Data from Selective Inhibition of Growth of <i>Tuberous Sclerosis Complex 2</i>–Null Cells by Atorvastatin Is Associated with Impaired Rheb and Rho GTPase Function and Reduced mTOR/S6 Kinase Activity

Abstract

<div>Abstract<p>Inactivating mutations in the <i>tuberous sclerosis complex 2</i> (<i>TSC2</i>) gene, which encodes tuberin, result in the development of TSC and lymphangioleiomyomatosis (LAM). The tumor suppressor effect of tuberin lies in its GTPase-activating protein activity toward Ras homologue enriched in brain (Rheb), a Ras GTPase superfamily member. The statins, 3-hydroxy-3-methylglutaryl CoA reductase inhibitors, have pleiotropic effects which may involve interference with the isoprenylation of Ras and Rho GTPases. We show that atorvastatin selectively inhibits the proliferation of <i>Tsc2<sup>−/−</sup></i> mouse embryo fibroblasts and ELT-3 smooth muscle cells in response to serum and estrogen, and under serum-free conditions. The isoprenoids farnesylpyrophosphate (FPP) and geranylgeranylpyrophosphate (GGPP) significantly reverse atorvastatin-induced inhibition of <i>Tsc2<sup>−/−</sup></i> cell growth, suggesting that atorvastatin dually targets a farnesylated protein, such as Rheb, and a geranylgeranylated protein, such as Rho, both of which have elevated activity in <i>Tsc2<sup>−/−</sup></i> cells. Atorvastatin reduced Rheb isoprenylation, GTP loading, and membrane localization. Atorvastatin also inhibited the constitutive phosphorylation of mammalian target of rapamycin, S6 kinase, and S6 found in <i>Tsc2<sup>−/−</sup></i> cells in an FPP-reversible manner and attenuated the high levels of phosphorylated S6 in <i>Tsc2</i>-heterozygous mice. Atorvastatin, but not rapamycin, attenuated the increased levels of activated RhoA in <i>Tsc2<sup>−/−</sup></i> cells, and this was reversed by GGPP. These results suggest that atorvastatin may inhibit both rapamycin-sensitive and rapamycin-insensitive mechanisms of tuberin-null cell growth, likely via Rheb and Rho inhibition, respectively. Atorvastatin may have potential therapeutic benefit in TSC syndromes, including LAM. [Cancer Res 2007;67(20):9878–86]</p></div>