Mechanistic investigation of an anticancer agent that damages DNA and interacts with the androgen receptor

Abstract

The 11[beta] molecule comprises a ligand for the androgen receptor (AR), which is crucial to progression and survival of many prostate cancers, tethered to a DNA-damaging aniline mustard. The compound was designed to exhibit selective toxicity toward prostate cancer cells by forming sites of 11[beta]P -DNA damage to which AR binds, physically blocking access of repair enzymes while becoming unavailable to activate transcription of pro-survival genes. Previous studies have demonstrated the ability of 11[beta] 3 to damage DNA, retain affinity for AR when covalently adducted to DNA, and prevent selectively the growth of prostate xenograft tumors implanted in mice. Here we demonstrate that 11[beta] promotes phosphorylation and nuclear localization of AR, resulting in receptor association with androgen response elements. However, 11[beta] 3 only weakly drives AR transcriptional activity, and instead moderately antagonizes AR-mediated transcription elicited by natural androgens. Furthermore, 11[beta]0 dramatically decreases steady-state levels of AR protein. Collectively, these activities limit the expression of androgen-regulated genes and could control toxicity in AR-expressing prostate cancers. Despite possessing an ability to modulate AR transcriptional activity, 11[beta]0 is not selectively toxic toward the AR-positive member of an otherwise isogenic pair of prostate cancer cell lines derived from PC3 cells, which do not depend on AR-mediated gene expression for growth or survival. Therefore, the extraneous presence of AR does not increase 11[beta]13 toxicity.

(cont.) LNCaP cells are not rescued from toxicity by addition of high-affinity AR ligand, raising doubts about AR involvement in the mechanism of toxicity in these cells as well. To further assess AR involvement in 11[beta]0 toxicity, an analogue with -I10-fold lower affinity for AR, 17a-OH- 13, was synthesized and shown to produce measurably reduced AR-driven transcription compared with 11[beta] 3. 17a-OH- 11[beta] 3 is less toxic to AR-positive cell lines; however, this differential toxicity persists in an AR-null cell line, further suggesting that AR is uninvolved in 11[beta] 3 toxicity. Global transcriptional profiling has been conducted to assess other potential mechanisms for 11[beta] toxicity and has uncovered an ability of 11[beta] 3 to activate cholesterol/lipid biosynthetic pathways and a response to unfolded protein while down-regulating genes related to DNA damage repair. The unfolded protein response represents an attractive potential mechanistic explanation for selective toxicity toward cancerous cells, which may be related to an ability of 11[beta] 3 to perturb biological membranes.