Expression of the splice variants of the p85α regulatory subunit of phosphoinositide 3-kinase in muscle and adipose tissue of healthy subjects and type 2 diabetic patients

The regulation by insulin of the expression of the p85α regulatory subunit of phosphoinositide 3-kinase (PI 3-kinase) is impaired in skeletal muscle and adipose tissue of type 2 diabetic patients. The gene encoding p85α (named grb-1) can generate several variants by alternative splicing, all being able to activate the p110 catalytic subunits of PI 3-kinase. Our aims were (i) to determine the mRNA expression profiles of these variants in human skeletal muscle and adipose tissue; (ii) to investigate the effect of insulin on their expression in vivo and in vitro in muscle and (iii) to verify whether this regulation is defective in type 2 diabetes. We determined the human genomic organization of grb-1 and set up reverse transcriptase competitive PCR assays for the quantification of each mRNA variant. In muscle, p85α and p50α mRNAs were the most abundant, and p55α represented less than 20% of all grb-1-derived mRNAs. In adipose tissue, p85α was expressed predominantly and p55α mRNA was not detectable. These expression profiles were not different in type 2 diabetics. During a 3h hyperinsulinaemic clamp, insulin increased the mRNA expression of the three variants in muscle of control subjects. In diabetic patients, the effect of insulin on p85α and p50α mRNAs was blunted, and largely reduced on p55α transcripts. In cultured human myotubes, up-regulation of p85α, p55α and p50α mRNAs by insulin was abolished by LY294002 (10μM) and by rapamycin (50nM), suggesting that the PI 3-kinase/protein kinase B/p70 S6 kinase pathway could be involved in the stimulation of grb-1 gene expression by insulin in human muscle cells.