Activation of the Mammalian Target of Rapamycin Pathway Acutely Inhibits Insulin Signaling to Akt and Glucose Transport in 3T3-L1 and Human Adipocytes

Abstract

The mammalian target of rapamycin (mTOR) pathway has recently emerged as a chronic modulator of insulin-mediated glucose metabolism. In this study, we evaluated the involvement of this pathway in the acute regulation of insulin action in both 3T3-L1 and human adipocytes. Insulin rapidly (t(1/2) = 5 min) stimulated the mTOR pathway, as reflected by a 10-fold stimulation of 70-kDa ribosomal S6 kinase 1 (S6K1) activity in 3T3-L1 adipocytes. Inhibition of mTOR/S6K1 by rapamycin increased insulin-stimulated glucose transport by as much as 45% in 3T3-L1 adipocytes. Activation of mTOR/S6K1 by insulin was associated with a rapamycin-sensitive increase in Ser636/639 phosphorylation of insulin receptor substrate (IRS)-1 but, surprisingly, did not result in impaired IRS-1-associated phosphatidylinositol (PI) 3-kinase activity. However, insulin-induced activation of Akt was increased by rapamycin. Insulin also activated S6K1 and increased phosphorylation of IRS-1 on Ser636/639 in human adipocytes. As in murine cells, rapamycin treatment of human adipocytes inhibited S6K1, blunted Ser636/639 phosphorylation of IRS-1, leading to increased Akt activation and glucose uptake by insulin. Further studies in 3T3-L1 adipocytes revealed that rapamycin prevented the relocalization of IRS-1 from the low-density membranes to the cytosol in response to insulin. Furthermore, inhibition of mTOR markedly potentiated the ability of insulin to increase PI 3,4,5-triphosphate levels concomitantly with an increased phosphorylation of Akt at the plasma membrane, low-density membranes, and cytosol. However, neither GLUT4 nor GLUT1 translocation induced by insulin were increased by rapamycin treatment. Taken together, these results indicate that the mTOR pathway is an important modulator of the signals involved in the acute regulation of insulin-stimulated glucose transport in 3T3-L1 and human adipocytes.

Inhibition of the mTOR/p70S6K pathway is not involved in the insulin-sensitizing effect of AMPK on cardiac glucose uptake

“…It should be mentioned that, at variance with our data, rapamycin was found to increase insulin-induced glucose uptake in both insulin-sensitive and insulin-resistant 3T3-L1 adipocytes (3,28,35). This discrepancy suggests that the effect of mTOR/p70S6K inhibition on glucose uptake could be cell or tissue specific.…”

Nutritional upregulation of p85α expression is an early molecular manifestation of insulin resistance

“…Even though in vitro and ex vivo experiments have demonstrated an acute inhibitory effect of mTOR on insulin signalling in 3T3-L1 and human adipocytes [23], the present study in human muscle did not reveal an impairment in this pathway after 3 days of overfeeding. It has been noted previously that regulation of mTOR by amino acids in isolated adipocytes differs from that found in other tissues [30].…”

“…A cellular nutrient sensor, mTOR, has been identified as a critical element integrating cellular metabolism with growth factor signalling [20][21][22][23]. In response to insulin and amino acids, mTOR phosphorylates and modulates the activities of p70 S6 kinase and an inhibitor of translational initiation, eIF-4E-binding protein [24][25][26].…”

Rapamycin and mTOR kinase inhibitors

“…Conversely, S6K1 −/− cells are hypersensitive to insulin activation of PI3K signaling [26,27]. Treatment of some cancer cells with rapamycin upregulates Akt, which can enhance survival under conditions that usually induce apoptosis [23,[28][29][30]. As a result, there is concern that reactivation of Akt in tumors following rapamycin treatment could lead to resistance to other chemotherapeutic agents.…”