Vascular smooth muscle cells (SMC) isolated from embyronic and early fetal (e13-e18) rat aortas exhibit an "embryonic growth phenotype" in culture (Cook, C. L., M. C. M. Weiser, P. E. Schwartz, C. L. Jones, and R. A. Majack. 1994. Circ. Res. 74:189-196). Cells in this growth phenotype exhibit autonomous, serum-independent replication, in contrast to SMC in the "adult" growth phenotype, whose proliferation in culture is dependent on exogenous mitogens. To determine which of these two phenotypes is genetically dominant, heterokaryons were constructed between adult and embryonic (day e17) rat aortic SMC. The fused cells were maintained in serum-free medium for 3 d, then were labeled with bromodeoxyuridine (BrdU) for an additional 24 h. Under these conditions, parental e17 SMC exhibited a high rate of self-driven DNA synthesis (73-85% BrdU-positive cells), while parental adult SMC showed minimal replication (13-21% BrdU-positive cells). Homokaryons of parental cells exhibited parental growth phenotypes and showed the expected mitogenic response when stimulated with serum. Heterokaryons between e17 and adult SMC exhibited a nonautonomous growth phenotype; the "adult" growth phenotype was calculated to be dominant in > 89% of all true heterokaryons. The data suggest that adult SMC express molecules capable of genetically extinguishing or otherwise inhibiting the autonomous replication of embryonic SMC.
R A Majack