High Skp2/Low p57(Kip2) Expression is Associated with Poor Prognosis in Human Breast Carcinoma

CIP/KIP and INK4 families as hostages of oncogenic signaling

CIP/KIP and INK4 families of Cyclin-dependent kinase inhibitors (CKIs) are well-established cell cycle regulatory proteins whose canonical function is binding to Cyclin-CDK complexes and altering their function. Initial experiments showed that these proteins negatively regulate cell cycle progression and thus are tumor suppressors in the context of molecular oncology. However, expanded research into the functions of these proteins showed that most of them have non-canonical functions, both cell cycle-dependent and independent, and can even act as tumor enhancers depending on their posttranslational modifications, subcellular localization, and cell state context. This review aims to provide an overview of canonical as well as non-canonical functions of CIP/KIP and INK4 families of CKIs, discuss the potential avenues to promote their tumor suppressor functions instead of tumor enhancing ones, and how they could be utilized to design improved treatment regimens for cancer patients.

Molecular targets and therapeutic strategies for triple-negative breast cancer

Triple-negative breast cancer (TNBC) is known for its heterogeneous complexity and is often difficult to treat. TNBC lacks the expression of major hormonal receptors like estrogen receptor, progesterone receptor, and human epidermal growth factor receptor-2 and is further subdivided into androgen receptor (AR) positive and AR negative. In contrast, AR negative is also known as quadruple-negative breast cancer (QNBC). Compared to AR-positive TNBC, QNBC has a great scarcity of prognostic biomarkers and therapeutic targets. QNBC shows excessive cellular growth and proliferation of tumor cells due to increased expression of growth factors like EGF and various surface proteins. This study briefly reviews the limited data available as protein biomarkers that can be used as molecular targets in treating TNBC as well as QNBC. Targeted therapy and immune checkpoint inhibitors have recently changed cancer treatment. Many studies in medicinal chemistry continue to focus on the synthesis of novel compounds to discover new antiproliferative medicines capable of treating TNBC despite the abundance of treatments currently on the market. Drug repurposing is one of the therapeutic methods for TNBC that has been examined. Moreover, some additional micronutrients, nutraceuticals, and functional foods may be able to lower cancer risk or slow the spread of malignant diseases that have already been diagnosed with cancer. Finally, nanomedicines, or applications of nanotechnology in medicine, introduce nanoparticles with variable chemistry and architecture for the treatment of cancer. This review emphasizes the most recent research on nutraceuticals, medication repositioning, and novel therapeutic strategies for the treatment of TNBC.

Gene S-phase kinase associated protein 2 is a novel prognostic marker in human neoplasms

BACKGROUND

Neoplasms are a series of diseases affecting human health. Prognostic and tumor status-related markers for various tumors should be identified.

METHODS

Based on 19,515 samples from multiple sources, for the first time, this study provided an overview of gene S-phase kinase associated protein 2 (SKP2) in pan-cancer. Differential SKP2 expression in multiple comparison groups was identified by the Kruskal-Wallis test and Wilcoxon rank-sum test. The prognosis significance of SKP2 in individuals with neoplasm was evaluated through univariate Cox regression analysis and Kaplan-Meier curves. The area under the curve was utilized to detect the accuracy of SKP2 in predicting cancer status. Spearman's rank correlation coefficients were calculated in all correlation analyses. Gene set enrichment analysis was used to identify essential signaling pathways of SKP2 in human neoplasms.

RESULTS

The study disclosed the upregulated SKP2 expression in 15 neoplasms and decreased SKP2 expression in three cancers (p < 0.05). The transcription factor Forkhead Box M1 may contribute to the increased expression levels of SKP2 in certain tumors. Over-expressed SKP2 represented a risk factor for the prognosis of most cancer patients (hazard ratio > 1, p < 0.05). SKP2 expression made it feasible to distinguish neoplasm and control tissues of 21 neoplasms (sensitivity = 0.79, specificity = 0.87, area under the curve = 0.90), implying its potential in screening a series of neoplasms. Further, the research revealed the close association of SKP2 expression with DNA methyltransferases, mismatch repair genes, microsatellite instability, tumor mutational burden, neoantigen count, and immunity.

CONCLUSIONS

SKP2 plays an essential role in multiple neoplasms and may serve as a marker for treating and identifying these neoplasms.

Inhibition of Skp2 enhances doxorubicin-induced cell death in B cell precursor acute lymphoblastic leukemia

S-phase kinase-associated protein 2 (Skp2) is a well-defined component of the Skp2-Culin1-F-box (SCF) E3 ubiquitin ligase complex, which is involved in cell cycle progression and considered a prognostic marker in cancers. Overexpression of Skp2 is frequently observed in patients with Acute lymphoblastic leukemia (ALL). Inhibition of this protein may be a valuable strategy to induce apoptosis in malignant cells. Less well known is the effect of Skp2 inhibition on the potentiation of the chemotherapeutic-induced cell death in B cell precursor acute lymphoblastic leukemia (BCP-ALL). Our results demonstrated that inhibition of the Skp2 using SZL P1-41, not only resulted in caspase-mediated apoptosis but also potentiated doxorubicin-induced apoptosis in BCP-ALL cell lines (NALM-6 and SUP-B15). SZL P1-41 in combination with doxorubicin altered cell cycle distribution and the level of cyclins and CDKs in BCP-ALL cells. DNA damage response genes were also up-regulated in presence of the doxorubicin and SZL P1-41 in both cell lines. In conclusion, our results indicated that inhibition of Skp2 either alone or in a combination with doxorubicin may hold promise in the future treatment of BCP-ALL. This article is protected by copyright. All rights reserved.

Emerging Roles of SKP2 in Cancer Drug Resistance

More than half of all cancer patients receive chemotherapy, however, some of them easily acquire drug resistance. Resistance to chemotherapy has become a massive obstacle to achieve high rates of pathological complete response during cancer therapy. S-phase kinase-associated protein 2 (Skp2), as an E3 ligase, was found to be highly correlated with drug resistance and poor prognosis. In this review, we summarize the mechanisms that Skp2 confers to drug resistance, including the Akt-Skp2 feedback loop, Skp2-p27 pathway, cell cycle and mitosis regulation, EMT (epithelial-mesenchymal transition) property, enhanced DNA damage response and repair, etc. We also addressed novel molecules that either inhibit Skp2 expression or target Skp2-centered interactions, which might have vast potential for application in clinics and benefit cancer patients in the future.

Quadruple-negative breast cancer: novel implications for a new disease

Based on the androgen receptor (AR) expression, triple-negative breast cancer (TNBC) can be subdivided into AR-positive TNBC and AR-negative TNBC, also known as quadruple-negative breast cancer (QNBC). QNBC characterization and treatment is fraught with many challenges. In QNBC, there is a greater paucity of prognostic biomarkers and therapeutic targets than AR-positive TNBC. Although the prognostic role of AR in TNBC remains controversial, many studies revealed that a lack of AR expression confers a more aggressive disease course. Literature characterizing QNBC tumor biology and uncovering novel biomarkers for improved management of the disease remains scarce. In this comprehensive review, we summarize the current QNBC landscape and propose avenues for future research, suggesting potential biomarkers and therapeutic strategies that warrant investigation.

A Derivate of Benzimidazole-Isoquinolinone Induces SKP2 Transcriptional Inhibition to Exert Anti-Tumor Activity in Glioblastoma Cells

We have previously shown that compound-7g inhibits colorectal cancer cell proliferation and survival by inducing cell cycle arrest and PI3K/AKT/mTOR pathway blockage. However, whether it has the ability to exert antitumor activity in other cancer cells and what is the exact molecular mechanism for its antiproliferation effect remained to be determined. In the present study, compound-7g exhibited strong activity in suppressing proliferation and growth of glioblastoma cells. The inhibitor selectively downregulated F-box protein SKP2 expression and upregulated cell cycle inhibitor p27, and then resulted in G1 cell cycle arrest. Mechanism analysis revealed that compound-7g also provokes the down-regulation of E2F-1, which acts as a transcriptional factor of SKP2. Further results indicated that compound-7g induced an increase of LC3B-II and p62, which causes a suppression of fusion between autophagosome and lysosome. Moreover, compound-7g mediated autophagic flux blockage promoted accumulation of ubiquitinated proteins and then led to endoplasmic reticulum stress. Our study thus demonstrated that pharmacological inactivation of E2F-1-SKP2-p27 axis is a promising target for restricting cancer progression.

SKP2 targeted inhibition suppresses human uveal melanoma progression by blocking ubiquitylation of p27

Background: SKP2 is considered an oncogene involved in various malignancies. SKP2 protein is a critical subunit of the SKP1-CUL1-F-box (SCF) E3 ligase complex which affects the cell cycle profoundly by specifically recognizing cell cycle regulators and mediating their ubiquitylation and proteasomal degradation. SKP2 dysfunction is characteristic of many tumor cells. However, its role in uveal melanoma (UM) has not been elucidated.

Materials and methods: We analyzed the expressions of SKP2 in different UM cell lines compared with normal pigment cell by RNA-seq, RT-qPCR and Western blot. We then knocked down SKP2 in OM431 and MUM2B cells and confirmed its roles in cell proliferation via CCK8 assay. The sensitivity of cells to SKP2 inhibitor C1 (SKPin C1) in vitro was evaluated by CCK8 assay and colony formation assay, and the sensitivity of MUM2B cells to SKPin C1 in vivo was estimated using the nude mouse-based xenograft model. Western blot and Immunoprecipitation assay were performed to detect the change of p27 and its ubiquitylation level in UM cells treated with SKPin C1, respectively.

Results: The results showed that SKP2 was significantly highly expressed in UM cells. SKP2 promoted the progression of UM and knockdown of SKP2 inhibited cell proliferation in UM cells. SKP2 inhibitor C1 that targets SKP2 essentially inhibits the growth of UM cells both in vivo and in vitro. SKP2 inhibitor C1 decreased the degradation of p27 by blocking ubiquitylation of p27, resulting in p27 accumulation and cell cycle arrest in UM cells.

Conclusion: Our findings demonstrated that SKP2 targeted inhibition suppresses UM cell proliferation and provides new options and possibilities for targeted therapies in UM.

Prognostic values of F-box members in breast cancer: an online database analysis and literature review

Introduction: F-box proteins are the substrate-recognizing subunits of SKP1 (S-phase kinase-associated protein 1)–cullin1–F-box protein (SCF) E3 ligase complexes that play pivotal roles in multiple cellular processes, including cell proliferation, apoptosis, angiogenesis, invasion, and metastasis. Dysregulation of F-box proteins may lead to an unbalanced proteolysis of numerous protein substrates, contributing to progression of human malignancies. However, the prognostic values of F-box members, especially at mRNA levels, in breast cancer (BC) are elusive. Methods: An online database, which is constructed based on the gene expression data and survival information downloaded from GEO (http://www.ncbi.nlm.nih.gov/geo/), was used to investigate the prognostic values of 15 members of F-box mRNA expression in BC. Results: We found that higher mRNA expression levels of FBXO1, FBXO31, SKP2, and FBXO5 were significantly associated with worse prognosis for BC patients. While FBXO4 and β-TrCP1 were found to be correlated to better overall survival (OS). Conclusion: The associated results provide new insights into F-box members in the development and progression of BC. Further researches to explore the F-box protein-targetting reagents for treating BC are needed.

Curcumin-Mediated Degradation of S-Phase Kinase Protein 2 Induces Cytotoxic Effects in Human Papillomavirus-Positive and Negative Squamous Carcinoma Cells

S-phase kinase-associated protein2 (Skp2), a proto-oncoprotein, plays an important role in development and progression of human malignancies. Skp2 is frequently overexpressed in many human malignancies. It targets cell cycle progression through ubiquitin mediated degradation of G1-checkpoint CDK inhibitors-p21 (CDKN1A) and p27 (CDKN1B). We investigated the role of Skp2 and its ubiquitin-proteasome pathway in head and neck squamous cell carcinoma (HNSCC) using a panel of cell lines with and without human papillomavirus (HPV+, HPV-). Treatment of HNSCC cell lines with curcumin, a natural compound isolated from rhizomes of the plant Curcuma longa, or transfection of small interfering RNA of Skp2, causes down-regulation of Skp2 with concomitant accumulation of p21 and p27 in HPV+, HPV- cells. Furthermore curcumin inhibits cell viability and induces apoptosis in a dose-dependent manner. Treatment of HPV+ and HPV- cells with curcumin induced apoptosis via mitochondrial pathway and activation of caspases. In addition, treatment of HPV+ and HPV- cell lines with curcumin down-regulated the expression of XIAP, cIAP1, and cIAP2. Interestingly, co-treatment of HNSCC cells with curcumin and cisplatin potentiated inhibition of cell viability and apoptotic effects. Altogether, these data suggest an important function for curcumin, acting as a suppressor of oncoprotein Skp2 in squamous cell carcinoma cells in both HPV+ and HPV- cells; raise the possibility that this agent may have a future therapeutic role in squamous cell carcinoma.

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.

Genistein inhibits the S-phase kinase-associated protein 2 expression in breast cancer cells

Breast cancer is one of the most lethal cancers affecting women worldwide and was estimated to account for ~30% of all new cancer diagnoses in women. Although available evidence has proved the tumor suppressor role of genistein in cancer, the underling mechanisms have remained to be fully elucidated. S-phase kinase-associated protein 2 (Skp2) has been revealed to critically enhance the pathogenesis of multiple human cancers. The present study determined whether genistein exerts its anti-tumor function by suppressing Skp2 in breast cancer cells. Genistein significantly inhibited the proliferation, invasion and migration of breast cancer cells. Furthermore, genistein treatment also induced marked apoptosis and a typical cell cycle arrest in G2/M phase. Mechanistically, genistein treatment was identified to cause a significant downregulation of Skp2. Two crucial tumor suppressors, p21 and p27, were upregulated in genistein-treated breast cancer cells. The present results revealed that genistein exerted its tumor suppressor effect at least partially via inhibition of Skp2 and promotion of its downstream targets p21 and p27. Therefore, inactivation of Skp2 by genistein may be a promising approach for breast cancer treatment.

CyclinD1 and p57kip2 as biomarkers in differentiation, metastasis and prognosis of gastric cardia adenocarcinoma

Objective

This study aims to investigate the expression and significance of p57^kip2 and cyclinD1 in gastric cardia adenocarcinoma (GCA). p57^kip2 is a negative regulator in the cell cycle. On the contrary, cyclinD1 is a positive regulator of cell cycle progression.

Methods

Thirty-two cases of GCA tissues and adjacent non-cancerous tissues were collected for this study. Immunohistochemistry and fluorescence qualitative PCR was used to determine the level of p57^kip2 and cyclinD1 in GCA and its adjacent non-cancerous tissues. Furthermore, the correlation between the mRNA/protein and GCA clinical pathologic parameters were analyzed, and the relationship of p57^kip2 and cyclinD1 in GCA were also evaluated.

Results

The expression of p57^kip2 significantly lower in GCA (P = 0.036), and there was a significant correlation in the different degrees of differentiation (P < 0.05). Furthermore, median survival time was 41 months for patients with high mRNA expression of p57^kip2. This was longer compared to patients with low mRNA expression of P57^kip2 (37 months, X² = 4.788, P = 0.029).The expression of cyclinD1 was significantly higher in GCA(P = 0.002), and was significant correlated to clinical stage(P<0.05). Median survival time was 34 months in patients with high mRNA expression of cyclinD1, which was shorter than in patients with low expression of cyclinD1 mRNA (41 months, X² = 4.071, P = 0.044). The protein expression of p57^kip2 was not correlated to the protein expression of cyclinD1 (P = 0.55).

Conclusion

The expression of p57^kip2 and cyclinD1 are likely to suppress or promote the tumorigenesis and progression of GCA.

Physical activity induced protection against breast cancer risk associated with delayed parity

Epidemiological evidence indicates that physical activity between menarche and first pregnancy is associated with a lower risk of breast cancer among women with at least 20years between these reproductive events. The mechanism by which physical activity during this interval confers protection is unknown. This study used a novel animal model to assess potentially protective effects of physical activity on tumor development in delayed parity. Thirty-six female Sprague Dawley rats received an i.p. injection of 50mg/kg N-methyl-N-nitrosourea (MNU) at 5weeks of age. Estrogen and progesterone pellets were implanted subcutaneously 1week (early parity, EP, n=8) or 4weeks (delayed parity, DP, n=11) following MNU injection. An additional group of DP rats were progressively exercise trained (Ex+DP, n=9) on a treadmill following MNU injection for 7weeks (up to 20m/min at 15% incline for 30min). We observed the greatest tumor latency and smallest tumor burden in Ex+DP animals. Ductal hyperplasia and inflammation of non-tumor bearing mammary glands were only found in DP, and we detected a significant increase in collagen for DP and Ex+DP compared to EP. Exercise induced differential gene expression of cyclin-dependent kinase-inhibitor 1C (Cdkn1c) and urokinase-plasminogen activator (Plau) in mammary tissue of Ex+DP animals compared to DP alone. While there are delayed parity-induced changes in mammary gland collagen and gene expression levels, Ex+DP animals had longer tumor latency, smaller tumor burden, and glandular tissue resistant to ductal hyperplasia. Exercise may induce protection through beneficial regulation of gene expression profiles.

Rottlerin exhibits anti-cancer effect through inactivation of S phase kinase-associated protein 2 in pancreatic cancer cells

Rottlerin, a natural product isolated from Mallotus philippinensis, has been characterized as an effective chemoprevention agent in inhibiting tumor cell growth. Although multiple studies have revealed the role of rottlerin in tumorigenesis, the molecular mechanism of rottlerin-mediated anti-tumor activity has not been fully elucidated. It has been reported that Skp2 (S-phase kinase associated protein 2) plays an oncogenic role in human malignancies, indicating that inactivation of Skp2 could be a promising approach for the treatment of cancers. Therefore, in this study, we aim to investigate whether rottlerin exhibits its anti-tumor activities via targeting Skp2 pathway in pancreatic cancer. We found that rottlerin inhibited cell growth, induced apoptosis, arrested cell cycle, and retarded cell invasion and migration. Notably, we observed that the expression of Skp2 was significantly decreased in rottlerin-treated pancreatic cancer cells. Importantly, overexpression of Skp2 abrogated the anti-tumor function induced by rottlerin in pancreatic cancer cells. Consistently, depletion of Skp2 promoted rottlerin-mediated inhibition of cell growth and invasion. Collectively, our study demonstrated that rottlerin could suppress Skp2 expression and subsequently exert its tumor suppressive function in pancreatic cancer cells, suggesting that rottlerin might be a potential therapeutic compound for treating pancreatic cancer.

Breast cancer molecular subtypes: from TNBC to QNBC

Treatment protocols for breast cancer depend predominantly on receptor status with respect to estrogen (estrogen receptor alpha), progesterone (progesterone receptor) and human epidermal growth factor [human epidermal growth factor receptor 2 (HER2)]. The presence of one or more of these receptors suggests that a treatment targeting these pathways might be effective, while the absence of, or in the case of HER2, lack of overexpression of, all of these receptors, termed triple negative breast cancer (TNBC), indicates a need for the more toxic chemotherapy. In an effort to develop targeted therapies for TNBC, it will be necessary to differentiate among specific TNBC subtypes. The subset of TNBC that expresses androgen receptor (AR) has been determined to express genes consistent with a luminal subtype and therefore may be amenable to therapies targeting either AR, itself, or other pathways typical of a luminal subtype. Recent investigations of the AR signal pathway within breast cancer lead to AR as a significant target for breast cancer therapy with several clinical trials currently in progress. The subclass of TNBC that lacks AR, which we have termed quadruple negative breast cancer (QNBC) currently lacks a defined targetable pathway. Unlike AR-positive TNBC, QNBC predominantly exhibits a basal-like molecular subtype. Several subtypes and related pathway proteins are preferentially expressed in QNBC that may serve as effective targets for treatment, such as ACSL4, SKP2 and EGFR. ACSL4 expression has been demonstrated to be inversely correlated with expression of hormone/growth factor receptors and may thus serve as a biomarker for QNBC as well as a target for therapy. In the following review we summarize some of the current efforts to develop alternatives to chemotherapy for TNBC and QNBC.

Ubiquitination-mediated degradation of cell cycle-related proteins by F-box proteins

F-box proteins, subunits of SKP1-cullin 1-F-box protein (SCF) type of E3 ubiquitin ligase complexes, have been validated to play a crucial role in governing various cellular processes such as cell cycle, cell proliferation, apoptosis, migration, invasion and metastasis. Recently, a wealth of evidence has emerged that F-box proteins is critically involved in tumorigenesis in part through governing the ubiquitination and subsequent degradation of cell cycle proteins, and dysregulation of this process leads to aberrant cell cycle progression and ultimately, tumorigenesis. Therefore, in this review, we describe the critical role of F-box proteins in the timely regulation of cell cycle. Moreover, we discuss how F-box proteins involve in tumorigenesis via targeting cell cycle-related proteins using biochemistry studies, engineered mouse models, and pathological gene alternations. We conclude that inhibitors of F-box proteins could have promising therapeutic potentials in part through controlling of aberrant cell cycle progression for cancer therapies.