Dietary γ-linolenic acid modulates macrophage-vascular smooth muscle cell interactions: Evidence for a macrophage-derived soluble factor that downregulates DNA synthesis in smooth muscle cells

Macrophages and smooth muscle cells (SMCs) are two of the major reactive cell types in atherosclerosis, a disease characterized by uncontrolled proliferation of SMCs. The present study was designed to determine how dietary oils containing γ-linolenic acid (GLA) (primrose oil [PO]) and long- chain n-3 fatty acids (fish oil) influence the ability of macrophages to modulate SMC DNA synthesis in vitro. Mice were fed one of four diets containing 10% (wt/wt) corn oil (CO), PO, fish oil-CO mix (FC; 9:1, wt/wt), or fish oil-PO mix (FP; 1:3, wt/wt) for 2 weeks. Resident peritoneal macrophages were isolated from these mice and seeded on a semipermeable membrane with a 30-kDa cutoff. Macrophages were preincubated with or without 50 μmol/L indomethacin (a cyclooxygenase inhibitor) or 50 μmol/L L655,238 (a 5-lipoxygenase inhibitor) for 30 minutes and subsequently cocultured with naive murine aortic SMCs grown on culture dishes. DNA synthesis in SMCs and prostaglandin formation in coculture supernatants were measured at the end of a 39-hour incubation period. SMC DNA synthesis was inhibited by 28% and 60% in PO and FP diets containing 10.1% and 8.2% GLA, respectively, relative to the control CO diet containing no GLA or long-chain n-3 fatty acid. A fourfold increase in the levels of PGE₁, a potent antiproliferative eicosanoid derived from GLA, was observed in the PO and FP groups relative to the control CO group. Although PGE₁ levels were not different between the CO and FC dietary groups, 15% inhibition of SMC DNA synthesis, relative to that in mice fed the control CO diet, was observed in mice fed the FC diet containing 13.3% 20:5n-3 and 7.6% 22:6n-3 fatty acids. Macrophage inhibition of SMC DNA synthesis and proliferation in mice consuming GLA-enriched diets was blocked by indomethacin but not by L655,238. Addition of exogenous PGE₁ (100 nmol/L) reversed the effect of indomethacin. In experiments in which mice were fed increasing levels of GLA-containing triglycerides, the ability of macrophages to downregulate SMC proliferation was modulated in a dose- dependent fashion. These data indicate that macrophages isolated from animals consuming diets supplemented with dietary oils containing GLA reduce SMC DNA synthesis and proliferation in a cyclooxygenase-dependent manner and therefore may favorably modulate the atherogenic process.