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J Biol Chem, Vol. 273, Issue 6, 3173-3179, February 6, 1998

Regulation of System A Amino Acid Transport in 3T3-L1 Adipocytes by Insulin

Ti-Zhi Su, Minghan Wang, Li-Jyun Syu^¶, Alan R. Saltiel^¶, and Dale L. Oxender^¶

From the Departments of  Molecular Biology and ^¶ Cell Biology, Parke-Davis Pharmaceutical Research Division of Warner Lambert Co., Ann Arbor, Michigan 48105

The insulin-stimulated uptake of 2-(methylamino)isobutyric acid (MeAIB), a nonmetabolizable substrate for system A, in 3T3-L1 adipocytes was investigated. As cells took on a more adipogenic phenotype, the insulin-stimulated versus the saturable basal MeAIB uptake increased by 5-fold. The induced transport activity showed properties characteristic of system A, with a K_m value of 190 µM. The half-life of the induced system A activity was independent of de novo mRNA and protein synthesis and was not accelerated by ambient amino acids, therefore, it was mechanistically distinct from the previously described adaptive and hormonal regulation of system A. Inhibition of mitogen-activated protein kinase kinase by PD98059, Ras farnesylation by PD152440 and B581, p70^S6K by rapamycin, and phosphatidylinositol 3-kinase (PI 3-K) by wortmannin and LY294002 revealed that only wortmannin and LY294002 inhibited the insulin-induced MeAIB uptake with IC₅₀ values close to that previously reported for inhibition of PI 3-K. These results suggest that the Ras/mitogen-activated protein kinase and pp70^S6K insulin signaling pathways are neither required nor sufficient for insulin stimulation of MeAIB uptake, and activation of PI 3-K or a wortmannin/LY294002-sensitive pathway may play an important role in regulation of system A transport by insulin in 3T3-L1 cells.

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