The RASSF8 candidate tumor suppressor inhibits cell growth and regulates the Wnt and NF-κB signaling pathways

The Ras-assocation domain family (RASSF) of tumor suppressor proteins until recently contained six proteins named RASSF1–6. Recently, four novel family members, RASSF7–10, have been identified by homology searches for RA-domain-containing proteins. These additional RASSF members are divergent and structurally distinct from RASSF1–6, containing an N-terminal RA domain and lacking the Sav/RASSF/Hpo (SARAH) domain. Here, we show that RASSF8 is ubiquitously expressed throughout the murine embryo and in normal human adult tissues. Functionally, RNAi-mediated knockdown of RASSF8 in non-small-cell lung cancer (NSCLC) cell lines, increased anchorage-independent growth in soft agar and enhanced tumor growth in severe combined immunodeficiency (SCID) mice. Furthermore, EdU staining of RASSF8-depleted cells showed growth suppression in a manner dependent on contact inhibition. We show that endogenous RASSF8 is not only found in the nucleus, but is also membrane associated at sites of cell–cell adhesion, co-localizing with the adherens junction (AJ) component β-catenin and binding to E-cadherin. Following RASSF8 depletion in two different lung cancer cell lines using alternative small interfering RNA (siRNA) sequences, we show that AJs are destabilized and E-cadherin is lost from the cell membrane. The AJ components β-catenin and p65 are also lost from sites of cell–cell contact and are relocalized to the nucleus with a concomitant increase in β-catenin-dependent and nuclear factor-κB (NF-κB)-dependent signaling following RASSF8 depletion. RASSF8 may also be required to maintain actin -cytoskeletal organization since immunofluorescence analysis shows a striking disorganization of the actin- cytoskeleton following RASSF8 depletion. Accordingly, scratch wound healing studies show increased cellular migration in RASSF8-deficient cells. These results implicate RASSF8 as a tumor suppressor gene that is essential for maintaining AJs function in epithelial cells and have a role in epithelial cell migration.