. Because PKC plays a key role in regulating CA contractility, in fetal and adult cerebral arteries, we tested the hypothesis that LTH differentially regulates the PKC-mediated Ca(2+) sensitization pathways and contractility. In four groups of sheep [fetal normoxic (FN), fetal hypoxic (FH), adult normoxic (AN), and adult hypoxic (AH)], we examined, simultaneously, responses of CA tension and intracellular Ca(2+) concentration and measured CA levels of PKC, ERK1/2, RhoA, 20-kDa myosin light chain, and the 17-kDa PKC-potentiated myosin phosphatase inhibitor CPI-17. The PKC activator phorbol 12,13-dibutyrate (PDBu) produced robust contractions in all four groups. However, PDBu-induced contractions were significantly greater in AH CA than in the other groups. In all CA groups except AH, in the presence of MEK inhibitor (U-0126), the PDBu-induced contractions were increased a further 20-30%. Furthermore, in adult CA, PDBu led to increased phosphorylation of ERK1, but not ERK2; in fetal CA, the reverse was the case. PDBu-stimulated ERK2 phosphorylation also was significantly greater in FH than FN CA. Also, although RhoA/Rho kinase played a significant role in PDBu-mediated contractions of FN CA, this was not the case in FH or either adult group. Also, whereas CPI-17 had a significant role in adult CA contractility, this was not the case for the fetus. Overall, in ovine CA, the present study demonstrates several important maturational and LTH acclimatization changes in PKC-induced contractile responses and downstream pathways. The latter may play a key role in the pathophysiologic disorders associated with acclimatization to high altitude.

Follistatin, induced by thyrotropin-releasing hormone (TRH), plays no role in prolactin expression but affects gonadotropin FSHβ expression as a paracrine factor in pituitary somatolactotroph GH3 cells

Follistatin regulates FSHβ gene expression by binding to and bioneutralizing activin effects. In this study, we found that thyrotropin-releasing hormone (TRH) increased follistatin gene expression in pituitary somatolactotroph GH3 cells. Treatment of GH3 with 100 nM TRH significantly increased follistatin mRNA expression as determined by real time PCR. TRH-induced follistatin expression was significantly abrogated in the presence of MEK inhibitor, U0126. Overexpression of constitutive active MEKK in GH3 cells dramatically increased follistatin expressions. Transfection of GH3 cells with follistatin siRNA reduced endogenous follistatin mRNA expression, but failed to modulate prolactin promoter activity. Prolactin mRNA levels were not affected by increasing the dose of follistatin, and TRH-induced prolactin promoter activity was not modulated in the presence of follistatin. In other experiments using pituitary gonadotroph LβT2 cells, activin increased FSHβ promoter activity and mRNA expression, and follistatin completely inhibited this activin-increased FSHβ gene expression. Treatment of GH3 cells with activin reduced the basal activity of prolactin promoter and follistatin prevented this effect. GH3 cells were co-cultured with LβT2 cells, which had been transfected with FSHβ promoter-linked luciferase vectors and treated with activin in the presence of TRH. Activin-induced FSHβ promoter activity was completely inhibited in the presence of TRH. In addition to that, FSHβ mRNA was not detected from LβT2 cells which were co-cultured with GH3 cells. Our current results suggest the possibility that TRH increases follistatin gene expression in prolactin-producing cells in association with ERK pathways. Somatolactotroph-derived follistatin affects gonadotrophs by countering activin-induced FSHβ gene expression in a paracrine fashion.

Germ Line Gain of Function with SOS1 Mutation in Hereditary Gingival Fibromatosis

Mutation of human SOS1 is responsible for hereditary gingival fibromatosis type 1, a benign overgrowth condition of the gingiva. Here, we investigated molecular mechanisms responsible for the increased rate of cell proliferation in gingival fibroblasts caused by mutant SOS1 in vitro. Using ectopic expression of wild-type and mutant SOS1 constructs, we found that truncated SOS1 could localize to the plasma membrane, without growth factor stimuli, leading to sustained activation of Ras/MAPK signaling. Additionally, we observed an increase in the magnitude and duration of ERK signaling in hereditary gingival fibromatosis gingival fibroblasts that was associated with phosphorylation of retinoblastoma tumor suppressor protein and the up-regulation of cell cycle regulators, including cyclins C, D, and E and the E2F/DP transcription factors. These factors promote cell cycle progression from G(1) to S phase, and their up-regulation may underlie the increased gingival fibroblast proliferation observed. Selective depletion of wild-type and mutant SOS1 through small interfering RNA demonstrates the link between mutation of SOS1, ERK signaling, cell proliferation rate, and the expression levels of Egr-1 and proliferating cell nuclear antigen. These findings elucidate the mechanisms for gingival overgrowth mediated by SOS1 gene mutation in humans.

MEK blockade converts AML differentiating response to retinoids into extensive apoptosis

AbstractThe aberrant function of transcription factors and/or kinase-based signaling pathways that regulate the ability of hematopoietic cells to proliferate, differentiate, and escape apoptosis accounts for the leukemic transformation of myeloid progenitors. Here, we demonstrate that simultaneous retinoid receptor ligation and blockade of the MEK/ERK signaling module, using the small-molecule inhibitor CI-1040, result in a strikingly synergistic induction of apoptosis in both acute myeloid leukemia (AML) and acute promyelocytic leukemia (APL) cells with constitutive ERK activation. This proapoptotic synergism requires functional RAR and RXR retinoid receptors, as demonstrated using RAR- and RXR-selective ligands and RAR-defective cells. In the presence of MEK inhibitors, however, retinoid-induced chromatin remodeling, target-gene transcription, and granulocytic differentiation are strikingly inhibited and apoptosis induction becomes independent of death-inducing ligand/receptor pairs; this suggests that apoptosis induction by combined retinoids and MEK inhibitors is entirely distinct from the classical “postmaturation” apoptosis induced by retinoids alone. Finally, we identify disruption of Bcl-2–dependent mitochondrial homeostasis as a possible point of convergence for the proapoptotic synergism observed with retinoids and MEK inhibitors. Taken together, these results indicate that combined retinoid treatment and MEK blockade exert powerful antileukemic effects and could be developed into a novel therapeutic strategy for both AML and APL.

A Novel BH3 Mimetic Reveals a Mitogen-Activated Protein Kinase–Dependent Mechanism of Melanoma Cell Death Controlled by p53 and Reactive Oxygen Species

The RAS/BRAF/MEK/ERK mitogen-activated protein kinase (MAPK) pathway is emerging as a key modulator of melanoma initiation and progression. However, a variety of clinical studies indicate that inhibiting the MAPK pathway is insufficient per se to effectively kill melanoma cells. Here, we report on a genetic and pharmacologic approach to identify survival factors responsible for the resistance of melanoma cells to MEK/ERK antagonists. In addition, we describe a new tumor cell-selective means to bypass this resistance in vitro and in vivo. By generating a panel of isogenic cell lines with specific defects in the apoptotic machinery, we found that the ability of melanoma cells to survive in the absence of functional MEK relies on an ERK-independent expression of the antiapoptotic factor Mcl-1 (and to a lesser extent, Bcl-x(L) and Bcl-2). Using computer-based modeling, we developed a novel Bcl-2 homology domain 3 (BH3) mimetic. This compound, named TW-37, is the first rationally designed small molecule with high affinity for Mcl-1, Bcl-x(L), and Bcl-2. Mechanistic analyses of the mode of action of TW-37 showed a synergistic tumor cell killing in the presence of MEK inhibitors. Importantly, TW-37 unveiled an unexpected role of the MAPK pathway in the control of reactive oxygen species (ROS). This function was critical to prevent the activation of proapoptotic functions of p53 in melanoma cells, but surprisingly, it was dispensable for normal melanocytes. Our results suggest that this MAPK-dependent ROS/p53 feedback loop is a point of vulnerability of melanoma cells that can be exploited for rational drug design.

MEK inhibitors impair insulin-stimulated glucose uptake in 3T3-L1 adipocytes.

In 3T3-L1 adipocytes, insulin activates three major signaling cascades, the phosphoinositide 3-kinase (PI3K) pathway, the Cbl pathway, and the mitogen-activated protein kinase (MAPK) pathway. Although PI3K and Cbl mediate insulin-stimulated glucose uptake by promoting the translocation of the insulin-responsive glucose transporter (GLUT4) to the plasma membrane, the MAPK pathway does not have an established role in insulin-stimulated glucose uptake. We demonstrate in this report that PI3K inhibitors also inhibit the MAPK pathway. To investigate the role of the MAPK pathway separately from that of the PI3K pathway in insulin-stimulated glucose uptake, we used two specific inhibitors of MAPK kinase (MEK) activity, PD-98059 and U-0126, which reduced insulin-stimulated glucose uptake by approximately 33 and 50%, respectively. Neither MEK inhibitor affected the activation of Akt or PKCzeta/lambda, downstream signaling molecules in the PI3K pathway. Inhibition of MEK with U-0126 did not prevent GLUT4 from translocating to the plasma membrane, nor did it inhibit the subsequent docking and fusion of GLUT4-myc with the plasma membrane. MEK inhibitors affected glucose transport mediated by GLUT4 but not GLUT1. Importantly, the presence of MEK inhibitors only at the time of the transport assay markedly impaired both insulin-stimulated glucose uptake and MAPK signaling. Conversely, removal of MEK inhibitors before the transport assay restored glucose uptake and MAPK signaling. Collectively, our studies suggest a possible role for MEK in the activation of GLUT4.

The extracellular signal-regulated kinase cascade suppresses amyloid β protein-induced promotion of glutamate clearance in cultured rat cortical astrocytes

We have recently found that Alzheimer’s disease amyloid β protein (Aβ) activates the extracellular signal-regulated kinase (ERK) and promotes l-glutamate uptake in astrocytes. To elucidate the relationship between the Aβ-induced ERK phosphorylation and promotion of l-glutamate uptake, we investigated the effects of U0126 and PD98059, specific inhibitors of the ERK-activating enzyme MEK, in cultured rat cortical astrocytes. Aβ-induced ERK phosphorylation was completely blocked by the MEK inhibitors, while Aβ-induced promotion of extracellular l-glutamate clearance was enhanced by the presence of the MEK inhibitors. Aβ-induced increase of the glutamate transporter GLAST expression was also enhanced by the presence of MEK inhibitors. The effective concentrations of MEK inhibitors in enhancing Aβ-induced promotion of glutamate clearance and GLAST expression were consistent with those in blocking Aβ-induced ERK phosphorylation. These results suggest that the MEK/ERK signal functions to suppress Aβ-induced upregulation of a glutamate uptake system in astrocytes.

Activation of the Extracellular Signal-Related Kinase/Mitogen-Activated Protein Kinase Pathway Discriminates CD4 Versus CD8 Lineage Commitment in the Thymus

We have investigated the role of the mitogen-activated protein kinase (MAPK) pathway in the differentiation of CD4+ and CD8+ T cells by looking specifically at the effects of inhibitors of MAPK-activating enzyme, MAPK/extracellular signal-related kinase (ERK) kinase (MEK), during the positive selection step from double-positive to single-positive (SP) thymocytes. Using a variety of transgenic/knockout mouse strain combinations that fail to differentiate individual lineages of SP thymocytes together with genetically engineered F(ab')2 reagents that induce maturation preferentially to either the CD4 or CD8 subpopulations, we show that induction of CD4 differentiation cells is highly sensitive to levels of MEK inhibition that have no effect on CD8 maturation. In addition, the presence of MEK inhibitor is able to modify signals that normally induce CD4 differentiation to instead promote CD8 differentiation. Finally, we show that continuous culture in the presence of inhibitor interferes with TCR up-regulation in SP thymocytes, suggesting that MAPK signaling may be involved in final maturation steps for both lineages. These data indicate that there is discrimination in the biochemical pathways that are necessary to specify CD4 and CD8 lineage commitment and can reconcile previously conflicting reports on the influence of MAPK activation in commitment and maturation of thymocytes.