Final answer:
In resting skeletal muscle fibers, Rheb activation of mTOR is inhibited by tuberous sclerosis complexes. In certain cancers, inhibition of RAS G-protein's GTPase activity means RAS cannot convert GTP to GDP and stays constantly active, resulting in unregulated activation of pathways controlling cell proliferation and survival.
Step-by-step explanation:
In resting skeletal muscle fibers, Ras homologue enriched brain (Rheb) activation of mTOR is inhibited by tuberous sclerosis complexes (TSC1/TSC2). When skeletal muscle is at rest, the mTOR pathway is suppressed to conserve energy and resources. However, in certain cancers, the GTPase activity of the RAS G-protein is inhibited. As a result, the RAS protein cannot hydrolyze GTP into GDP, leading to a state where it remains constantly active. This constant activation of the RAS pathway can lead to unregulated cell growth, as RAS is involved in signaling pathways that control cell proliferation and survival.
The RAS pathway involves several key steps in which RAS, when bound to GTP, activates RAF, which then phosphorylates MEK, followed by phosphorylation of ERK. Once activated, ERK can enter the nucleus and trigger a cell response that promotes division and proliferation. Therefore, when the RAS G-protein is locked in an active state, it leads to continuous signaling through this pathway, which can cause cells to divide uncontrollably, contributing to the development and progression of cancer.