Consequences of interrupted Rheb-to-AMPK feedback signaling in tuberous sclerosis complex and cancer.

Rheb is a small GTPase primarily known for activating mammalian target of rapamycin complex 1 (mTORC1) and promoting cell growth in response to insulin and nutrients (amino acids, glucose). Shortage of glucose activates adenosine 5'-monophosphate-activated protein kinase (AMPK), which induces catabolic processes that produce ATP and suppresses energy-consuming anabolic reactions. As part of the latter response, AMPK activates the TSC1-TSC2 tumor suppressor complex, which in turn inhibits Rheb, thereby reducing mTORC1 activity and consequently suppressing protein synthesis. We recently identified an mTORC1-independent Rheb-to-AMPK feedback signaling loop in Tsc2-null in vitro models of Tuberous Sclerosis Complex (TSC). In addition to activating AMPK, Rheb reduced the nuclear levels of the cyclin-dependent kinase inhibitor p27(KIP1) (p27). Importantly, Rheb-mediated repression of p27 correlated with activation of Cdk2 and cell proliferation, and with tumor formation by TSC cells. Considering that AMPK was previously reported to regulate stability and subcellular localization of p27, we hypothesize that Rheb regulates p27 in TSC cells by activating AMPK. This article discusses how Rheb-to-AMPK, and p27 signaling may impact on disease progression and treatment of TSC, including sporadic lymphangioleiomyomatosis (S-LAM) and malignancies.