RhoA modulates signaling through the mechanistic target of rapamycin complex 1 (mTORC1) in mammalian cells

Area of Science:

• Cellular Biology
• Molecular Biology
• Signal Transduction

Background:

• The mechanistic target of rapamycin (mTOR) pathway, specifically mTOR complex 1 (mTORC1), is crucial for controlling cell growth and metabolism.
• mTORC1 activity is modulated by GTPases like Rheb and Rags, integrating signals from nutrients and hormones.
• In yeast, Rho1 interacts with TORC1 and represses its activity.

Purpose of the Study:

• To investigate whether the RhoA GTPase influences signaling through mTORC1 in mammalian cells.
• To elucidate the mechanism by which RhoA might regulate mTORC1 activity.

Main Methods:

• Overexpression of wild-type or constitutively active RhoA in mammalian cells.
• Assessment of mTORC1 signaling by measuring phosphorylation of key substrates like p70S6K1, 4E-BP1, and ULK1.
• Investigation of RhoA interaction with mTORC1 and its effect on upstream regulators like Rheb, Rags, TSC2, PRAS40, and Akt.

Main Results:

• Exogenous RhoA overexpression repressed mTORC1 signaling, evidenced by decreased phosphorylation of p70S6K1, 4E-BP1, and ULK1.
• RhoA·GTP repressed mTORC1 signaling independently of insulin or leucine stimulation.
• RhoA functions upstream of Rheb to inhibit mTORC1, as constitutively active Rheb rescued the repression.
• RhoA·GTP repressed phosphorylation of TSC2, PRAS40, Akt, and mTORC2, suggesting a role upstream of Akt and mTORC2.

Conclusions:

• RhoA GTPase acts as a negative regulator of mTORC1 signaling in mammalian cells.
• The inhibitory mechanism involves RhoA acting upstream of Rheb, and potentially impacting the Akt and mTORC2 pathways.
• This study establishes a novel role for RhoA in modulating mTORC1-dependent cellular processes.