C-Myc And Bcl-2 Expression In Supraglottic Squamous Cell Carcinoma of the Larynx

Development, synthesis and validation of improved c-Myc/Max inhibitors

The pathophysiological foundations of various diseases are often subject to alteration through the utilization of small compounds, rendering them invaluable tools for the exploration and advancement of novel therapeutic strategies. Within the scope of this study, we meticulously curated a diverse library of novel small compounds meticulously designed to specifically target the c-Myc/Max complex. We conducted in vitro examinations of novel c-Myc inhibitors across a spectrum of cancer cell lines, including PANC1 (pancreatic adenocarcinoma), MCF7 (breast carcinoma), DU-145 (prostate carcinoma), and A549 (lung cancer). The initial analysis involved a 25 μM dose, which enabled the identification of potent anticancer compounds effective against a variety of tumour types. We identified c-Myc inhibitors with remarkable potency, featuring IC50 values as low as 1.6 μM and up to 40 times more effective than the reference molecule in diminishing cancer cell viability. Notably, c-Myc-i7 exhibited exceptional selectivity, displaying 37-fold and 59-fold preference for targeting prostate and breast cancers, respectively, over healthy cells. Additionally, we constructed drug-likeness models. This study underscores the potential for in vitro investigations of various tumour types using novel c-Myc inhibitors to yield ground-breaking and efficacious anticancer compounds.

Biomarkers of laryngeal squamous cell carcinoma: a review

Laryngeal carcinoma is the second common malignancy of the upper aerodigestive tract after lung cancer; in most cases is a squamous cell carcinoma, whose risk factors include tobacco smoking and alcohol consumption. Despite therapeutic progress, the five-year overall survival rate for this malignancy has remained nearly 50% and many patients already present metastasis at the time of diagnosis. To date, there are no tools that predict the evolution of laryngeal carcinoma: in this light, during the last years, many studies were planned with the aim to investigate the role played by different biomarkers expressed by larynx cancer, which can help make an early diagnosis, predict disease evolution and direct therapeutic choice. This review aims to summarize these markers and correlating them with disease evolution.

Resveratrol Modulation of Apoptosis and Cell Cycle Response to Cisplatin in Head and Neck Cancer Cell Lines

In head and neck cancers, the effectiveness of cisplatin (CisPt) treatment is limited by its toxicity, especially when higher doses are necessary, and the possible occurrence of cisplatin resistance. This study evaluated the effects of resveratrol (RSV) on the expression of different genes involved in the response of human tumor cells (FaDu, PE/CA-PJ49) to cisplatin therapy. Our results revealed that RSV induced apoptosis amplification in both FaDu and PE/CA-PJ49 cells and modulated the expression of specific genes differently than in normal HaCaT cells. In FaDu cells, combined CisPt + RSV treatment induced an increase in apoptosis, which was associated with an increase in c-MYC and TP53 and a decrease in BCL-2 expression. While CisPt + RSV treatment induced apoptosis in PE/CA-PJ49 cells by inhibition of BCL-2 associated with high levels of MDM-2 and subsequently led to inhibition of TP53 gene expression. Decreased c-MYC expression in PE/CA-PJ49 treated with CisPt + RSV was accompanied by cell cycle blockage in G0/G1 phase. In conclusion, RSV influences tumor cell response to CisPt by inducing apoptosis and modulating gene expression. In addition, in normal HaCaT cells, RSV was able to reduce the harmful effects of CisPt.

Altering MYC phosphorylation in the epidermis increases the stem cell population and contributes to the development, progression, and metastasis of squamous cell carcinoma

cMYC (MYC) is a potent oncoprotein that is subject to post-translational modifications that affect its stability and activity. Here, we show that Serine 62 phosphorylation, which increases MYC stability and oncogenic activity, is elevated while Threonine 58 phosphorylation, which targets MYC for degradation, is decreased in squamous cell carcinoma (SCC). The oncogenic role of MYC in the development of SCC is unclear since studies have shown in normal skin that wild-type MYC overexpression can drive loss of stem cells and epidermal differentiation. To investigate whether and how altered MYC phosphorylation might affect SCC development, progression, and metastasis, we generated mice with inducible expression of MYC^WT or MYC^T58A in the basal layer of the skin epidermis. In the T58A mutant, MYC is stabilized with constitutive S62 phosphorylation. When challenged with DMBA/TPA-mediated carcinogenesis, MYC^T58A mice had accelerated development of papillomas, increased conversion to malignant lesions, and increased metastasis as compared to MYC^WT mice. In addition, MYC^T58A-driven SCC displayed stem cell gene expression not observed with MYC^WT, including increased expression of Lgr6, Sox2, and CD34. In support of MYC^T58A enhancing stem cell phenotypes, its expression was associated with an increased number of BrdU long-term label-retaining cells, increased CD34 expression in hair follicles, and increased colony formation from neonatal keratinocytes. Together, these results indicate that altering MYC phosphorylation changes its oncogenic activity—instead of diminishing establishment and/or maintenance of epidermal stem cell populations like wild-type MYC, pS62-MYC enhances these populations and, under carcinogenic conditions, pS62-MYC expression results in aggressive tumor phenotypes.

Predicting transition from oral pre-malignancy to malignancy via Bcl-2 immuno-expression: Evidence and lacunae

Bcl-2 (B cell Lymphoma -2) family comprises of both anti-apoptotic and pro-apoptotic proteins whose altered expression or change in ratio inhibits apoptosis, and promotes tumor progression. The aim of this study is to assess the usefulness of Bcl-2 in distinguishing dysplastic or malignant epithelium from non-dysplastic or normal epithelium to aid in prediction of malignant transformation potential.

Material and method

Study group comprised of 30 cases of clinically diagnosed leukoplakia (OPMD), 15 cases of Oral Squamous Cell Carcinoma (OSCC) and 5 normal tissue samples. The labeling index of Bcl-2 was analyzed in immunohistochemically stained sections. Different statistical tools were used to analyze the data and to compare Bcl-2 expression qualitatively and quantitatively among all the groups.

Results

An increasing trend of Bcl-2 immunoexpression was observed from normal epithelium to non-dysplastic and from non-dysplastic to dysplastic lesions. In OSCC, the peripheral cells in the differentiating epithelial islands (within the connective tissue) showed Bcl-2 immuno-reactivity, which gradually decreased towards the center. In contrast, intense and diffuse Bcl-2 immuno-reactivity was seen in poorly differentiated carcinoma. But the overall Bcl-2 positivity was less in OSCC as compared to dysplastic lesions.

Conclusion

Increased expression of Bcl-2 oncoprotein in sequentially progressing epithelial dysplasia and down-regulation in differentiating carcinoma (well and moderately differentiating OSCC) unveils the clinical relevance of Bcl-2 in early stages of OSCC tumorigenesis. The heterogenous expression of Bcl-2 in carcinoma with different grades of differentiation renders them unable to be used as an independent tool for predicting transition from oral pre-malignancy to malignancy.

Research Progress Regarding Surgical Margins, Molecular Margins, and Prognosis of Laryngeal Carcinoma

BACKGROUND

Laryngeal carcinoma is one of the most common malignant tumors of the head and neck. Researchers have refined the study of surgical margin in the anatomical subarea of the larynx to determine the most appropriate distance of the surgical margin for laryngeal carcinoma, to achieve accurate resection of laryngeal carcinoma and to improve the possibility of retention of laryngeal function.

METHODS

A comprehensive review of the primary literature was performed from 2009 to 2019 utilizing keywords laryngeal carcinoma, surgical margin, molecular margin, and prognostic factor. Articles were included at the discretion of the authors based on novel and/or contributions to the literature.

RESULTS

The prognosis of laryngeal carcinoma significantly correlates with the status of surgical margins. Patients with positive surgical margins have higher recurrence and metastasis rates and worse prognosis. Patients with negative pathological surgical margin but with the expression or altered expression levels of one or more tumor-related molecular biomarkers had high rates of recurrence and metastasis, and poor prognosis.

CONCLUSIONS

Clinical intervention can improve the prognosis of patients with positive surgical margins. Patients with close margins should be followed closely. Among patients with negative surgical margins, patients with abnormal molecular margin results should be closely followed up. However, the specific selection of one or several molecular biomarkers as the detection index of molecular margin currently requires multicenter prospective or retrospective large sample study as guidance.

Low expression of pRB predicts disease relapse in early glottic cancer treated with transoral laser microsurgery

OBJECTIVES/HYPOTHESIS

To elucidate the associations among the immunohistochemical expression of tumor markers, clinicopathological variables, and disease-free survival (DFS) in patients with early-stage glottic squamous cell carcinoma (SCC) who underwent transoral laser microsurgery (TLM) as the primary treatment.

STUDY DESIGN

Retrospective chart review.

METHODS

The records of consecutive patients with Tis-T2N0 glottic SCC who underwent TLM between August 1, 2012 and October 31, 2015 were reviewed. Expression of Bcl-2, pRB, p16^INK4A , p53, c-Myc, E-cadherin, and EGFR was examined using tissue microarrays containing tumor specimens through immunohistochemistry. Three-year DFS rates were calculated.

RESULTS

A total of 65 consecutive patients were identified, of which 28 were excluded due to insufficient tissue (n = 22) and low biomarker quality (n = 6). Therefore, 37 patients with complete records were included. The included patients were significantly older and had a more advanced type of cordectomy than did the excluded patients (P = .015 and .009, respectively). According to the findings of univariate analysis, age, betel quid chewing, type of cordectomy, BCL-2 expression, and pRB expression significantly predicted 3-year DFS. According to the findings of multivariate analysis, age (adjusted hazard ratio: 0.94, 95% CI: 0.88-1.00), betel quid chewing (adjusted hazard ratio: 5.07, 95% CI: 1.32-19.44), and pRB expression (adjusted hazard ratio: 0.02, 95% CI: 0.00-0.28) were independent predictors of 3-year DFS.

CONCLUSIONS

Low pRB expression is a potential biomarker for predicting disease relapse after primary TLM for early-stage glottic SCC and may help to identify high-risk patients who can subsequently undergo intensive management.

LEVEL OF EVIDENCE

4 Laryngoscope, 129:E220-E226, 2019.

Circular RNA PVT1 expression and its roles in acute lymphoblastic leukemia

AIM

The roles of circular RNA PVT1 (circPVT1) are explored in the patients with acute lymphoblastic leukemia (ALL).

METHODS

The circPVT1 level was detected by qRT-PCR and western blot. The apoptotic cells were examined by the annexin V assay in lentiviral shRNA knockdown cells.

RESULTS

circPVT1 was highly expressed in ALL compared with normal bone marrow samples. circPVT1 expression was also significantly higher in ALL cell lines. circPVT1 knockdown inhibited cell proliferation and induced cell apoptosis through suppression of its neighbor gene c-Myc, and antiapoptotic Bcl-2 protein expression.

CONCLUSION

circPVT1 is upregulated in ALL. Silencing circPVT1 results in cell growth arrest and apoptosis of the cells. Our results also suggested a therapeutic potential of targeting circPVT1 in ALL.

The evaluation of bcl-2 expression as a prognostic marker in early stage laryngeal cancer

Aims and background

To evaluate the effect of bcl-2 expression on the local control and overall survival of patients with early stage laryngeal cancer treated with radiotherapy alone.

Methods and study design

We included 53 patients with stage Tis, T1, and T2 laryngeal cancer who were irradiated in our department. Paraffin blocks of all biopsy specimens were subjected to immunohistochemical analysis with a bcl-2 oncoprotein mouse clone 124 Scytek kit.

Results

The mean follow-up time was 61 months (range, 7–166). Local-regional recurrence was observed in 10 (19%) patients. Forty-three patients (81%) had negative bcl-2 staining, 5 patients (9%) had + staining, 3 patients (6%) ++ staining, and 2 patients (4%) +++ staining. No relationship was detected between bcl-2 expression and local control or overall survival. The emergence of a recurrence and a younger age (<50 years) were significantly related to poor overall survival (P = 0.000 and P = 0.021, respectively). Patients with hemoglobin levels in the middle of radiotherapy and at the end of radiotherapy higher than 13 g/dl had improved overall survival in multivariate analyses (P = 0.002 and P = 0.001, respectively). Regarding local control, the following were poor prognostic factors: smoking more than 20 cigarettes a day (P = 0.001) and being younger than 50 years of age (P = 0.001).

Conclusions

No correlation was observed between bcl-2 expression and local control or overall survival. Whereas hemoglobin level, age and existence of a recurrence had a prognostic impact on overall survival, patient age and smoking status influenced local control rates.

Bcl-2 and c-Myc expression in oral dysplasia and oral squamous cell carcinoma: An immunohistochemical study to assess tumor progression

Introduction

Oral cancer is a multistep process involving foul play of proto-oncogenes that induce cell proliferation, inactivation of tumor suppressor gene and cessation of programmed cell death. Among various proto-oncogenes, the nature and behavior of Bcl-2 and c-Myc in oral precancerous/cancerous lesions were obscured which require further assessment for better understanding of etiology, treatment and prognosis.

Aim

The aim of the study is to assess the expression of Bcl-2 and c-Myc in oral epithelial dysplasia and oral squamous cell carcinoma (OSCC).

Materials and Methods

This retrospective study of 70 (oral dysplasia [30], OSCC [30] and normal gingiva [10]) is immunohistochemically assessed for Bcl-2 and c-Myc for distribution, intensity, percentage of positive cells, localization and immunoreactive scores using ImageJ software.

Results

Bcl-2 showed 60% and 37% positivity within dysplasia and OSCC, respectively (P = 0.03); c-Myc showed 87% and 90% positivity within dysplasia and OSCC, respectively. In OSCC, c-Myc showed moderate intensity (P = 0.04). Average percentage of positive cells expressing c-Myc and Bcl-2 increased proportionally within grades of dysplasia (P = 0.000 and P = 0.008, respectively), whereas in OSCC, only c-Myc showed significant expression (P = 0.021). Localization of c-Myc was seen in the nucleus among OSCC (P = 0.01). c-Myc and Bcl-2 showed moderate immunoreactivity in dysplasia (P = 0.005 and P = 0.013, respectively), whereas in OSCC, moderate immunoreactivity of c-Myc (P = 0.05) was observed.

Conclusion

Variable expression of c-Myc and Bcl-2 reveals that these proteins act in synergism in early phases of carcinogenesis, whereas in later stages, due to the diminished activity of Bcl-2, c-Myc interacts incoordination with other oncogenes contributing to tumor progression.

mTORC2 promotes cell survival through c-Myc-dependent up-regulation of E2F1

Inactivation of mTORC2 reduces PP2A activity toward c-Myc serine 62 (S62), leading to enhancement of c-Myc phosphorylation and expression and increased transcription of pri-miR-9-2/miR-9-3p, which in turn suppresses E2F1 and enhances apoptosis.

Previous studies have reported that mTORC2 promotes cell survival through phosphorylating AKT and enhancing its activity. We reveal another mechanism by which mTORC2 controls apoptosis. Inactivation of mTORC2 promotes binding of CIP2A to PP2A, leading to reduced PP2A activity toward c-Myc serine 62 and, consequently, enhancement of c-Myc phosphorylation and expression. Increased c-Myc activity induces transcription of pri-miR-9-2/miR-9-3p, in turn inhibiting expression of E2F1, a transcriptional factor critical for cancer cell survival and tumor progression, resulting in enhanced apoptosis. In vivo experiments using B cell–specific mTORC2 (rapamycin-insensitive companion of mTOR) deletion mice and a xenograft tumor model confirmed that inactivation of mTORC2 causes up-regulation of c-Myc and miR-9-3p, down-regulation of E2F1, and consequent reduction in cell survival. Conversely, Antagomir-9-3p reversed mTORC1/2 inhibitor–potentiated E2F1 suppression and resultant apoptosis in xenograft tumors. Our in vitro and in vivo findings collectively demonstrate that mTORC2 promotes cell survival by stimulating E2F1 expression through a c-Myc– and miR-9-3p–dependent mechanism.

FOXQ1 is overexpressed in laryngeal carcinoma and affects cell growth, cell cycle progression and cell invasion

Forkhead box Q1 (FOXQ1) is a forkhead transcription factor that is involved in numerous biological processes and has been shown to participate in tumorigenesis. However, the clinical significance of the expression of this protein in laryngeal carcinoma, and the mechanisms underlying its regulation in this disease remain unclear. The aim of present study was to measure the expression of FOXQ1 in laryngeal carcinoma, and to examine its effect on tumorigenesis. In the present study, reverse transcription-quantitative polymerase chain reaction and western blotting were employed to measure FOXQ1 expression in laryngeal carcinoma tissue samples, small interfering RNA specific to FOXQ1, was transfected into Hep2 cells and its effect on cell proliferation, cell cycle progression and cell migration was examined, using a CCK-8 assay, flow cytometry and a transwell migration assay, respectively. The results showed overexpression of FOXQ1 mRNA and protein in laryngeal cancer tissue samples. Inhibition of FOXQ1 suppressed cell growth and invasion, and arrested cells in the G0/G1 phase. Overexpression of FOXQ1 is associated with the development of laryngeal carcinoma and may enhance tumorigenesis through its effects on cell proliferation, cell cycle progression and cell migration.

Prognostic value of Bcl-2 expression in squamous cell carcinoma of the larynx: a systematic review

The aim of this systematic review was to determine the prognostic value of Bcl-2 immunostaining in patients affected by laryngeal squamous cell carcinoma. An appropriate search was conducted on PubMed to retrieve articles dealing with this topic. A double cross-check was performed on citations and full-text articles by 2 investigators independently to review all manuscripts and perform a comprehensive quality assessment. Of 115 abstracts identified, 15 articles were included. These studies reported on 1,150 patients with histologically confirmed diagnosis of laryngeal squamous cell carcinoma. Only a few studies showed a statistical correlation between Bcl-2 immunohistochemical expression and at least 1 of the clinical and histopathological parameters considered by the authors. Moreover, these findings were also discordant between them. Overall the studies analyzed suggested that Bcl-2 expression was statistically connected with N stage (2/14), grading (2/14), disease-free survival (3/14) and overall survival (5/14). Interestingly, all of the 3 studies investigating the relation between Bcl-2 and radioresistance showed significant results in terms of recurrence-free survival and overall survival. Our review strongly suggests that the immunohistochemical staining of Bcl-2 does not correlate with tumoral aggressiveness and prognosis of patients affected by laryngeal squamous cell carcinoma and treated with primary surgery. However, an interesting connection of this protein could be demonstrated with tumoral radioresistance. Further, high-quality prospective studies should be carried out to confirm this hypothesis.

MYC and the control of apoptosis

MYC expression is tightly correlated with cell-cycle progression in normal tissues, whereas unchecked MYC expression is among the most prominent hallmarks of the hyperproliferation associated with most forms of cancer. At first glance it might seem counterintuitive that MYC is also among the most robust agents of programmed cell death (apoptosis) in mammalian cells. However it is clearly beneficial for a multicellular organism to have a mechanism for triggering death in cells that express potentially oncogenic levels of MYC. Decades of intense study have begun to provide an understanding of the mechanisms that regulate MYC's seemingly split personality. Key features of MYC-induced apoptosis will be discussed here along with examples of how our understanding of this pathway might be exploited for the therapeutic benefit of cancer patients.

The clinical relevance of Ki-67 expression in laryngeal squamous cell carcinoma

The aim of this study was to determine the prognostic value of Ki-67 immunostaining in patients affected by laryngeal squamous cell carcinoma. A systematic review was carried out in a tertiary university referral center. An appropriate string was run on PubMed to retrieve articles dealing with Ki-67 immunohistochemical staining and laryngeal squamous cell carcinoma. A double cross-check was performed on citations and full-text articles by two investigators independently to review all manuscripts and perform a comprehensive quality assessment. Of 85 abstracts identified, 18 articles were included. These studies reported on 1,342 patients with histological confirmed diagnosis of laryngeal squamous cell carcinoma. Most studies showed a statistical association between Ki-67 immunohistochemical expression and at least one of the clinical and histopathological parameters considered by the authors. Overall the studies analyzed suggested that the tumoral proliferative index was statistically connected respectively with T stage (2/18), N stage (4/18), grading (6/18), disease-free survival (10/18) and overall survival (4/18). Our review strongly suggests that immunohistochemical staining of Ki-67 correlates with tumoral aggressiveness and worse prognosis in patients affected by laryngeal squamous cell carcinoma. Further high-quality prospective studies should be carried out to confirm our finding and determine the eventual differences between cancers of specific laryngeal subsites.

MYCT1-TV, a novel MYCT1 transcript, is regulated by c-Myc and may participate in laryngeal carcinogenesis

BACKGROUND

MYCT1, a putative target of c-Myc, is a novel candidate tumor suppressor gene cloned from laryngeal squamous cell carcinoma (LSCC). Its transcriptional regulation and biological effects on LSCC have not been clarified.

METHODOLOGY/PRINCIPAL FINDINGS

Using RACE assay, we cloned a 1106 bp transcript named Myc target 1 transcript variant 1 (MYCT1-TV) and confirmed its transcriptional start site was located at 140 bp upstream of the ATG start codon of MYCT1-TV. Luciferase, electrophoretic mobility shift and chromatin immunoprecipitation assays confirmed c-Myc could regulate the promoter activity of MYCT1-TV by specifically binding to the E-box elements within -886 to -655 bp region. These results were further verified by site-directed mutagenesis and RNA interference (RNAi) assays. MYCT1-TV and MYCT1 expressed lower in LSCC than those in paired adjacent normal laryngeal tissues, and overexpression of MYCT1-TV and MYCT1 could inhibit cell proliferation and invasion and promote apoptosis in LSCC cells.

CONCLUSIONS/SIGNIFICANCE

Our data indicate that MYCT1-TV, a novel MYCT1 transcript, is regulated by c-Myc and down-regulation of MYCT1-TV/MYCT1 could contribute to LSCC development and function.

BCL2 is a downstream effector of MIZ-1 essential for blocking c-MYC-induced apoptosis

The c-MYC oncoprotein is among the most potent transforming agents in human cells. Ironically, c-MYC is also capable of inducing massive apoptosis under certain conditions. A clear understanding of the distinct pathways activated by c-MYC during apoptosis induction and transformation is crucial to the design of therapeutic strategies aimed at selectively reactivating the apoptotic potential of c-MYC in cancer cells. We recently demonstrated that apoptosis induction in primary human cells strictly requires that c-MYC bind and inactivate the transcription factor MIZ-1. This presumably blocked the ability of MIZ-1 to activate the transcription of an unidentified pro-survival gene. Here we report that MIZ-1 activates the transcription of BCL2. More importantly, inhibition of the MIZ-1/BCL2 signal is an essential event during the apoptotic response. Furthermore, targeting BCL2 with short hairpin RNA or small molecule inhibitors restores the apoptotic potential of a c-MYC mutant that is defective for MIZ-1 inhibition. These observations suggest that repression of BCL2 transcription is the single essential consequence of targeting the MIZ-1 pathway during apoptosis induction. These data define a genetic pathway that helps to explain historical observations documenting cooperation between c-MYC and BCL2 overexpression in human cancer.

Spindle basaloid squamous carcinoma of the upper aerodigestive tract: immunohistochemical and clinicopathological study of three cases

We describe the clinicopathological and immunohistochemical features of three spindle (sarcomatoid) basaloid squamous carcinomas in three men aged 73, 69, and 59 years with a history of tobacco and alcohol abuse. Two tumors were located in the hypopharynx and one was located in the nasal cavity. The three tumors have a pedunculated polypoid appearance. Histologically, they were composed of conventional basaloid squamous carcinomas with extensive malignant spindle cell proliferation, comprising more than 50% of the tumor. The sarcomatoid component demonstrated immunoreactivity with one or more epithelial markers. One case in addition expressed CD99 and Bcl-2 and was originally diagnosed as monophasic synovial sarcoma; however, a subsequent biopsy disclosed basaloid squamous cell carcinoma with sarcomatoid stroma. Two patients were treated with surgery and radiation whereas one refused therapy. The patients were alive 14 (case patient 1), 10 (case patient 2), and 8 (case patient 3) months after diagnosis. In the absence of evidence from immunohistochemical or electron microscopy studies, a polypoid malignant spindle cell tumor of a mucosal surface of the upper aerodigestive tract should be considered a sarcomatoid carcinoma until proven otherwise. The type of epithelial component would determine the subtype of sarcomatoid carcinoma.

The loss of PIN1 deregulates cyclin E and sensitizes mouse embryo fibroblasts to genomic instability

During the G0/G1-S phase transition, the timely synthesis and degradation of key regulatory proteins is required for normal cell cycle progression. Two of these proteins, c-Myc and cyclin E, are recognized by the Cdc4 E3 ligase of the Skp1/Cul1/Rbx1 (SCF) complex. SCF(Cdc4) binds to a similar phosphodegron sequence in c-Myc and cyclin E proteins resulting in ubiquitylation and degradation of both proteins via the 26 S proteosome. Since the prolyl isomerase Pin1 binds the c-Myc phosphodegron and participates in regulation of c-Myc turnover, we hypothesized that Pin1 would bind to and regulate cyclin E turnover in a similar manner. Here we show that Pin1 regulates the turnover of cyclin E in mouse embryo fibroblasts. Pin1 binds to the cyclin E-Cdk2 complex in a manner that depends on Ser384 of cyclin E, which is phosphorylated by Cdk2. The absence of Pin1 results in an increased steady-state level of cyclin E and stalling of the cells in the G1/S phase of the cell cycle. The cellular changes that result from the loss of Pin1 predispose Pin1 null mouse embryo fibroblasts to undergo more rapid genomic instability when immortalized by conditional inactivation of p53 and sensitizes these cells to more aggressive Ras-dependent transformation and tumorigenesis.