Uncoupling protein 3 (UCP3) stimulates glucose uptake in muscle cells through a phosphoinositide 3-kinase-dependent mechanism

UCP3 is a mitochondrial membrane protein expressed in humans selectively in skeletal muscle. To determine the mechanisms by which UCP3 plays a role in regulating glucose metabolism, we expressed human UCP3 in L6 myotubes by a denovirus-mediated gene transfer and in H9C2 cardiomyoblasts by stable tran sfection with a tetracycline-repressible UCP3 construct. Expression of UCP3 in L6 myotubes increased 2-deoxyglucose uptake S-fold and cell surface GLU T4 2.3-fold, thereby reaching maximally insulin-stimulated levels in contro l myotubes. Wortmannin, LY 294002, or the tyrosine kinase inhibitor geniste in abolished the effect of UCP3 on glucose uptake, and wortmannin inhibited UCP3-induced GLUT4 cell surface recruitment. UCP3 overexpression increased phosphotyrosine-associated phosphoinositide 3-kinase (PI3K) activity 2.2-f old compared with control cells (p < 0.05). UCP3 overexpression increased l actate release 1.5- to 2-fold above control cells, indicating increased glu cose metabolism. In H9C2 cardiomyoblasts stably transfected with UCP3 under control of a tetracycline-repressible promotor, removal of doxycycline res ulted in detectable levels of UCP3 at 12 h and 2.2-fold induction at 7 days compared with 12 h, In parallel, glucose transport increased 1.3- and 8-fo ld at 12 h and 7 days, respectively, and the stimulation was inhibited by w ortmannin or genistein, p85 association with membranes was increased 5.5-fo ld and phosphotyrosine-associated PI3K activity 3.8-fold. In contrast, over expression of UCP3 in 3T3-L1 adipocytes did not alter glucose uptake, sugge sting tissue-specific effects of human UCP3. Thus, UCP3 stimulates glucose transport and GLUT4 translocation to the cell surface in cardiac and skelet al muscle cells by activating a PI3K dependent pathway.