Wild-type p16INK4a suppresses cell growth, telomerase activity and DNA repair in human breast cancer MCF-7 cells

p16(INK4a), a cell cycle inhibitor that inhibits cyclin-dependent kinase 4 (cdk4) and cdk6, has been found as the tumor suppressor gene and is frequently deleted, methylated or mutated in many malignancies. Since p16(INK4a) is also a key element controlling cellular senescence and other functions, we hypothesized that p16(INK4a) induced tumor suppression may not be limited to the inhibition of cdks. To investigate the role of p16(INK4a) in tumor suppression and the potential interaction between p16(INK4a) and other cellular controlling elements, such as telomerase activity and DNA repair ability, the full-length of p16(INK4a) cDNA was cloned into a retroviral vector and introduced into human breast cancer MCF-7 cells that were previously demonstrated to harbor homozygous deletions of the p16(INK4a) gene. Stable expression of p16(INK4a) suppressed the malignant phenotype in MCF-7 cells, including cell proliferation, anchorage-independent growth, G1/G0 cell cycle arrest, and the blockage of pRB phosphorylation. In addition, expression of p16(INK4a) suppressed telomerase activity and restored the telomere shortening process, and decreased cell DNA repair ability and sensitized cells to the DNA damage reagent. Our data suggest that the wild-type p16(INK4a) plays an important role in suppression of tumor malignancy, not only by inhibiting cell proliferation through cell cycle arrest, but also by inhibiting other cellular controlling mechanisms, such as telomerase activity and DNA repair capacity.