Raptor-rictor axis in TGFbeta-induced protein synthesis

Area of Science:

• Cell Biology
• Molecular Biology
• Renal Physiology

Background:

• Transforming growth factor-beta (TGFbeta) induces renal cell hypertrophy, a process critical for pathological kidney conditions.
• Increased protein synthesis is a hallmark of TGFbeta-stimulated renal cell hypertrophy.
• The precise molecular mechanisms underlying TGFbeta-induced protein synthesis remain incompletely understood, though PI 3-kinase/Akt signaling is implicated.

Purpose of the Study:

• To elucidate the downstream effectors of Akt involved in TGFbeta-stimulated protein synthesis in renal cells.
• To investigate the role of mTOR kinase and its associated complexes (mTORC1 and mTORC2) in this process.
• To determine how rictor influences TGFbeta signaling and protein synthesis.

Main Methods:

• Utilized Western blotting to assess protein phosphorylation and activation states.
• Employed RNA interference (shRNA) to downregulate specific proteins, including raptor and rictor.
• Measured protein synthesis rates and analyzed the activation of key signaling molecules like Akt, mTOR, S6 kinase, and 4EBP-1.

Main Results:

• TGFbeta activated mTOR kinase activity via PI 3-kinase/Akt-dependent phosphorylation of tuberin.
• mTORC1, containing raptor, was essential for TGFbeta-induced phosphorylation of S6 kinase and 4EBP-1, and subsequent protein synthesis.
• Rictor, a component of mTORC2, negatively regulated mTORC1 activity and basal protein synthesis, independent of TGFbeta stimulation.

Conclusions:

• mTORC1 signaling is indispensable for TGFbeta-induced renal cell hypertrophy through enhanced protein synthesis.
• Rictor plays a crucial role in controlling basal protein synthesis by negatively regulating mTORC1 activity.
• These findings highlight a novel regulatory mechanism for cellular hypertrophy and protein synthesis in the kidney.