No abstract

The cDNAs for two putative glucose transporters from mouse 3T3-L1 adipocytes were isolated and sequenced. One of these cDNAs encodes the murine homolog of the human hepG2/erythrocyte glucose transporter, termed GT1. GT1 mRNA is most abundant in mouse brain and is expressed in both 3T3-L1 preadipocytes and adipocytes. The other cDNA encodes a glucose transporter-like protein, termed GT2, that has a unique amino acid sequence and tissue distribution. GT2 cDNA encodes a protein with 63% amino acid sequence identity and a similar structural organization to GT1. GT2 mRNA is found at high levels in mouse skeletal muscle, heart, and adipose tissue, all of which exhibit insulinstimulated glucose uptake. GT2 mRNA is absent from 3T3-L1 preadipocytes but is induced dramatically during differentiation into adipocytes. This increase in mRNA content correlates closely with the acquisition of insulin-stimulated glucose uptake. We propose that GT2 is an insulin-regulated glucose transporter.

The effect of insulin on expression of CCAAT/enhancer binding protein (C/EBP) alpha, beta, and delta was investigated in fully-differentiated 3T3-L1 adipocytes. Treatment of adipocytes with insulin stimulated rapid dephosphorylation of C/EBP alpha, and repressed the expression of C/EBP alpha within 2-4 h, with > 90% suppression occurring at 24 h. While insulin induced expression of C/EBP beta and C/EBP delta within 1 h and caused a > 20-fold increase by 4 h, expression returned to nearly pretreatment levels by 24 h. The insulin concentration dependence of these effects was consistent with involvement of the insulin receptor. Gel shift analysis revealed that 6 h of insulin treatment decreased the binding of nuclear C/EBP alpha while increasing binding of nuclear C/EBP beta and C/EBP delta. The reciprocal effects of insulin on the steady-state levels of C/EBP transcription factors can be accounted for kinetically and quantitatively by changes in their mRNA levels, which can be accounted for by effects on gene transcription. The effects of insulin on adipocyte gene transcription (e.g. GLUT4) may be mediated, at least in part, by down-regulation of C/EBP alpha and/or its dephosphorylation.