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, ERα and ERβ, 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 ERα and ERβ 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 ERβ but was extremely reduced in ERα^-/- mice and elevated in ArKO mice. RT-PCR confirmed a parallel reduction in GLUT4 mRNA levels in ERα^-/- mice. Upon treatment of ArKO mice with the ERβ 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 ERβ^-/- and ERα^-/- mice than in WT littermates. GLUT4 and caveolin-1 were colocalized in WT and ArKO mice but not in ERβ^-/- and ERα^-/- mice. These results reveal that ERa is a positive regulator of GLUT4 expression, whereas ERβ has a suppressive role. Both ERβ and ERα are necessary for optimal caveolin-1 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 J resistance observed in ERα^-/- mice.