Abstract
The TOR pathway is a vital component of cellular homeostasis that controls the synthesis of proteins, nucleic acids and lipids. Its core is the TOR kinase. Activation of the TOR pathway suppresses autophagy, which plays a vital but complex role in tumorigenesis. The TOR pathway is regulated by activation of the Ras-related protein Rheb, which can bind mTOR. The Hippo pathway is a major growth control module that regulates cell growth, differentiation and apoptosis. Its core consists of an MST/LATS kinase cascade that can be activated by the RASSF1A tumor suppressor. The TOR and Hippo pathways may be coordinately regulated to promote cellular homeostasis. However, the links between the pathways remain only partially understood. We now demonstrate that in addition to mTOR regulation, Rheb also impacts the Hippo pathway by forming a complex with RASSF1A. Using stable clones of two human lung tumor cell lines (NCI-H1792 and NCI-H1299) with shRNA-mediated silencing or ectopic overexpression of RASSF1A, we show that activated Rheb stimulates the Hippo pathway, but is suppressed in its ability to stimulate the TOR pathway. Moreover, by selectively labeling autophagic vacuoles we show that RASSF1A inhibits the ability of Rheb to suppress autophagy and enhance cell growth. Thus, we identify a new connection that impacts coordination of Hippo and TOR signaling. As RASSF1A expression is frequently lost in human tumors, the RASSF1A status of a tumor may impact not just its Hippo pathway status, but also its TOR pathway status.
Highlights
Rheb is a Ras-related small GTPase which is broadly expressed in human tissue [1][2]
We used exogenous expression experiments to show that an activated form of Rheb, co-precipitated with RASSF1A when the proteins were co-expressed in HEK-293T cells
The mTOR kinase plays a key role in cellular homeostasis [45]. mTOR kinase forms two functional complexes TORC1 and TORC2
Summary
Rheb is a Ras-related small GTPase which is broadly expressed in human tissue [1][2]. It is negatively regulated by the TSC1/TSC2 tumor suppressor complex, which exhibits GAP activity against Rheb [3,4,5]. Hereditary defects in the TSC complex lead to Tuberous Sclerosis, a genetic disease characterized by constitutively active Rheb and a predisposition to CNS and Renal neoplasms [7,8,9]. Rheb can promote the transformed phenotype [10]. It can induce apoptosis [11, 12]. This suggests that the action of Rheb in a cell may be context dependent [13]
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