PDGF beta-receptor kinase activity and ERK1/2 mediate glycosaminoglycan elongation on biglycan and increases binding to LDL.

The initiation of atherosclerosis involves the subendothelial retention of lipoproteins by proteoglycans (PGs). Structural characteristics of glycosaminoglycan (GAG) chains on PGs influence lipoprotein binding and are altered adversely by platelet-derived growth factor (PDGF). The signaling pathway for PDGF-mediated GAG elongation via the PDGF receptor (PDGFR) was investigated. In human vascular smooth muscle cells, PDGF significantly increased (35)S-sulfate incorporation into PGs and GAG chain size. PGs from PDGF-stimulated cells showed increased binding low-density lipoprotein (P < 0.001) in gel mobility shift assays. Knockdown of PDGFRbeta using small interfering RNA demonstrated that PDGF mediated changes in PGs via PDGFRbeta. GAG synthesis and hyperelongation was blocked by inhibition of receptor tyrosine kinase autophosphorylation site Tyr857 activity using Ki11502 or imatinib. Downstream signaling to GAG hyperelongation was mediated through ERK MAPK and not phosphatidylinositol-3 kinase or phospholipase Cgamma. In high-fat-fed apolipoprotein E(-/-) mice, inhibition of PDGFRbeta activity by imatinib reduced aortic total lipid staining area by 35% (P < 0.05). Inhibition of PDGFRbeta tyrosine kinase activity leads to inhibition of GAG synthesis on vascular PGs and aortic lipid area in vivo. PDGFRbeta and its signaling pathways are potential targets for novel therapeutic agents to prevent the earliest stages atherosclerosis.

Chronic rosiglitazone treatment restores AMPKalpha2 activity in insulin-resistant rat skeletal muscle.

Rosiglitazone (RSG) is an insulin-sensitizing thiazolidinedione (TZD) that exerts peroxisome proliferator-activated receptor-gamma (PPARgamma)-dependent and -independent effects. We tested the hypothesis that part of the insulin-sensitizing effect of RSG is mediated through the action of AMP-activated protein kinase (AMPK). First, we determined the effect of acute (30-60 min) incubation of L6 myotubes with RSG on AMPK regulation and palmitate oxidation. Compared with control (DMSO), 200 microM RSG increased (P < 0.05) AMPKalpha1 activity and phosphorylation of AMPK (Thr172). In addition, acetyl-CoA carboxylase (Ser218) phosphorylation and palmitate oxidation were increased (P < 0.05) in these cells. To investigate the effects of chronic RSG treatment on AMPK regulation in skeletal muscle in vivo, obese Zucker rats were randomly allocated into two experimental groups: control and RSG. Lean Zucker rats were treated with vehicle and acted as a control group for obese Zucker rats. Rats were dosed daily for 6 wk with either vehicle (0.5% carboxymethylcellulose, 100 microl/100 g body mass), or 3 mg/kg RSG. AMPKalpha1 activity was similar in muscle from lean and obese animals and was unaffected by RSG treatment. AMPKalpha2 activity was approximately 25% lower in obese vs. lean animals (P < 0.05) but was normalized to control values after RSG treatment. ACC phosphorylation was decreased with obesity (P < 0.05) but restored to the level of lean controls with RSG treatment. Our data demonstrate that RSG restores AMPK signaling in skeletal muscle of insulin-resistant obese Zucker rats.

Rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle.

We hypothesized that improved glucose tolerance with rosiglitazone treatment would coincide with decreased levels of i.m. triacylglycerol (IMTG), diacylglycerol, and ceramide. Obese Zucker rats were randomly divided into two experimental groups: control (n = 9) and rosiglitazone (n = 9), with lean Zucker rats (n = 9) acting as a control group for obese controls. Rats received either vehicle or 3 mg/kg rosiglitazone for 6 wk. Glucose tolerance was impaired (P < 0.01) in obese compared with lean rats, but was normalized after rosiglitazone treatment. IMTG content was higher in obese compared with lean rats (70.5 +/- 5.1 vs. 27.5 +/- 2.0 micromol/g dry mass; P < 0.05) and increased an additional 30% (P < 0.05) with rosiglitazone treatment. Intramuscular fatty acid composition shifted toward a higher proportion of monounsaturates (P < 0.05) in obese rosiglitazone-treated rats due to an increase in palmitoleate (16:1; P < 0.05). Rosiglitazone treatment increased (P < 0.05) skeletal muscle diacylglycerol and ceramide levels by 65% and 100%, respectively, compared with obese rats, but elevated muscle diacylglycerol was not associated with changes in the total or membrane contents of the diacylglycerol-sensitive protein kinase C isoforms theta;, delta, alpha, and beta. In summary, we observed a disassociation among skeletal muscle IMTG, diacylglycerol and ceramide content, and glucose tolerance with rosiglitazone treatment in obese Zucker rats. Our data suggest, therefore, that rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle.