Salvador Protein Is a Tumor Suppressor Effector of RASSF1A with Hippo Pathway-independent Functions

Abstract

The RASSF1A tumor suppressor binds and activates proapoptotic MST kinases. The Salvador adaptor protein couples MST kinases to the LATS kinases to form the hippo pathway. Upon activation by RASSF1A, LATS1 phosphorylates the transcriptional regulator YAP, which binds to p73 and activates its proapoptotic effects. However, although serving as an adaptor for MST and LATS, Salvador can also bind RASSF1A. The functional role of the RASSF1A/Salvador interaction is unclear. Although Salvador is a novel tumor suppressor in Drosophila and mice, its role in human systems remains largely unknown. Here we show that Salvador promotes apoptosis in human cells and that Salvador inactivation deregulates the cell cycle and enhances the transformed phenotype. Moreover, we show that although the salvador gene is seldom mutated or epigenetically inactivated in human cancers, it is frequently down-regulated posttranscriptionally. Surprisingly, we also find that although RASSF1A requires the presence of Salvador for full apoptotic activity and to activate p73, this effect does not require a direct interaction of RASSF1A with MST kinases or the activation of the hippo pathway. Thus, we confirm a role for Salvador as a human tumor suppressor and RASSF1A effector and show that Salvador allows RASSF1A to modulate p73 independently of the hippo pathway.

Hippo signalling in the G2/M cell cycle phase: Lessons learned from the yeast MEN and SIN pathways

“…Whatever the case, RASSF1A has been repeatedly reported as an activator of MST1/2 signalling [105][106][107][108]110]. RASSF1A can form complexes with MST1/2 and WW45 through conserved SARAH domains [71,82,83,111], which seems to play a role in Hippo dependent and independent apoptotic signalling [112]. Significantly, RASSF1A null cells display cytokinesis failure [82], which is a similar phenotype as observed in LATS2 null cells [92,93].…”

The RASSF1A Tumor Suppressor Regulates XPA-Mediated DNA Repair

“…However, the A(133)S SNP variant of RASSF1A exhibits the binding profile opposite to that of XPA as the wild-type pro- and is mediated by SIRT1. HEK-293 cells were transiently cotransfected with expression constructs for HA-tagged XPA and GFP-tagged RASSF1A or a RASSF1A mutant that is defective for binding to Mst1 (42) [RASSF1A L(308)P]. Equal amounts of protein were immunoprecipitated with anti-acetyl-Lys beads, and the immunoprecipitates were fractionated on an SDS-polyacrylamide gel and Western blotted with anti-HA and anti-GFP antibodies.…”

“…RASSF1A activates the MST1/ Hippo pathway (10). To determine whether the effects of RASSF1A were via Hippo signaling, we examined the ability of a RASSF1A point mutant that is defective for MST1 binding (42) to modulate XPA deacetylation. Figure 11A shows that the point mutant retains the ability to promote XPA deacetylation, suggesting that the effect is Hippo independent.…”

Cell Cycle Restriction Is More Important Than Apoptosis Induction for RASSF1A Protein Tumor Suppression

“…In addition to microtubules and the cell cycle, RASSF1A also controls at least two apoptotic pathways, Hippo and Bax (13)(14)(15)(16). RASSF1A activates the Hippo pathway by directly binding the kinases MST1 and MST2 (17,18).…”