The HACE1 E3 ligase mediates RAC1-dependent control of mTOR signaling complexes

Area of Science:

• Oncology
• Molecular Biology
• Biochemistry

Background:

• HACE1 (HECT domain E3 ubiquitin ligase 1) is a tumor suppressor with incompletely defined mechanisms.
• The mammalian target of rapamycin (mTOR) pathway is crucial in cell growth and is often dysregulated in cancer.

Purpose of the Study:

• To investigate the link between HACE1 and mTOR signaling.
• To elucidate the mechanism by which HACE1 regulates mTOR activity.

Main Methods:

• Co-immunoprecipitation assays to detect protein interactions.
• Ubiquitination assays to assess protein modification.
• Western blotting to measure protein levels.
• Cellular and in vivo models (including mouse xenografts) to study tumor development and signaling pathway activity.

Main Results:

• HACE1 directly interacts with and ubiquitylates Ras-related C3 botulinum toxin substrate 1 (RAC1) within mTOR-associated complexes.
• This ubiquitination leads to proteasomal degradation of RAC1, consequently reducing the stability and activity of mTORC1 and mTORC2.
• HACE1 deficiency enhances mTORC1/2 activity, which is rescued by RAC1 inhibition.
• In vivo, Rac1 deletion reverses enhanced mTOR expression in HACE1-deficient, KRas-driven lung tumors.

Conclusions:

• HACE1 functions as a negative regulator of mTOR signaling by targeting RAC1 for degradation.
• This mechanism highlights a novel ubiquitin-dependent pathway controlling mTOR complex activity and tumor suppression.
• HACE1's role in destabilizing mTOR via RAC1 offers potential therapeutic targets for cancers with mTOR pathway dysregulation.