A Phosphatidylinositol 3-Kinase/p70 Ribosomal S6 Protein Kinase Pathway Is Required for the Regulation by Insulin of the p85α Regulatory Subunit of Phosphatidylinositol 3-Kinase Gene Expression in Human Muscle Cells

Abstract

Insulin acutely up-regulates p85␣ phosphatidylinositol 3-kinase (p85␣PI 3-K) mRNA levels in human skeletal muscle (Laville, M., Auboeuf, D., Khalfallah, Y., Vega, N., Riou, J. P., and Vidal, H. (1996) J. Clin. Invest. 98, 43-49). In the present work, we attempted to elucidate the mechanism of action of insulin in primary cultures of human muscle cells. Insulin (10 ؊7 M, 6 h of incubation) induced a 2-fold increase in p85␣PI 3-K mRNA abundances (118 ؎ 12 versus 233 ؎ 35 amol/g total RNA, n ‫؍‬ 5, p < 0.01) without changing the expression levels of insulin receptor, IRS-1, glycogen synthase, and Glut 4 mRNAs in differentiated myotubes from healthy subjects. The effect is most probably due to a transcriptional activation of the p85␣PI 3-K gene because the half-life of the mRNA was not affected by insulin treatment (4.0 ؎ 0.8 versus 3.1 ؎ 0.4 h). PD98059 (50 M) did not modify the insulin response but increased p85␣PI 3-K mRNA levels in the absence of insulin, suggesting that the mitogen-activated protein kinase pathway exerts a negative effect on p85␣PI 3-K mRNA expression in the absence of the hormone. On the other hand, the insulin effect was totally abolished by LY294002 (10 M) and rapamycin (50 nM). In addition, overexpression of a constitutively active protein kinase B increased p85␣PI 3-K mRNA levels. These results indicate that the phosphatidylinositol 3-kinase/PKB/p70S6 kinase pathway is required for the stimulation by insulin of p85␣PI 3-K gene expression in human muscle cells. Phosphatidylinositol (PI)1 3-kinase (EC 2.7.1.67) is one of the key components of insulin signaling (1, 2). This lipid kinase is activated when the SH2 domains of its p85 regulatory subunit bind to the insulin receptor substrates (IRS) on specific tyrosine-phosphorylated sites (1-3). PI 3-kinase phosphorylates the D-3 position of the inositol ring of phosphoinositides (1, 2), generating potential second messengers that participate in the activation of protein kinase B (PKB) and p70 S6 kinase (p70S6K) (4, 5). Mainly using inhibitors, like LY294002 and wortmannin (inhibitors of PI 3-kinase) and rapamycin (inhibitor of p70S6K) (6 -8), the PI 3-kinase pathway was demonstrated to participate in a variety of insulin effects. These include (but are not limited to) the regulation of glucose uptake, the activation of glycogen synthesis, the inhibition of lipolysis, the control of protein synthesis, the inhibition of apoptosis, and the insulin-induced membrane ruffling (1, 2).The PI 3-kinase involved in insulin actions belongs to the class 1a of heterodimeric p85/p110 PI 3-kinases (9). The p85 regulatory subunit is an adaptator protein that links the p110 catalytic subunit to upstream signaling molecules. Two different genes coding highly homologous p85 (␣ and ␤) have been identified in mammalians, but the p85␣ regulatory subunit (p85␣PI 3-K) has been studied in more detail (2, 9). In cultured cells, expression of mutants of p85␣PI 3-K dramatically altered the response to insulin, supporting a major role of this subunit in the transduction of ...

Region-specific transcriptional response to chronic nicotine in rat brain

“…Recently, PTEN has gained enormous attention from the scientific community since it was found to be a tumor suppressor gene whose inactivation constitutively activates PI3 /AKT pathway [17,4,40]. Involvement of PI3 /AKT pathway in response to nicotine was also supported by our observation that the expression of p70S6, a downstream target of AKT gene [42], was downregulated in VTA ( Table 2). The observed effects of nicotine on the mRNA expression of PTEN and AKT/ p70S6 warrant further investigation, which may shed light on the associations between nicotine and cancer [32,47,2].…”

Phosphatidylinositol 3-Kinase Suppresses Glucose-Stimulated Insulin Secretion by Affecting Post-Cytosolic [Ca2+] Elevation Signals

“…Essentially identical effects on glucose-stimulated insulin releases were observed with LY294002, another inhibitor of PI 3-kinase (46,47). The 22.2 mmol/l glucosestimulated insulin secretion was potentiated by ϳ100% with 25-50 mol/l LY294002 (data not shown).…”

Insulin Regulation of Insulin-like Growth Factor-binding Protein-1 Gene Expression Is Dependent on the Mammalian Target of Rapamycin, but Independent of Ribosomal S6 Kinase Activity

“…The mTOR pathway has been linked to the regulation of expression of the insulin gene in pancreatic ␤-cells (19), the Na ϩ /P i cotransporter-1 gene in H4IIE cells (20), the hexokinase II gene in L6 myotubes (21), and the p85 ␣-regulatory subunit of PI3K in isolated muscle cells (22). The molecular connection between mTOR and these gene promoters is not clear.…”