RASSF1A inactivation unleashes a tumor suppressor/oncogene cascade with context-dependent consequences on cell cycle progression.

The RASSF1A gene is one of the most frequently inactivated genes in over 30 different types of cancers (H. Donninger, M. D. Vos, and G. J. Clark, J. Cell Sci. 120:3163-3172, 2007, http://dx.doi.org/10.1242/jcs.010389). Despite the prevalence of RASSF1A silencing in human cancer, the mechanism by which RASSF1A functions as a tumor suppressor is not well understood. Characterization of the consequences of RASSF1A loss on epithelial cell proliferation revealed that RASSF1A expression suppresses both microRNA 21 (miR-21) expression and extracellular signal-regulated kinase 1/2 (ERK1/2) activation. The mechanism of the former is through restraint of SCF(ßTrCP)-dependent destruction of the repressor element 1 silencing transcription factor (REST) tumor suppressor and consequent inhibition of miR-21 promoter activation. The mechanism of the latter is through physical sequestration of MST2, which results in accumulation of inactivating S259 phosphorylation of RAF1. Whether or not inactivation of these RASSF1A regulatory relationships can unleash enhanced proliferative capacity is dependent upon the coupling of SCF(ßTrCP) and miR-21 to suppression of SKP2 protein translation and stability. Airway epithelial cultures retain this coupling and therefore respond to RASSF1A inactivation by p27-dependent cell cycle arrest. In contrast, colonic crypt-derived epithelial cells have uncoupled SCF(ßTrCP) from SKP2 and respond to RASSF1A inactivation by enhanced proliferation rates. These observations help account for context-specific molecular etiology of oncogenic transformation and suggest intervention strategies for recently developed SKP2 inhibitors.

RalB and the exocyst mediate the cellular starvation response by direct activation of autophagosome assembly.

The study of macroautophagy in mammalian cells has described induction, vesicle nucleation, and membrane elongation complexes as key signaling intermediates driving autophagosome biogenesis. How these components are recruited to nascent autophagosomes is poorly understood, and although much is known about signaling mechanisms that restrain autophagy, the nature of positive inductive signals that can promote autophagy remain cryptic. We find that the Ras-like small G protein, RalB, is localized to nascent autophagosomes and is activated on nutrient deprivation. RalB and its effector Exo84 are required for nutrient starvation-induced autophagocytosis, and RalB activation is sufficient to promote autophagosome formation. Through direct binding to Exo84, RalB induces the assembly of catalytically active ULK1 and Beclin1-VPS34 complexes on the exocyst, which are required for isolation membrane formation and maturation. Thus, RalB signaling is a primary adaptive response to nutrient limitation that directly engages autophagocytosis through mobilization of the core vesicle nucleation machinery.