p27Kip1 and p21Cip1-independent proliferative inhibition of vascular smooth muscle cells cultured in type-I collagen matrix honeycombs

The proliferation of vascular smooth muscle cells (SMCs) contributes to atherosclerotic plaque formation and restenosis. Cyclin-dependent kinase inhibitors, such as p27Kip1 and p21Cip1, are known to play significant roles in the control of the aberrant proliferation of SMCs. Primary cultured SMCs stop proliferating immediately when cultured in three-dimensional matrices of type-I collagen “honeycombs” structures. To clarify whether p27Kip1 and p21Cip1 are involved in the proliferative inhibition of SMCs cultured in honeycombs, the characteristics of SMCs derived from the aorta of both wild-type mice (p27[+/+] SMCs) and p27Kip1 knockout mice (p27[−/−] SMCs) were investigated. Although the growth of p27(−/−) SMCs cultured on plates was faster than that of p27(+/+) SMCs, the number of both p27(+/+) and p27(−/−) SMCs did not change when they were cultured in honeycombs. p21Cip1 expression was decreased but maintained in p27(−/−) SMCs cultured on plates and in honeycombs. Knockdown of p21Cip1 in p27(−/−) SMCs promoted proliferation on plates. On the contrary, p21Cip1 knockdown had no effect on the proliferation of p27(−/−) SMCs cultured in honeycombs. In conclusion, p27Kip1 and p21Cip1 are insufficient for the proliferative inhibition of SMCs cultured in honeycombs.