Muscle GLUT4 regulation by estrogen receptors ERβ and ERα

Estrogen is known to influence glucose homeostasis with dominant effects in the liver, but the role of estrogen receptors in muscle glucose metabolism is unknown. In the present study, we investigated the expression of the two estrogen receptors, ER alpha and ER beta, and their influence on regulation of the glucose transporter, GLUT4, and its associated structural protein, caveolin-1, in mouse gastrocnemius muscle. Immunohistochemical analysis revealed that ER alpha and ER beta are coexpressed in the nuclei of most muscle cells, and that their levels were not affected by absence of estradiol [in aromatase-knockout (ArKO) mice]. GLUT4 expression on the muscle cell membrane was not affected by loss of ER beta but was extremely reduced in ER alpha(-/-) mice and elevated in ArKO mice. RT-PCR confirmed a parallel reduction in GLUT4 mRNA levels in ER alpha(-/-) mice. Upon treatment of ArKO mice with the ER beta agonist 2,3-bis(4-hydroxyphenyl)propionitrile, GLUT4 expression was reduced. By immunofluorescence and Western blotting, caveolin-1 expression was higher in ArKO mice and lower in ER beta(-/-) and ER alpha(-/-) mice than in WT littermates. GLUT4 and caveolin-1 were colocalized in WT and ArKO mice but not in ER beta(-/-) and ER alpha(-/-) mice. These results reveal that ERa is a positive regulator of GLUT4 expression, whereas ER beta has a suppressive role. Both ER beta and ER alpha are necessary for optimal caveolin-11 expression. Taken together, these results indicate that colocalization of caveolin-1 and GLUT4 is not an absolute requirement for muscle glucose metabolism but that reduction in GLUT4 could be contributing to the insulin resistance observed in ER alpha(-/-) mice.