RASSF5: An MST activator and tumor suppressor in vivo but opposite in vitro

Area of Science:

• Oncology
• Molecular Biology
• Cell Signaling

Background:

• The Ras association domain family member 5 (RASSF5) protein plays a complex role in cancer, with conflicting reports on its tumor suppressor or activator functions.
• RASSF5 acts as a molecular link between K-Ras signaling and the Hippo pathway, which regulates cell proliferation and apoptosis.
• Overexpression of YAP1, a downstream component of the Hippo pathway, is frequently observed in various cancers.

Purpose of the Study:

• To elucidate the mechanistic basis for the dual role of RASSF5 in cancer.
• To investigate how RASSF5 influences MST1/2 kinase activity in different cellular contexts.
• To reconcile conflicting findings regarding RASSF5's function as a tumor suppressor or activator.

Main Methods:

• Mechanistic investigation of RASSF5 interactions with K-Ras and MST1/2.
• In vivo and in vitro assays to assess MST1/2 kinase activity and dimerization.
• Analysis of RASSF5-mediated signaling in the context of membrane localization and Ras dimerization.

Main Results:

• RASSF5 activates MST1/2 and suppresses cancer in vivo, dependent on membrane-anchored Ras dimers.
• RASSF5 inhibits MST1/2 in vitro, where K-Ras binding releases RASSF5 SARAH domain but lacks membrane localization for MST1/2 kinase domain homodimerization.
• Both activation and inhibition of MST1/2 by RASSF5 occur through SARAH domain heterodimerization, with distinct outcomes based on cellular context.

Conclusions:

• RASSF5's function as a tumor suppressor or activator is context-dependent, influenced by K-Ras and membrane localization.
• Membrane-anchored Ras dimers promote RASSF5-mediated MST1/2 activation and tumor suppression in vivo.
• Understanding these mechanisms provides insights into targeted cancer therapies.