Inhibition of MAPK pathway is essential for suppressing Rheb-Y35N driven tumor growth

Rheb is a Ras family GTPase, which binds to and activates mammalian target of rapamycin complex 1 (mTORC1) when GTP loaded. Recently, cancer genome sequencing efforts have identified recurrent Rheb Tyr35Asn mutations in kidney and endometrial carcinoma. Here we show that Rheb-Y35N causes not only constitutive mTORC1 activation, but sustained activation of the MEK-ERK pathway in a TSC1/TSC2/TBC1D7 protein complex and mTORC1-independent manner, contributing to intrinsic resistance to rapamycin. Rheb-Y35N transforms NIH3T3 cells, resulting in aggressive tumor formation in xenograft nude mice, which could be suppressed by combined treatment with rapamycin and an extracellular signal-regulated kinase (ERK) inhibitor. Furthermore, Rheb-Y35N inhibits AMPKa activation in response to nutrient depletion or 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR), leading to attenuated phosphorylation of BRAF-S729 and retained mitogen-activated protein kinase (MAPK) activation. Finally, we demonstrate that Rheb-WT can bind AMPK to facilitate AMPK activation, whereas Rheb-Y35N competitively binds AMPK, impairing AMPK phosphorylation. In summary, our findings indicate that Rheb-Y35N is a dominantly active tumor driver that activates both mTORC1 and MAPK to promote tumor growth, suggesting a combination of mTORC1 and MAPK inhibitors may be of therapeutic value in patients whose cancers sustain this mutation