Identification of a novel AS160 splice variant that regulates GLUT4 translocation and glucose-uptake in rat muscle cells.

AS160 (AKT substrate of 160 kDa) is an important mediator of GLUT4 (glucose transporter 4) translocation and glucose-uptake in adipocytes and muscle cells. In our study we have identified a novel splice variant of AS160 (variant 2 of AS160, AS160_v2) that lacks exon 11 and 12. The protein is phosphorylated in response to insulin via the PI3K/AKT pathway. Expression of this splice variant in human tissues from different donors was examined with quantitative RT-PCR. Our data reveal a tissue specific distribution pattern of both isoforms with highest overall expression of AS160_v2. To investigate the function of the novel splice variant we established the doxycycline-inducible expression of the protein in a rat myoblast cell line co-expressing GLUT4-myc. In contrast to data reported for the full-length AS160 protein, over expression and activation of transcript variant 2 in this cell line increased GLUT4 translocation and glucose-uptake rates in response to insulin and IGF-1 but not in response to AICAR or metformin. Immunofluorescence based studies indicated a direct association of AS160_v2 with GLUT4 under basal but not under insulin-stimulated conditions. Additionally, over expression of AS160_v2 slightly improved glucose-uptake rates in a model of insulin resistance but was not able to fully prevent induction of insulin resistance. This was accompanied with decreased phosphorylation of AS160_v2 and AKT. Taken together, our data suggest a tissue specific distribution of full-length AS160 and the novel AS160 splice variant (AS160_v2) indicating different functions. In contrast to full-length AS160, transcript variant 2 of AS160 seems to be a novel regulator of glucose transport that positively influences glucose-uptake rates.