PTEN-mediated G1 cell-cycle arrest in LNCaP prostate cancer cells is associated with altered expression of cell-cycle regulators.

BACKGROUND: The tumor suppressor PTEN regulates many biological processes. A well-known downstream effector of PTEN is phospho-Akt. Although PTEN is the most frequently inactivated gene in prostate cancer, its mode of action is not fully understood. We studied the association of regulated PTEN expression with changes in biological function and gene expression profiles. METHODS: PTEN-negative LNCaP cells were stably transfected with wild-type PTEN cDNA under inducible control, resulting in LNCaP/PTEN cells. Microarray analysis was used to monitor gene expression changes upon induction of PTEN. Expression of selected individual genes was studied in Q-PCR and siRNA experiments. Cell-cycle distribution was analyzed by flow cytometry. RESULTS: Induced expression of PTEN in LNCaP/PTEN cells significantly inhibited cell proliferation, at least partly due to cell-cycle arrest at the G1 phase. Expression profiling combined with pathway analysis revealed that PTEN-dependent G1 growth arrest was associated with an altered mRNA expression of the G1 cell-cycle regulators Cdc25a, E2F2, cyclin G2, and RBL2/p130. Specific inhibition of Akt signaling by siRNA resulted in downregulation of both E2F2 and Cdc25a mRNA expression and upregulation of the FOXO target cyclin G2, similar to the effect observed by PTEN induction. However, Akt did not mediate the PTEN-dependent RBL2/p130 mRNA expression in LNCaP/PTEN cells. CONCLUSIONS: The results indicate that PTEN dependent gene expression is important in cell-cycle regulation and is mediated by both Akt-dependent and -independent mechanisms.

The PTEN gene in locally progressive prostate cancer is preferentially inactivated by bi-allelic gene deletion.

PTEN is frequently inactivated during the development of many cancers, including prostate cancer, and both bi-allelic and mono-allelic PTEN inactivation may contribute to tumorigenesis. PTEN mutations in clinical cancer specimens can easily be recorded but mono- or bi-allelic gene deletions are often difficult to assess. We performed a comprehensive study to detect PTEN inactivation in 40 locally progressive clinical prostate cancer specimens obtained by transurethral resection of the prostate, utilizing a variety of complementary technical approaches. The methods to detect PTEN deletion included allelotype analysis, dual-colour FISH and array-based CGH. We also applied a novel semi-quantitative approach, assessing the PTEN-WT (wild-type): PTEN-Psi (pseudogene) ratio (WPR). Structural analysis of PTEN was performed by single-strand conformational polymorphism (PCR-SSCP) and sequencing. PTEN protein expression was assessed by immunohistochemistry. Our data predict complete PTEN inactivation in 12 samples (30%), nine of these by bi-allelic deletion. Loss of one PTEN copy was also detected by several methodologies but the number could not be accurately assessed. Immunohistochemistry indicated the absence of PTEN protein in 15 samples, and heterogeneous expression of the protein in eight tumours. Taken together, these data show that bi-allelic deletion is a major mechanism of PTEN inactivation in locally progressive prostate cancer.