Endothelial cell dysfunction and cytoskeletal changes associated with repression of p16(INK4a) during immortalization

The immortalization process is a fundamental step in the development of most (if not all) human cancers, including the aggressive endothelial cell (EC)-derived malignancy angiosarcoma. Inactivation of the tumor suppressor p16(INK4a) and the development of multiple chromosomal abnormalities are features of angiosarcoma that are recapitulated during telomerase-mediated immortalization of human ECs in vitro. The present study used a panel of telomerase-immortalized bone marrow EC (BMEC) lines to define the consequences of inactivation of p16(INK4a) on EC function and to identify molecular changes associated with repression of p16(INK4a). In a comparison of two immortalized BMEC mass cultures and six clones, the cell lines that repressed p16(INK4a) showed a higher rate of proliferation and an impaired ability to undergo morphogenic differentiation and form vessel-like structures in vitro. Proteomic comparison of a p16(INK4a)-negative and a p16(INK4a)-positive BMEC mass culture at early- and late-passage time points following transduction with telomerase reverse transcriptase (hTERT) revealed altered expression of cytoskeletal proteins, including vimentin and α-tropomyosin (αTm), in the immortal cells. Immunoblot analyses of a panel of 11 immortal clones showed that cells that lacked p16(INK4a) expression tended to accumulate more dramatic changes in these cytoskeletal proteins than cells that retained p16(INK4a) expression. This corresponded with aberrant cytoskeletal architectures among p16(INK4a)-negative clones, which featured thicker actin stress fibers and less fluid membrane ruffles than p16(INK4a)-positive cells. A direct link between p16(INK4a) repression and defective EC function was confirmed by analysis of normal cells transfected with small interfering RNA (siRNA) targeting p16(INK4a). siRNA-mediated repression of p16(INK4a) significantly impaired random motility and vessel formation in vitro. This report is the first to demonstrate that ECs that repress the expression of p16(INK4a) are prone to defects in motility, morphogenesis and cytoskeletal organization. These defects are likely to reflect alterations that occur during the development of EC-derived malignancies.