Telomeres are specialized heterochromatin at the ends of linear chromosomes. Telomeres are crucial for maintaining genome stability and play important roles in cellular senescence and tumor biology. Six core proteins-TRF1, TRF2, TIN2, POT1, TPP1 and Rap1 (termed the telosome or shelterin complex)-regulate telomere structure and function. One of these proteins, TIN2, regulates telomere length and structure indirectly by interacting with TRF1, TRF2 and TPP1, but no direct function has been attributed to TIN2. Here we present evidence for a TIN2 isoform (TIN2L) that differs from the originally described TIN2 isoform (TIN2S) in two ways: TIN2L contains an additional 97 amino acids, and TIN2L associates strongly with the nuclear matrix. Stringent salt and detergent conditions failed to extract TIN2L from the nuclear matrix, despite removing other telomere components, including TIN2S. In human mammary epithelial cells, each isoform showed a distinct nuclear distribution both as a function of cell cycle position and telomere length. Our results suggest a dual role for TIN2 in mediating the function of the shelterin complex and tethering telomeres to the nuclear matrix.

Telomere stability is important for cell viability, as cells with telomere DNA damage that is not repaired do not survive. We reported previously that androgen receptor (AR) antagonist induces telomere DNA damage in androgen-sensitive LNCaP prostate cancer cells; this triggers a DNA damage response (DDR) at telomeres that includes activation of ATM, and blocking ATM activation prevents telomere DNA repair and leads to cell death. Remarkably, AR antagonist induces telomere DNA damage and triggers ATM activation at telomeres also in 22Rv1 castration-resistant prostate cancer (CRPC) cells that are not growth inhibited by AR antagonist. Treatment with AR antagonist enzalutamide (ENZ) or ATM inhibitor (ATMi) by itself had no effect on growth in vitro or in vivo, but combined treatment with ENZ plus ATMi significantly inhibited cell survival in vitro and tumor growth in vivo. By inducing telomere DNA damage and activating a telomere DDR, an opportunity to inhibit DNA repair and promote cell death was created, even in CRPC cells. 22Rv1 cells express both full-length AR and AR splice variant AR-V7, but full-length AR was found to be the predominant form of AR associated with telomeres and required for telomere stability. Although 22Rv1 growth of untreated 22Rv1 cells appears to be driven by AR-V7, it is, ironically, expression of full-length AR that makes them sensitive to growth inhibition by combined treatment with ENZ plus ATMi. Notably, this combined treatment approach to induce telomere DNA damage and inhibit the DDR was effective in inducing cell death also in other CRPC cell lines (LNCaP/AR and C4-2B). Thus, the use of ENZ in combination with a DDR inhibitor, such as ATMi, may be effective in prolonging disease-free survival of patients with AR-positive metastatic CRPC, even those that co-express AR splice variant.

Background Despite multiple treatment advances for castration‐resistant prostate cancer (CRPC), there are currently no curative therapies and patients ultimately to succumb to the disease. Docetaxel (DTX) is the standard first‐line chemotherapy for patients with metastatic CRPC; however, drug resistance is inevitable and often develops rapidly, leading to disease progression in nearly all patients. In contrast, when DTX is deployed with androgen deprivation therapy in castration‐sensitive disease, more durable responses and improved outcomes are observed, suggesting that aberrant androgen receptor (AR) signaling accelerates DTX resistance in CRPC. In this study, we demonstrate that AR dysregulates the mitotic checkpoint, a critical pathway involved in the anticancer action of DTX. Methods Androgen‐dependent and independent cell lines were used to evaluate the role of AR in DTX resistance. Impact of drug treatment on cell viability, survival, and cell‐cycle distribution were determined by plate‐based viability assay, clonogenic assay, and cell‐cycle analysis by flow cytometry, respectively. Mitotic checkpoint kinase signal transduction and apoptosis activation was evaluated by Western blotting. Pathway gene expression analysis was evaluated by RT‐PCR. A Bliss independence model was used to calculate synergy scores for drug combination studies. Results Activation of AR in hormone‐sensitive cells induces a rescue phenotype by increasing cell viability and survival and attenuating G2/M arrest in response to DTX. Analysis of mitotic checkpoint signaling shows that AR negatively regulates spindle checkpoint signaling, resulting in premature mitotic progression and evasion of apoptosis. This phenotype is characteristic of mitotic slippage and is also observed in CRPC cell lines where we demonstrate involvement of AR splice variant AR‐v7 in dysregulation of checkpoint signaling. Our findings suggest that DTX resistance is mediated through mechanisms that drive premature mitotic exit. Using pharmacologic inhibitors of anaphase‐promoting complex/cyclosome and polo‐like kinase 1, we show that blocking mitotic exit induces mitotic arrest, apoptosis, and synergistically inhibits cell survival in combination with DTX. Conclusion Our results suggest that targeting the mechanisms of dysregulated mitotic checkpoint signaling in AR‐reactivated tumors has significant clinical potential to extend treatment benefit with DTX and improve outcomes in patients with lethal prostate cancer.

Abstract. Identification of sensitive and specific biomarkers for early detection and prognosis of prostate cancer is essential for timely and appropriate treatment of the disease in individual patients. We identified an RNA transcript with sequence homology to TRPM8 (melastatin-related transient receptor potential member 8) that was overexpressed in tumor vs. patient-matched non-tumor prostate tissues by RT-PCR differential display (DD). Semi-quantitative RT-PCR analysis revealed that TRPM8 levels were higher in tumor than in non-tumor tissue from 31 of 40 (>75%) patients examined. Overexpression of TRPM8 was independent of changes in androgen receptor (AR) mRNA levels in tumor tissue. However, in studies with established cell lines, TRPM8 expression was detectable only in AR-positive, but not in AR-negative cells, and it was suppressed by steroid deprivation or anti-androgen bicalutamide (Casodex) treatment, suggesting the requirement of AR activity for TRPM8 expression in prostate cancer cells. TRPM8 mRNA was also detected in body fluids of men. Most importantly, its levels were significantly higher (p<0.001, n=18) in urine and blood of patients with metastatic disease than in those of healthy men. However, there was no significant difference (p>0.05, n=10) in its levels between prostate cancer patients with localized disease and healthy men. Together, these studies demonstrate that TRPM8 expression is androgen regulated in prostate cancer cells and that, while tissue TRPM8 mRNA levels can be used for detection of prostate cancer, urine and blood TRPM8 mRNA levels may prove to be useful for distinguishing metastatic disease from clinically localized prostate cancer at the time of diagnosis.

Prostate-specific membrane antigen (PSMA) expression increases with prostate cancer (PCa) grade and progression; however, the role of PSMA in PCa progression remains poorly understood. Telomere stability is essential for the survival and genome stability of cancer cells. We found massive telomere DNA damage in PSMA-negative PCa cells (PC-3 and DU145) compared to PSMA-positive PCa (LNCaP) cells. The ectopic expression of PSMA suppressed telomere DNA damage in PC3 cells. PSMA inhibitor, 2-PMPA, and PSMA knockdown induced telomere DNA damage in PSMA-positive LNCaP cells but not in PSMA-negative PC-3 cells, suggesting that PSMA plays a critical role in telomere stability in PCa cells. In addition, we observed that inhibition of PSMA or inhibition of glutamate receptor, which mediates PSMA-dependent activation of AKT, suppressed AKT phosphorylation and caused telomere DNA damage. Furthermore, 2-PMPA-induced telomere DNA damage in LNCaP cells was associated with telomere aberrations, such as telomere-telomere fusions, sister-chromatid telomere fusions, and telomere breakages. AKT is reported to promote cell growth by stabilizing telomere association with telomere-binding proteins TRF1 and TPP1. We observed that TRF1 and TPP1 transfection of LNCaP cells attenuated the inhibitory effect of 2-PMPA on cell growth and telomere DNA damage. Together, these observations indicate that PSMA role in maintaining telomere stability in PCa cells is mediated by AKT. Thus, these studies reveal an important role of PSMA in maintaining telomere stability that can promote cell survival and, thereby, PCa progression. Implications: the role of PSMA in telomere stability suggests a strong correlation between PSMA expression and prostate cancer progression.