CCAAT/Enhancer binding protein β controls androgen-deprivation-induced senescence in prostate cancer cells

Abstract

The processes associated with transition to castration-resistant prostate cancer (PC) growth are not well understood. Cellular senescence is a stable cell cycle arrest that occurs in response to sublethal stress. It is often overcome in malignant transformation to confer a survival advantage. CCAAT/Enhancer Binding Protein (C/EBP) β function is frequently deregulated in human malignancies and interestingly, androgen-sensitive PC cells express primarily the liver-enriched inhibitory protein isoform. We found that C/EBPβ expression is negatively regulated by androgen receptor (AR) activity and that treatment of androgen-sensitive cell lines with anti-androgens increases C/EBPβ mRNA and protein levels. Accordingly, we also find that C/EBPβ levels are significantly elevated in primary PC samples from castration-resistant compared with therapy-naive patients. Chromatin immunoprecipitation demonstrated enhanced binding of the AR to the proximal promoter of the CEBPB gene in the presence of dihydroxytestosterone. Upon androgen deprivation, induction of C/EBPβ is facilitated by active transcription as evident by increased histone 3 acetylation at the C/EBPβ promoter. Also, the androgen agonist R1881 suppresses the activity of a CEBPB promoter reporter. Loss of C/EBPβ expression prevents growth arrest following androgen deprivation or anti-androgen challenge. Accordingly, suppression of C/EBPβ under low androgen conditions results in reduced expression of senescence-associated secretory genes, significantly decreased number of cells displaying heterochromatin foci and increased numbers of Ki67-positive cells. Ectopic expression of C/EBPβ caused pronounced morphological changes, reduced PC cell growth and increased the number of senescent LNCaP cells. Lastly, we found that senescence contributes to PC cell survival under androgen deprivation, and C/EBPβ-deficient cells were significantly more susceptible to killing by cytotoxic chemotherapy following androgen deprivation. Our data demonstrate that upregulation of C/EBPβ is critical for complete maintenance of androgen deprivation-induced senescence and that targeting C/EBPβ expression may synergize with anti-androgen or chemotherapy in eradicating PC.