Overexpression Of Skp2 Research Articles

E3 ligase SCFSKP2 ubiquitinates and degrades tumor suppressor C/EBPα in acute myeloid leukemia

AimTranscription factor CCAAT/Enhancer binding protein alpha (C/EBPα) is a key regulator of myeloid differentiation, granulopoiesis in particular. Although CEBPA mutations are found in more than 10% in AML, functional inhibition of C/EBPα protein is also widely observed in AML. Here, we sought to examine if SKP2, an aberrantly enhanced E3 ubiquitin ligase in primary AMLs inhibits C/EBPα stability to induce differentiation block. Main methodsHere we employed cell based assays such as transfections, immunoblotting, co-immunoprecipitation, luciferase and gel shift assays along with differentiation assays to investigate SKP2 regulated C/EBPα protein stability in acute myeloid leukemia. Key findingsHere we discovered that oncogenic E3 ubiquitin ligase SCFskp2 ubiquitinates and destabilizes C/EBPα in a proteasome-dependent manner. Our data demonstrates that SKP2 physically interacts with C-terminal of C/EBPα and promotes its K48-linked ubiquitination-mediated degradation leading to its reduced transactivation potential, DNA binding ability and cellular functions. We further show that while overexpression of SKP2 inhibits both ectopic as well as endogenous C/EBPα in heterologous (HEK293T) as well as myeloid leukemia cells respectively, SKP2 depletion restores endogenous C/EBPα leading to reduced colony formation and enhanced myeloid differentiation of myeloid leukemia cells. Using Estradiol-inducible K562-C/EBPα-ER cells as yet another model of granulocytic differentiation, we further confirmed that SKP2 overexpression indeed inhibits granulocytic differentiation by mitigating C/EBPα stability. SignificanceOur findings identify SKP2 as a potential negative regulator of C/EBPα stability and function in AML which suggests that SKP2 can be potentially targeted in AML to restore C/EBPα and overcome differentiation block.

Paeoniflorin inhibits cell viability and invasion of liver cancer cells via inhibition of Skp2.

Paeoniflorin (PF) has been demonstrated to exert tumor suppressive functions in various types of human cancer. However, the mechanisms of PF-mediated anti-tumor activity have not been fully elucidated. S-phase kinase associated protein 2 (Skp2) has been characterized as an oncoprotein that contributes to carcinogenesis. Therefore, the inhibition of Skp2 may be a useful approach for the treatment of various types of human cancer. The present study explored whether PF inhibited the expression of Skp2 in liver cancer cells, leading to cell viability inhibition, induction of apoptosis, and suppression of migration and invasion. PF treatment led to inhibition of Skp2 expression in liver cancer cells. The overexpression of Skp2 abolished PF-mediated anti-cancer activity, whereas the downregulation of Skp2 enhanced this type of activity. The data indicated that PF may be considered as a novel inhibitor of Skp2 in liver cancer cells.

S-phase kinase-associated protein-2 (Skp2) promotes nucleus pulposus cell proliferation by inhibition of p27 in attenuating intervertebral disc degeneration.

Nucleus pulposus (NP) cell proliferation plays a key role during the process of intervertebral disc degeneration (IDD). S-phase kinase-associated protein-2 (Skp2) has been proved as an important regulator for cell growth factors in vitro. Nonetheless, whether Skp2 attenuates IDD by mediating NP cell proliferation still remains unclear. Therefore, the aim of this study was to explore how Skp2 affected NP cell proliferation and the potential mechanism in vitro. In this study, we first collected different degenerated human NP samples and isolated NP cells from these tissues. NP cell degenerated model was established with IL-1β, and the cells were transfected with lentivirus to achieve Skp2 overexpression. Besides, SKPinC1 was used to suppress Skp2 expression in vitro. Western blot, RT-PCR, and immunocytofluorescence were applied to detect genetic differences among groups. Furthermore, cell viability and cell cycle were determined by CCK-8 assay and flow cytometry, respectively. Skp2 expression decreased significantly in degenerated disc samples (p<0.05). IL-1β stimulation significantly promoted NP cell degeneration, which could be reversed by Skp2 overexpression (p<0.05). Meanwhile, Skp2 in IDD significantly inhibited the expression level of medial p27 and promoted cell cycle by CDK2 activation (p<0.05). In addition, Skp2 suppression affected NP cell proliferation in vitro. NP cells exhibited significantly inhibited proliferation ability when down-regulated the expression level of Skp2. Our findings provided a more meritorious viewpoint of Skp2 in NP cell proliferation. Furthermore, the above results suggested that Skp2 was a novel target in the treatment of IDD.

The Skp2 Pathway: A Critical Target for Cancer Therapy.

Strictly regulated protein degradation by ubiquitin-proteasome system (UPS) is essential for various cellular processes whose dysregulation is linked to serious diseases including cancer. Skp2, a well characterized component of Skp2-SCF E3 ligase complex, is able to conjugate both K48-linked ubiquitin chains and K63-linked ubiquitin chains on its diverse substrates, inducing proteasome mediated proteolysis or modulating the function of tagged substrates respectively. Overexpression of Skp2 is observed in various human cancers associated with poor survival and adverse therapeutic outcomes, which in turn suggests that Skp2 engages in tumorigenic activity. To that end, the oncogenic properties of Skp2 are demonstrated by various genetic mouse models, highlighting the potential of Skp2 as a target for tackling cancer. In this article, we will describe the downstream substrates of Skp2 as well as upstream regulators for Skp2-SCF complex activity. We will further summarize the comprehensive oncogenic functions of Skp2 while describing diverse strategies and therapeutic platforms currently available for developing Skp2 inhibitors.

BRAFV600E dictates cell survival via c-Myc-dependent induction of Skp2 in human melanoma

BRAFV600E mutation is frequently observed in melanoma, and contributes to tumor malignancy. Despite inhibition of BRAF causes a profound cell growth inhibition and a strong clinical benefit in BRAFV600E melanoma, acquired drug resistance is still the major hurdle. In this study, we demonstrate that BRAFV600E drives cell growth and glycolysis in melanoma cells but does so by a previously unappreciated mechanism that involves direct induction of Skp2. Skp2 is highly expressed in melanoma tissues and particularly in tissues with BRAFV600E mutation. The inhibition of BRAFV600E by either siRNA or inhibitor vemurafenib suppressed Skp2 expression and cell growth. Mechanistic study shows that BRAFV600E suppression of Skp2 is dependent on c-Myc transcription factor via specifically bounding to the E-box region on SKP2 promoter. Further, the overexpression of Skp2 resulted in a markedly increase in cell growth, cell cycle progression and glycolysis which were repressed by BRAFV600E inhibition. Supporting the biological significance, Skp2 is specifically correlated with poor patient outcome in BRAFV600E but did not in BRAFWT melanomas. Thus, as a downstream target of BRAFV600E, Skp2 is critical for responses to BRAF inhibition, indicating targeting Skp2 might be a promising strategy for the treatment of BRAFi resistant melanomas.

Skp2-mediated ubiquitination and mitochondrial localization of Akt drive tumor growth and chemoresistance to cisplatin.

The E3 ligase S-phase kinase-associated protein 2(Skp2) is overexpressed in human cancers and correlated with poor prognosis, but its contributions to tumorigenesis and chemoresistance in nasopharyngeal carcinoma (NPC) are not evident. Herein we show that Skp2 is highly expressed in NPC tumor tissues and cell lines. Knockdown of Skp2 suppresses tumor cell growth, colony formation, glycolysis, and in vivo tumor growth. Skp2 promotes Akt K63-mediated ubiquitination and activation, which is required for EGF-induced Akt mitochondrial localization. Importantly, K63-linked ubiquitination enhances the interaction between Akt and HK2 and eventually increases HK2 phosphorylation on Thr473 and mitochondrial localization. Overexpression of Skp2 impaired the intrinsic apoptotic pathway and confers cisplatin resistance. Moreover, ectopic expression of Myr-Akt1 or phosphomimetic HK2-T473D rescued cisplatin-induced tumor suppression in Skp2 knockdown stable cells. Also, depletion of Akt ubiquitination enhances the antitumor efficacy of cisplatin in vitro and in vivo. Finally, we demonstrated that Skp2 is positively correlated with p-Akt and HK2 in NPC tumor tissues. This study highlights the clinical value of Skp2 targeting in NPC treatment.

Abstract 181A: PIN1 and FOXP3 regulate SKP2 expression

Abstract S-phase kinase-associated protein 2 (SKP2), a component of the E3 ubiquitin ligase SKP1-CUL1-Fbox complex, acts as an oncogene/oncoprotein as evidence suggests that overexpression of SKP2 results in cell cycle dysregulation and tumorigenesis as well as associates with poor prognosis in numerous cancers. The unique peptidyl-propyl isomerase PIN1, which isomerizes the cis-trans peptide bond between pSer/pThr and proline, regulates its target proteins’ cell localization, stability, and function. However, whether SKP2 is the target protein of PIN1 remains unknown. Herein, we demonstrate that SKP2 expression is regulated by PIN1 and FOXP3. First, we observed that knock-down PIN1 using siRNAs reduced SKP2 protein level in several cancer cell lines (MCF7, PC3, DU145, HepG2, and JEG3 cells). Secondly, we found that overexpression of PIN1 increased SKP2 protein level. Furthermore, PIN1 dose-dependently enhanced SKP2 promoter activity in gene reporter assay. Since FOXP3 is a known transcriptional repressor for the oncogene SKP2, we investigated whether PIN1 affects FOXP3-mediated SKP2 expression. Interestingly, we observed that PIN1 dose-dependently counteracted FOXP3-mediated SKP2 promoter activity. Finally, we demonstrated that mutations on PIN1 (W34A) and FOXP3 (Y342F) abolished PIN1-mediated and FOXP3-mediated SKP2 expression, respectively. Taken together, our results suggest that PIN1 functions as a novel regulator of SKP2 and with FOXP3 may be involved in tumor development and progression. Citation Format: Wei-Hsiung Yang, Chiung-Min Wang, William H. Yang. PIN1 and FOXP3 regulate SKP2 expression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 181A.

Abstract 5865: Gartanin, a 4-prenylated xanthone in the mangosteen fruit, inhibits the growth of prostate cancer cells associated with Skp2 degradation via neddylation inhibition and FBXW2 induction

Abstract The S-phase kinase-associated protein 2 (Skp2) is overexpressed in prostate cancer and associated with tumor stage, disease recurrence, and worse patient survival. Studies also have demonstrated that Skp2 overexpression plays a critical role in development and progression of prostate cancer in several prostate cancer models, including PTEN knockout and TRAMP carcinogenesis and CWR22 xenograft models. Therefore, Skp2 is a promising target for developing new therapeutic and preventive approaches for prostate cancer. Here, we have found that gartanin, 4-prenylated xanthone from the Garcinia mangostana fruit (mangosteen), promoted Skp2 degradation in a proteasome-dependent manner in prostate cancer cells. In addition, gartanin induced the expression of FBXW2, a novel E3 ligase of SKP2 for targeted degradation, and downregulated the expression of NEDD8, leading to reduced neddylation levels of UBC12 and Cullin1. Molecular modelling analysis predicts that gartanin reasonably docks onto the regulatory subunit of NEDD8 activating enzyme E1 (NAE1) and next to the NEDD8 binding complex. More interestingly, gartanin selectively inhibits the growth of Skp2 overexpressing PC3 cells compared to parental PC cells (IC50 values for PC3/Skp2 vs. PC3 cells are approximately 4 μM vs. 14 μM). Together, our data indicate that gartanin is a novel natural agent for targeting Skp2 degradation via induction of FBXW2 expression and inhibition of neddylation. Gartanin deserves further investigation for its usefulness in prostate cancer prevention and treatment. Citation Format: Victor Pham, Michelle Bui, Raymond Rendon, Ericka Agredano, Thanh Le, Liankun Song, Xiaolin Zi. Gartanin, a 4-prenylated xanthone in the mangosteen fruit, inhibits the growth of prostate cancer cells associated with Skp2 degradation via neddylation inhibition and FBXW2 induction [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5865.

Abstract 275: Flavokawain B, a kava chalcone, acts synergistically with proteasome inhibitors (i.e., bortezomib and MG132) to reduce the growth of prostate cancer cells via downregulation of Skp2

Abstract Flavokawain B, a chalcone isolated from kava root extracts, has been shown to inhibit the in vitro and in vivo growth of various cancer cell lines via induction of apoptosis. However, the mechanism of flavokawain B's action remains largely unknown. Here we have demonstrated that flavokawain B caused Skp2 degradation in an ubiquitin and proteasome dependent manner. In addition, flavokawain B inhibited NEDD8 conjugations to both Cullin1 and Ubc12 in PC3 cells and Ubc12 NEDDylation in an in vitro assay. Overexpression of dominant-negative cullin1 (1-452) or K720R mutant cullin1 rescued the effect of flavokawain B on Skp2 degradation, but further increased the expression of p27. However, siRNA knock-down of Skp1, CSN5, and UBC12 expression resulted in down-regulation of Skp2 expression and further enhanced the effect of flavokawain B on Skp2 degradation. siRNA knock-down of Cdh1, a known E3 ligase of Skp2 for targeted degradation, also didn't rescue the effect of flavokawain B on Skp2 degradation, but overexpression of F-box deleted Skp2 can reverse the effect of flavokawain B on Skp2 degradation. These results suggest that degradation of Skp2 by flavokawain B is involved in Cullin1. Furthermore, flavokawain B selectively inhibited the growth of pRb deficient cell lines and PC3 cells with Skp2 overexpression. Flavokawain B also acts synergistically with Bortezomib and MG132 to inhibit the growth of prostate cancer cell lines via down-regulation of Skp2 and up-regulation of p27 and p21 expression. These finding provide rationale for combination of flavokawain B and Bortezomib in treatment of prostate cancer. Citation Format: Xuesen Li, Victor Pham, Matthew Tippin, Shan Xu, Dongjun Fu, Liankun Song, Bang H. Hoang, Xiaolin Zi. Flavokawain B, a kava chalcone, acts synergistically with proteasome inhibitors (i.e., bortezomib and MG132) to reduce the growth of prostate cancer cells via downregulation of Skp2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 275.

SKP2- and OTUD1-regulated non-proteolytic ubiquitination of YAP promotes YAP nuclear localization and activity

Dysregulation of YAP localization and activity is associated with pathological conditions such as cancer. Although activation of the Hippo phosphorylation cascade is known to cause cytoplasmic retention and inactivation of YAP, emerging evidence suggests that YAP can be regulated in a Hippo-independent manner. Here, we report that YAP is subject to non-proteolytic, K63-linked polyubiquitination by the SCFSKP2 E3 ligase complex (SKP2), which is reversed by the deubiquitinase OTUD1. The non-proteolytic ubiquitination of YAP enhances its interaction with its nuclear binding partner TEAD, thereby inducing YAP’s nuclear localization, transcriptional activity, and growth-promoting function. Independently of Hippo signaling, mutation of YAP’s K63-linkage specific ubiquitination sites K321 and K497, depletion of SKP2, or overexpression of OTUD1 retains YAP in the cytoplasm and inhibits its activity. Conversely, overexpression of SKP2 or loss of OTUD1 leads to nuclear localization and activation of YAP. Altogether, our study sheds light on the ubiquitination-mediated, Hippo-independent regulation of YAP.

Tanshinone IIA regulates colorectal cancer apoptosis via attenuation of Parkin‑mediated mitophagy by suppressing AMPK/Skp2 pathways.

Mitophagy is important for cancer development. Notably, the role of Parkin‑mediated mitophagy in colorectal cancer (CRC) mortality has not been fully determined. Therefore, the present study aimed to investigate the effect of Parkin‑mediated mitophagy on CRC apoptosis. In addition, the present study investigated the therapeutic effects of TanshinoneIIA (TanIIA) on the regulation of CRC cell death via mitophagy. Cellular apoptosis was measured following TanIIA treatment. In addition, mitophagy activity was evaluated by immunofluorescence and western blotting. The results of the present study revealed that TanIIA may enhance CRC cell death. In addition, the results demonstrated that TanIIA enhanced mitochondrial apoptosis, as demonstrated by reduced mitochondrial membrane potential, elevated mitochondrial permeability transition pore opening, and increased oxidative stress, mitochondrial energy disorder and proapoptotic factor expression. Furthermore, the results of the present study demonstrated that TanIIA induced mitochondrial apoptosis via inhibition of mitophagy. In addition, it was revealed that mitophagy could suppress mitochondrial apoptosis. Functional assays revealed that TanIIA suppressed the adenosine monophosphate‑activated protein kinase (AMPK) pathway, resulting in the inactivation of S‑phase kinase associated protein2 (Skp2). Furthermore, reduced levels of Skp2 failed to activate Parkin, thus resulting in inhibition of mitophagy. Conversely, reactivation of AMPK and overexpression of Skp2 rescued mitophagy activity and thus attenuated the TanIIA‑induced apoptosis of CRC cells. In conclusion, the results of the present study demonstrated the beneficial role of mitophagy in CRC cell survival and suggested that TanIIA may be an effective therapeutic agent, which suppresses mitophagy activity and enhances CRC apoptosis.

Paeoniflorin exerts antitumor effects by inactivating S phase kinase-associated protein 2 in glioma cells.

Paeoniflorin (PF), a natural compound isolated from Paeoniae radix, has been shown to exert antitumor effects in various types of human cancers including glioma. However, the mechanism of action is not well understood. S-phase kinase-associated protein (Skp)2 functions as an oncogene in many cancers. In the present study, we investigated whether Skp2 mediates the anti-glioma activity of PF. We found that PF inhibited glioma cell proliferation, migration and invasion, and induced G2/M arrest and apoptosis. Skp2 expression was downregulated in glioma cells treated with PF. PF-induced antitumor effects in glioma cells were abolished by Skp2 overexpression but were enhanced by RNA interference of Skp2. Moreover, PF treatment inhibited U87 cell-derived tumor growth in a xenograft mouse model. These results demonstrate that PF exerts its antitumor effects in part by inhibiting Skp2 expression in glioma cells and could be a promising therapeutic agent for glioma therapy.

MicroRNA-365 accelerates cardiac hypertrophy by inhibiting autophagy via the modulation of Skp2 expression

Evidence is emerging of a tight link between cardiomyocyte autophagy and cardiac hypertrophy (CH). Sustained exposure to stress leads CH to progress to heart failure. Several miRNAs have been described in heart failure, and miRNA-based therapeutic approaches are being pursued. Although microRNA-365 (miR-365) has been testified as a positive modulator of CH, the specific mechanism remains unclear. In the present study, we observed that miR-365 expression was up-regulated in hypertrophic cardiomyocytes both in vivo and in vitro, and was accompanied by dysregulation of autophagy. We found that miR-365 negatively modulates autophagy in hypertrophic cardiomyocytes by targeting Skp2. Overexpression of Skp2 promoted autophagy and rescued CH induced by Ang-II; conversely, Skp2 knockdown further inhibited autophagy and CH. Furthermore, we found that the activation of mammalian target of rapamycin (mTOR) signaling was regulated by Skp2 following Ang-II treatment, as indicated by the up-regulation of p-S6K and p-4EBP1 levels. The inactivation of mTOR by rapamycin completely abolished the Ang-II-induced inhibition of autophagy. In conclusion, our study provides substantial evidence that miR-365 and its target gene Skp2 play a functional role in CH and suggests the development of novel therapeutic options based on miR-365 and Skp2.

Inhibition of Skp2 sensitizes lung cancer cells to paclitaxel.

S-phase kinase-associated protein 2 (Skp2) is an E3 ubiquitin ligase and plays an important role in the control of cell cycle progression. Skp2 is upregulated in several cancers, including lung cancers, but the role of Skp2 in the tumorigenesis and anticancer drug resistance in human lung cancer remains to be determined. We report here that Skp2 positively regulated mitotic arrest deficient 2 (MAD2) expression and that inhibition of Skp2 sensitizes human lung cancer cells to paclitaxel. Knockdown of Skp2 by small interfering RNA (siRNA) decreased Mad2 messenger RNA (mRNA) and protein levels in A549 and NCI-H1975 cells, accompanied with upregulation of p27 but decrease of the phosphorylation of retinoblastoma (Rb). In contrast, ectopic overexpression of Skp2 increased Mad2 mRNA and protein levels and phosphorylation of Rb, while it decreased p27. Pharmacological inhibition of CDK1/2 by flavopiridol or E2F1 with HLM006474 led to downregulation of Mad2 expression and prevented the increase of Mad2 expression by Skp2. Most importantly, pharmacological inhibition of Skp2 sensitized A549 and NCI-H1299 cells to paclitaxel. Our results demonstrated that SKP2 positively regulates the gene expression of MAD2 through p27-CDKs-E2F1 signaling pathway and that inhibition of Skp2 sensitizes A549 and NCI-H1299 cells to paclitaxel, suggesting that small molecule inhibitors of Skp2 are potential agents for the treatment of lung cancer with upregulation of Skp2.

Soluble Egg Antigens of Schistosoma japonicum Induce Senescence of Activated Hepatic Stellate Cells by Activation of the FoxO3a/SKP2/P27 Pathway.

BackgroundLiver fibrosis was viewed as a reversible process. The activation of hepatic stellate cells (HSCs) is a key event in the process of liver fibrosis. The induction of senescence of HSCs would accelerate the clearance of the activated HSCs. Previously, we demonstrated that soluble egg antigens (SEA) of Schistosoma japonicum promoted the senescence of HSCs via STAT3/P53/P21 pathway. In this paper, our study was aimed to explore whether there are other signaling pathways in the process of SEA-induced HSCs aging and the underlying effect of SKP2/P27 signal on senescent HSCs.Methodology/Principal findingsHuman hepatic stellate cell line, LX-2 cells, were cultured and stimulated with SEA. Western blot and cellular immunofluorescence analysis were performed to determine the expression of senescence-associated protein, such as P27, SKP2 and FoxO3a. Besides, RNA interfering was applied to knockdown the expression of related protein. The senescence of HSCs was determined by senescence-associated β-gal staining. We found that SEA increased the expression of P27 protein, whereas it inhibited the expression of SKP2 and FoxO3a. Knockdown of P27 as well as overexpression of SKP2 both suppressed the SEA-induced senescence of HSCs. In addition, the nuclear translocation of FoxO3a from the nucleus to the cytoplasm was induced by SEA stimulation.Conclusions/SignificanceThe present study demonstrates that SEA promotes HSCs senescence through the FoxO3a/SKP2/P27 pathway.

Oncogenic Actions of SKP2 Involves Deregulation of CDK1 Turnover Mediated by FOXM1.

Cyclin-dependent kinases (cdks) are central catalytic units of cell division cycle. Among the cdk family members, cdk1 has critical roles in multiple phases of the cell cycle. Aberrant expression or hyper-actions of cdk1 are tumorigenic and yet the complex oncogenic network that regulates its turnover is poorly understood. We found a hitherto unexplored functional connection between skp2 and cdk1 turn over. In vitro knockdown or overexpression of skp2 in cultured cells reduced or induced cdk1 expression indicating skp2 as a positive driver for cdk1. A partial inhibitory role for p27 was identified in this context. Interestingly, concurrent overexpression of skp2 and p27 favored cdk1 upregulation in vitro, which correlated well with similar observations in clinical tumor samples. We found that the transcription factor FOXM1 may play a central role in the skp2-cdk1 loop. Additional molecular involvement in the skp2-cdk1 loop was also explored. In conclusion, our results revealed hitherto unexplored p27 independent molecular mechanisms for skp2 driven tumor progression. Our results support the previous findings that skp2 may be a potential therapeutic target for the management of tumors. J. Cell. Biochem. 118: 797-807, 2017. © 2016 Wiley Periodicals, Inc.

Rottlerin exerts its anti-tumor activity through inhibition of Skp2 in breast cancer cells.

Studies have investigated the tumor suppressive role of rottlerin in carcinogenesis. However, the molecular mechanisms of rottlerin-induced anti-tumor activity are largely unclear. Skp2 (S-phase kinase associated protein 2) has been validated to play an oncogenic role in a variety of human malignancies. Therefore, inactivation of Skp2 could be helpful for the treatment of human cancers. In the current study, we explore whether rottlerin could inhibit Skp2 expression, leading to inhibition of cell growth, migration and invasion in breast cancer cells. We found that rottlerin treatment inhibited cell growth, induced apoptosis and cell cycle arrest. We also revealed that rottlerin suppressed cell migration and invasion in breast cancer cells. Mechanically, we observed that rottlerin significantly down-regulated the expression of Skp2 in breast cancer cells. Importantly, overexpression of Skp2 abrogated rottlerin-mediated tumor suppressive activity, whereas down-regulation of Skp2 enhanced rottlerin-triggered anti-tumor function. Strikingly, we identified that rottlerin exhibited its anti-tumor potential partly through inactivation of Skp2 in breast cancer. Our findings indicate that rottlerin could be a potential safe agent for the treatment of breast cancer.

Associations and prognostic significance of p27Kip1, Jab1 and Skp2 in non-Hodgkin lymphoma.

Non-Hodgkin lymphoma (NHL) is a primary tumor arising in lymph nodes and lymphoid tissue. The incidence of NHL is increasing at an annual rate of 3%. The human Jun activation domain-binding protein 1/COP9 signalosome subunit 5 (Jab1/CSN5) is a negative regulator of the cell cycle inhibitor p27Kip1 and abnormal expression of Jab1 is correlated with reduced p27 expression and associated with advanced tumor stage and poor prognosis in several human cancers. F-box protein S-phase kinase-interacting protein-2 (Skp2), the substrate recognition subunit of the Skp1-Cul1-F-box protein ubiquitin protein ligase complex, is required for the ubiquitination and consequent degradation of p27. The Skp2 protein is overexpressed in several human cancers and is associated with the degree of differentiation and the prognosis. The aim of the present study was to investigate the expression status of p27Kip1, Jab1 and Skp2 by immunohistochemistry, and assess their prognostic significance in patients with NHL. Immunohistochemical analysis revealed an inverse association between Jab1 and p27 in NHL tissue samples. Kaplan-Meier analysis demonstrated that Jab1 overexpression, Skp2 overexpression and low p27 expression were significantly associated with poor prognosis. Among clinicopathological parameters, overexpression of Jab1 was significantly associated with tumor size and International Prognostic Index (IPI), whereas Skp2 expression was significantly associated with metastasis and IPI. These findings suggest that the overexpression of Jab1 or Skp2 plays an important role in the pathogenesis of NHL. Thus, the expression of p27Kip1, Jab1 and Skp2 provided a clinical reference for the treatment of NHL.

Definition of a Skp2-c-Myc Pathway to Expand Human Beta-cells

Type 2 diabetes (T2D) is characterized by insulin resistance and reduced functional β-cell mass. Developmental differences, failure of adaptive expansion and loss of β-cells via β-cell death or de-differentiation have emerged as the possible causes of this reduced β-cell mass. We hypothesized that the proliferative response to mitogens of human β-cells from T2D donors is reduced, and that this might contribute to the development and progression of T2D. Here, we demonstrate that the proliferative response of human β-cells from T2D donors in response to cdk6 and cyclin D3 is indeed dramatically impaired. We show that this is accompanied by increased nuclear abundance of the cell cycle inhibitor, p27kip1. Increasing nuclear abundance of p27kip1 by adenoviral delivery decreases the proliferative response of β-cells from non-diabetic donors, mimicking T2D β-cells. However, while both p27kip1 gene silencing and downregulation by Skp2 overexpression increased similarly the proliferative response of human β-cells, only Skp2 was capable of inducing a significant human β-cell expansion. Skp2 was also able to double the proliferative response of T2D β-cells. These studies define c-Myc as a central Skp2 target for the induction of cell cycle entry, expansion and regeneration of human T2D β-cells.

P27T187A knockin identifies Skp2/Cks1 pocket inhibitors for advanced prostate cancer.

SCFSkp2/Cks1 ubiquitinates Thr187-phosphorylated p27 for degradation. Overexpression of Skp2 coupled with underexpression of p27 are frequent characteristics of cancer cells. When the role of SCFSkp2/Cks1-mediated p27 ubiquitination in cancer was specifically tested by p27 Thr187-to-Ala knockin (p27T187A KI), it was found dispensable for KrasG12D-induced lung tumorigenesis but essential for Rb1-deficient pituitary tumorigenesis. Here we identify pRb and p53 doubly deficient (DKO) prostate tumorigenesis as a context in which p27 ubiquitination by SCFSkp2/Cks1 is required for p27 downregulation. p27 protein accumulated in prostate when p27T187A KI mice underwent DKO prostate tumorigenesis. p27T187A KI or Skp2 knockdown (KD) induced similar degrees of p27 protein accumulation in DKO prostate cells, and Skp2 KD did not further increase p27 protein in DKO prostate cells that contained p27T187A KI (AADKO prostate cells). p27T187A KI activated an E2F1-p73-apoptosis axis in DKO prostate tumorigenesis, slowed disease progression and significantly extended survival. Querying co-occurrence relationships among RB1, TP53, PTEN, NKX3-1 and MYC in TCGA of prostate cancer identified co-inactivation of RB1 and TP53 as the only statistically significant co-occurrences in metastatic castration-resistant prostate cancer (mCRPC). Together, our study identifies Skp2/Cks1 pocket inhibitors as potential therapeutics for mCRPC. Procedures for establishing mCRPC organoid cultures from contemporary patients were recently established. An Skp2/Cks1 pocket inhibitor preferentially collapsed DKO prostate tumor organoids over AADKO organoids, which spontaneously disintegrated over time when DKO prostate tumor organoids grew larger, setting the stage to translate mouse model findings to precision medicine in the clinic on the organoid platform.