Castration-resistant prostate cancer: Androgen receptor inactivation induces telomere DNA damage, and damage response inhibition leads to cell death

Abstract

Telomere stability is important for cell viability, as cells with telomere DNA damage that 24 is not repaired do not survive. We reported previously that androgen receptor (AR) antagonist 25 induces telomere DNA damage in androgen-sensitive LNCaP prostate cancer cells; this triggers 26 a DNA damage response (DDR) at telomeres that includes activation of ATM, and blocking 27 ATM activation prevents telomere DNA repair and leads to cell death. Remarkably, AR 28 antagonist induces telomere DNA damage and triggers ATM activation at telomeres also in 29 22Rv1 castration-resistant prostate cancer (CRPC) cells that are not growth inhibited by AR 30 antagonist. Treatment with AR antagonist enzalutamide (ENZ) or ATM inhibitor (ATMi) by itself 31 had no effect on growth in vitro or in vivo, but combined treatment with ENZ plus ATMi 32 significantly inhibited cell survival in vitro and tumor growth in vivo. By inducing telomere DNA 33 damage and activating a telomere DDR, an opportunity to inhibit DNA repair and promote cell 34 death was created, even in CRPC cells. 22Rv1 cells express both full-length AR and AR splice 35 variant AR-V7, but full-length AR was found to be the predominant form of AR associated with 36 telomeres and required for telomere stability. Although 22Rv1 growth of untreated 22Rv1 cells 37 appears to be driven by AR-V7, it is, ironically, expression of full-length AR that makes them 38 sensitive to growth inhibition by combined treatment with ENZ plus ATMi. Notably, this 39 combined treatment approach to induce telomere DNA damage and inhibit the DDR was 40 effective in inducing cell death also in other CRPC cell lines (LNCaP/AR and C4-2B). Thus, the 41 use of ENZ in combination with a DDR inhibitor, such as ATMi, may be effective in prolonging 42 disease-free survival of patients with AR-positive metastatic CRPC, even those that co-express 43 AR splice variant. 44 45 3 46 Introduction: 47The critical role of the androgen receptor (AR) in prostate cancer cell proliferation and 48 survival is the enduring basis for treating advanced prostate cancer with drugs that block AR 49 function or androgen biosynthesis (1, 2). However, a relentless challenge is the development of 50 resistance to these treatments, referred to as castration-resistant prostate cancer (CRPC) (3). 51Remarkably, CRPC still relies on AR (4, 5), indicating a need to more fully understand the role 52 of AR in cell survival. In this regard, we have discovered a role of AR in prostate cancer cell 53 telomere stability (6, 7). Notably, inactivation of this role of AR creates a DNA damage 54 response (DDR) target, inactivation of which blocks repair and promotes cell death (8). 55Telomeres are the DNA-protein structures that cap the ends of linear chromosomes, 56 which are double-stranded DNA with a single-stranded overhang (9). Telomeres contain many 57 different proteins that play a role in the maintenance of telomere stability; the best characterized 58 are the six proteins (TRF1, TRF2, Rap1, TIN2, POT1 and TPP1) that comprise a complex 5...