PubMed · 11782439
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Mitchell A Lazar. 2002-01-01. Becoming fat.. https://doi.org/10.1101/gad.964002
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Insulin regulates cellular metabolism and growth through activation of insulin receptors (IRs). We recently identified a non-peptide small-molecule IR activator (compound 2), which induced human IR tyrosine kinase activity in Chinese-hamster ovary cells expressing human IR [Qureshi, Ding, Li, Szalkowski, Biazzo-Ashnault, Xie, Saperstein, Brady, Huskey, Shen et al. (2000) J. Biol. Chem. 275, 36590-36595]. Oral treatment with this compound resulted in correction of hyperglycaemia, hypertriacylglycerolaemia and hyperinsulinaemia in several rodent models of diabetes. In the present study, we have found that this compound increased tyrosine phosphorylation of the IR beta-subunit and IR substrate 1 in primary rat adipocytes as well as induced phosphorylation of Akt, the 70 kDa ribosomal protein S6 kinase and glycogen synthase-3 (deactivation) in Chinese-hamster ovary cells expressing human IR. Similar to insulin, compound 2 stimulated glucose uptake, glycogen synthesis and inhibited isoprenaline-stimulated lipolysis in adipocytes. A structurally related analogue (compound 3) was devoid of the above activities suggesting that the activity of compound 2 is specifically mediated by targeted IR activation. The effects of compound 2 on stimulation of glucose uptake, glycogen synthesis and inhibition of lipolysis were blocked by wortmannin, consistent with the involvement of a phosphoinositide 3-kinase-dependent pathway. In addition, compound 2, but not compound 3, exhibited additive or synergistic effects with sub-maximal concentrations of insulin in rat adipocytes. Thus the IR activator was capable of activating insulin-mediated signalling and metabolic pathways in primary adipocytes. These results demonstrate that IR activators have implications for the future development of new therapeutic approaches to Type I and Type II diabetes.
Adiponectin (ACRP30), an adipocyte-secreted protein encoded by the APM1 gene, is known to modulate insulin sensitivity and glucose homeostasis, those effects protecting obese mice from diabetes. Plasma adiponectin levels correlate well with insulin sensitivity in humans, and are decreased in both type 2 diabetes (T2D) and obesity. We screened for single-nucleotide polymorphisms (SNPs) the APM1 gene coding and 5' sequences in 40 French Caucasians: 12 SNPs and 4 rare non-synonymous mutations of exon 3 were detected. The 10 most frequent SNPs were genotyped in 1373 T2D and obese French Caucasian subjects and in all subjects available from 148 T2D multiplex families. The screening for rare mutations of exon 3 was extended to 1246 T2D and obese French subjects and to the members of the 148 T2D multiplex families. A haplotype including SNPs -11391 and -11377, both located in the 5' sequences, was associated with adiponectin levels (P<0.0001) and with T2D (P=0.004). The presence of at least one non-synonymous mutation in exon 3 showed evidence of association with adiponectin levels (P=0.0009) and with T2D (P=0.005). We failed to detect an association with insulin resistance indexes. Although family-based association analysis with T2D did not reach significance, our results suggest that an at-risk haplotype of common variants located in the promoter and rare mutations in exon 3 contribute to the variation of the adipocyte-secreted adiponectin hormone level, and may be part of the genetic determinants for T2D in the French Caucasian population.
Thiazolidinediones (TZDs) are effective therapies for type 2 diabetes, which has reached epidemic proportions in industrialized societies. TZD treatment reduces circulating free fatty acids (FFAs), which oppose insulin actions in skeletal muscle and other insulin target tissues. Here we report that TZDs, acting as ligands for the nuclear receptor peroxisome proliferator-activated receptor (PPAR)-gamma, markedly induce adipocyte glycerol kinase (GyK) gene expression. This is surprising, as standard textbooks indicate that adipocytes lack GyK and thereby avoid futile cycles of triglyceride breakdown and resynthesis from glycerol and FFAs. By inducing GyK, TZDs markedly stimulate glycerol incorporation into triglyceride and reduce FFA secretion from adipocytes. The 'futile' fuel cycle resulting from expression of GyK in adipocytes is thus a novel mechanism contributing to reduced FFA levels and perhaps insulin sensitization by antidiabetic therapies.