Age-dependent increase in c-fos activity and cyclin A expression in vascular smooth muscle cells A potential link between aging, smooth muscle cell proliferation and atherosclerosis

OBJECTIVE: Aging can be defined as a progressive deterioration of biological functions after the organism has attained its maximal reproductive competence, which is usually associated with a decrease in proliferative ability in most cell types. However, in certain pathological situations such as atherosclerosis and restenosis, aging has been shown to be associated with a higher level of vascular smooth muscle cell (VSMC) proliferation and neointimal lesion formation after angioplasty. In the present study, we investigated potential mechanisms involved in the age-dependent increase in VSMC proliferation. METHODS AND RESULTS: Primary cultures of VSMCs were isolated from young (6-8-month-old) and old (4-5-year-old) New Zealand rabbits. Results from cell counting assays and FACS analysis were consistent with a shortening of the cell cycle in old VSMCs. Western blot analysis in serum stimulated cells showed a significant increase in the level of cyclin A and cyclin-dependent kinase 2 proteins in the old vs. young VSMCs. In marked contrast, expression of cyclin E in VSMCs was not influenced by aging. Transient transfection assays showed an age-dependent increase in transcription from the human cyclin A promoter. Parallel studies demonstrated that the expression of the AP1 transcription factor c-fos, which interacts with the cyclin A promoter and stimulates VSMC proliferation, was also increased in old VSMCs. Consistent with this notion, electrophoretic mobility shift assays demonstrated an increase in AP1 DNA-binding activity in old VSMCs. CONCLUSIONS: These studies suggest that age-associated increase in c-fos activity contributes to augmented cyclin A expression and VSMC proliferation in old animals. These mechanisms might contribute to the higher prevalence and severity of atherosclerosis in the elderly.