Aging increases p16^INK4a expression in vascular smooth-muscle cells

Alteration of VSMC (vascular smooth-muscle cell) physiology is associated with the development of atherosclerosis and restenosis. We hypothesize that aging up-regulates the expression of p16^INK4a in VSMCs, which may increase the susceptibility of blood vessels to vascular occlusive diseases. Aortic VSMCs were obtained from young and aged mice. Cells from aged mice grew more slowly than those from their younger counterparts. Progression of cell cycle in response to serum stimulation was significantly inhibited in those cells with aging, as determined by FACS after propidium iodide staining. A significant up-regulation of p16^INK4a (2.5-fold, P=0.0012) was found in VSMC from aged animals using gene arrays. The up-regulation of this gene was further confirmed by quantitative RT–PCR (reverse transcription–PCR) and Western-blot experiments. Immunostaining for p16^INK4a confirmed that aortas from aged mice contained more p16^INK4a+ SMA (smooth-muscle cell actin)⁺ cells than aortas from young animals (26.79±2.45 versus 7.06±1.44, P=0.00027, n=4). In conclusion, we have shown that aging up-regulates the expression of p16^INK4a in VSMC in both cultures and arteries. The increase in p16^INK4a in the vasculature with aging may modify VSMC's response to post-injury stress and therefore accelerate the development of age-related cardiovascular diseases.