Transforming growth factor-β1 inhibits human arterial smooth-muscle cell proliferation in a growth-rate-Dependent manner

Background: Proliferation of arterial smoothmuscle cells is central in the development of both atherosclerosis and intimal hyperplasia. The cytokine transforming growth factor-beta 1 (TGF-β1) is known to have variable effects on smooth-muscle cell proliferation. Using human arterial smooth-muscle cells, we sought (1) to define the serum concentrations required to maintain cellular proliferation; and (2) to define the effects of TGF-β1 on smooth-muscle cell proliferation. Methods: Smooth-muscle cell cultures were established from the normal aorta of transplant donors. Cells were grown to subconfluent and confluent densities, then incubated in either serum-free media, or 1% or 10% fetal bovine serum (FBS) enhanced media. Cellular proliferation was assayed by cell counting at 24, 48, and 96 hours to establish growth rate. Identical experiments with the addition of recombinant human TGF-β1 (5 ng/mL) were also performed. Studies were done in triplicate for each group, and results expressed as the mean ± SE. Groups were compared by analysis of variance. Results: In subconfluent cultures, only smoothmuscle cells in 10% FBS proliferated, whereas growth arrest occurred in serum-free media and 1% FBS. In confluent cultures, cells in all media conditions proliferated. TGF-β1 had an inhibitory effect in actively proliferating cultures. There was a positive correlation between the inhibitory effects of TGF-β1 and smooth-muscle cell growth rate (r = .65; P = 0.005). Conclusions: When confluent, human arterial smooth-muscle cells continue to proliferate after serum deprivation, suggesting that these cells are capable of conditioning their own medium. TGF-β1 inhibits smooth-muscle cell proliferation in a growth-rate-dependent manner. These data suggest that TGF-B1 may have a growth-regulatory role in vascular disease by counteracting states of arterial smooth-muscle cell proliferation.