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

Objective: Aging can be defined as a progressive deterioration of biologica l functions after the organism has attained its maximal reproductive compet ence, which is usually associated with a decrease in proliferative ability in most cell types. However, in certain pathological situations such as ath erosclerosis and restenosis, aging has been shown to be associated with a h igher level of vascular smooth muscle cell (VSMC) proliferation and neointi mal lesion formation after angioplasty. In the present study, we investigat ed potential mechanisms involved in the age-dependent increase in VSMC prol iferation. Methods and results: Primary cultures of VSMCs were isolated fro m young (6-8-month-old) and old (4-5-year-old) New Zealand rabbits. Results from cell counting assays and FAGS analysis were consistent with a shorten ing of the cell cycle in old VSMCs. Western blot analysis in serum stimulat ed 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 tra nsfection assays showed an age-dependent increase in transcription from the human cyclin A promoter. Parallel studies demonstrated that the expression of the API transcription factor c-fos, which interacts with the cyclin A p romoter and stimulates VSMC proliferation, was also increased in old VSMCs. Consistent with this notion, electrophoretic mobility shift assays demonst rated an increase in AP1 DNA-binding activity in old VSMCs. Conclusions: Th ese studies suggest that age-associated increase in c-fos activity contribu tes 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. (C) 2000 Elsevier Science B.V. All rights reserved.