Relationship between C-SRC Tyrosine Kinase Activity and the Control of Glucose Transporter Gene Expression

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Abstract
It has been shown previously that the rate of glucose transport in fibroblasts is accelerated by oncoproteins such as v-src, ras, and the transforming protein of feline sarcoma virus. This induction of glucose transport is associated with, and presumably caused by, induction of Hep-G2/rat brain glucose transporter gene expression. To determine the mechanism underlying the induction of glucose transporter gene expression by the v-src oncogene we studied cell lines that overexpress the normal counterpart of the v-src protein (c-src), or various mutants of the c-src protein. In these mutants, the tyrosines at positions 416, 527, or 519, or various combinations of these, have been replaced by phenylalanine by site directed mutagenesis, resulting in mutated c-src proteins that possess varying tyrosine kinase activity and transforming potential. Cells that overexpress the c-src protein show no changes in glucose transporter gene expression. However, when Tyr 527 in the COOH terminus of the c-src protein is replaced with Phe, the tyrosine kinase activity and transforming potential of the protein are increased and the protein acquires a potent ability to increase levels of glucose transporter mRNA and protein, as well as the rate of 2-deoxy-Dglucose uptake. This ability is abolished by the double mutation of Tyr to Phe in positions 416 and 527, which reduces the tyrosine kinase activity of the 527 single mutant. Thus, the ability of src proteins to induce expression of the glucose transport system is linked to the tyrosine kinase activity of the protein. However, comparisons between cells expressing the 527 mutant of c-src and v-src indicate that the control of transporter expression is not absolutely predicted by the kinase activity of the mutant src proteins. Thus, the 527 Tyr-Phe src mutant which has lowered kinase activity on exogenous substrates compared to v-src, induces a pattern of tyrosine phosphorylated proteins that is distinct from that induced by v-src, and has a greater ability to induce transporter expression than does v-src. This suggests that specific tyrosine phosphorylations induced by src are responsible for regulating glucose transporter gene expression and function of the glucose transport system.