Inhibition of smooth muscle cell proliferation by adiponectin requires proteolytic conversion to its globular form.

Accelerated atherosclerosis is the primary cardiovascular manifestation of diabetes and correlates inversely with levels of circulating adiponectin, an anti-atherosclerotic adipokine that declines in diabetes. We therefore initiated a study to examine the mechanisms by which adiponectin, a hormone released from adipose tissue, influences the proliferation of vascular smooth muscle cells (SMCs). Addition of adiponectin to quiescent porcine coronary artery SMCs increased both protein and DNA synthesis and concurrently activated ERK1/2 and Akt. By contrast, globular adiponectin, a truncated form of this protein, exhibited anti-mitogenic properties as indicated by the inhibition of protein and DNA synthesis in SMCs stimulated with platelet-derived growth factor (PDGF). Whereas globular adiponectin did not stimulate growth-related signal transduction pathways, it was able to block the PDGF-dependent phosphorylation of eukaryotic elongation factor 2 kinase, a regulator of protein synthesis. Proteolysis of adiponectin with trypsin, which produces globular adiponectin, reversed the growth-stimulating actions of the undigested protein. As the existence of globular adiponectin remains controversial, western blotting was used to establish its presence in rat serum. We found that globular adiponectin was detectable in rat serum, but this result was not obtained with all antibodies. The contrasting properties of adiponectin and its globular form with respect to SMC proliferation suggest that protection against atherosclerosis may therefore be mediated, in part, by the level of globular adiponectin.

Peroxisome proliferator-activated receptor alpha and gamma ligands differentially affect smooth muscle cell proliferation and migration.

Peroxisome proliferator-activated receptors (PPARs) alpha and gamma are expressed in smooth muscle cells (SMCs). This study was designed to compare the effects of PPARalpha and PPARgamma on SMC proliferation and migration and to determine how they operate. Treatment of SMCs from porcine coronary artery revealed that mitogen-stimulated DNA synthesis was blocked by the PPARalpha ligand 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (WY14,643) and 15-deoxy-Delta(12,14) prostaglandin J(2) (15d-PGJ(2)) (a putative PPARgamma agonist) but not by the PPARgamma agonist rosiglitazone or the PPARbeta/delta ligand 2-methyl-4-((4-methyl-2-(4-trifluoromethylphenyl)-1,3-thiazol-5-yl)-methylsulfanyl)phenoxy acetic acid (GW501516). Inhibition of DNA synthesis by clofibrate and 2-(4-(2-(1-cyclohexanebutyl-3-cyclohexylureido)ethyl)phenylthio)-2-methylproprionic acid (GW7647) confirmed that SMC proliferation is affected by PPARalpha. This conclusion was supported by the fact that WY14,643 also inhibited the proliferation of H4IIE hepatoma cells (expressing only PPARalpha) but not A10 SMCs (expressing only PPARgamma1). In contrast, the effective inhibition of all cell types with 15d-PGJ(2) indicated that this compound probably operates via a PPARgamma-independent mechanism. Interestingly, rosiglitazone did not inhibit DNA synthesis of either H4IIE or A10 cells, suggesting that the activation of PPARgamma does not influence cell proliferation. Phosphorylation of cyclin-dependent kinase 2 and expression of proliferating cell nuclear antigen were inhibited by WY14,643 but not by rosiglitazone or 15d-PGJ(2), indicating that PPARalpha prevents progression into S phase. Although rosiglitazone did not block SMC proliferation, it (like WY14,643) reduced neointimal hyperplasia in vitro. This observation can be rationalized by the fact that both WY14,643 and rosiglitazone inhibit SMC migration, probably through matrix metalloproteinase 9. Our study therefore shows that selective interference with mediators of cell cycle progression and cell migration via activation of PPARs may prevent growth-related vascular diseases such as restenosis and atherosclerosis.