Estrogen-induced smooth muscle cell growth is regulated by tuberin and associated with altered activation of platelet-derived growth factor receptor-β and ERK-1/2

The mechanisms that regulate the diverse responses to estrogen (E-2) are unknown. Loss of function of the tuberous sclerosis 2 gene (TSC2), a tumor suppressor gene, has been associated with a growth-promoting effect of E-2. We hypothesized that tuberin, the protein product of TSC2, binds to estrogen receptors ( ER) and regulates the growth effect of E-2. An in vivo association between full-length tuberin and ERalpha was observed in HEK 293 cells and ELT-3 smooth muscle cells. In contrast, poor association was observed between tuberin and ERbeta. Complex formation with ERalpha and the C-terminal end of tuberin was also observed in vivo and in vitro, indicating that binding between ERalpha and tuberin occurs at the C-terminal end of the tuberin molecule. We examined the effect of tuberin expression in ELT-3 smooth muscle cells on the growth response to E-2. The growth-promoting effect of E-2 in tuberin-null ELT-3 smooth muscle cells was ERalpha-specific, associated with up-regulation and activation of platelet-derived growth factor receptor-beta (PDGFRbeta) and activation of the signaling intermediate, extracellular signal-regulated kinase-1/-2 (ERK-1/ 2). In contrast, the expression of tuberin in ELT-3 smooth muscle cells resulted in significant abrogation of E-2-stimulated growth. In parallel with this observation, the expression of tuberin in ELT-3 cells also resulted in significant inhibition of PDGFRbeta and ERK-1/ 2 activation in response to E-2. These results demonstrate that tuberin binds specifically to ERalpha and inhibits E-2-induced proliferation of ELT-3 cells. Furthermore, the opposing effects of tuberin on estrogen-induced activation of PDGFRbeta and ERK-1/-2 suggest a pivotal role for tuberin in directing the signaling events that dictate the growth response to E-2.