Role of cyclin-dependent kinase inhibitors in the growth arrest at senescence in human prostate epithelial and uroepithelial cells

Cellular senescence has been proposed to be an in vitro and in vivo block t hat cells must overcome in order to immortalize and become tumorigenic. To characterize these pathways, we focused on changes in the cyclin-dependent kinase inhibitors and their binding partners that underlie the cell cycle a rrest at senescence. As a model, we utilized normal human prostate epitheli al cell (HPEC) and human uroepithelial cell (HUC) cultures. After 30-40 pop ulation doublings cells became growth-arrested in G0/1 with a threefold dec rease in Cdk2-associated activity, a point defined as pre-senescence. Tempo rally following this growth arrest, the cells develop a senescence morpholo gy and express senescence-associated beta -galactosidase (SA-beta -gal). Le vels of p16(INK4a) and p57(KIP2) rise in HUCs during progressive passages, whereas only p16 increases in HPEC cultures. The induced expression of p57, similar to p16, produces a senescent-like phenotype. pRB, cyclin D, p19(IN K4d) and p27(KIP1) decrease in both cell types. We find that p53, p21(CIP1) and p15(INK4b) are transiently elevated in HPECs and HUCs at the pre-senes cent growth arrest, then return to low proliferating levels at terminal sen escence. Analysis of p53, p21(CIP1), P15(INK4b), p16(INK4a), and p57(KIP2) reveals altered expression in immortalized, non-tumorigenic HPV16 E6 and E7 prostate lines and in tumorigenic prostate cancer cells. These results ind icate: (i) the existence of a subset of growth inhibiting genes elevated at the onset of the senescence, (ii) a distinct class of genes involved in th e maintenance of senescence, and (iii) the frequent inactivation of these p athways during immortalization.