EXTINCTION OF AUTONOMOUS GROWTH-POTENTIAL IN EMBRYONIC-ADULT VASCULARSMOOTH-MUSCLE CELL HETEROKARYONS

Vascular smooth muscle cells (SMC) isolated from embryonic and early f etal (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 p henotype exhibit autonomous, serum-independent replication, in contras t to SMC in the ''adult'' growth phenotype, whose proliferation in cul ture is dependent on exogenous mitogens. To determine which of these t wo 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 b romodeoxyuridine (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 cell s exhibited parental growth phenotypes and showed the expected mitogen ic response when stimulated with serum. Heterokaryons between e17 and adult SMC exhibited a nonautonomous growth phenotype; the ''adult'' gr owth phenotype was calculated to be dominant in >89% of all true heter okaryons. The data suggest that adult SMC express molecules capable of genetically extinguishing or otherwise inhibiting the autonomous repl ication of embryonic SMC.