HIPPO SIGNALING PROTEIN MST1 REGULATES OSTEOCLAST DIFFERENTIATION BY INTERACTING WITH INTEGRIN LINKED KINASE (ILK) AND MODULATING ACTIN STRUCTURES

Hippo signaling is implicated in balancing cell proliferation, differentiation and death in multiple organs. However, its role in specific bone cell types such as osteoclasts, and its significance in maintaining overall bone tissue homeostasis remain largely unknown. In this study, we investigated the role of the Hippo pathway in osteoclast differentiation. Human primary monocyte cells were treated with receptor activator nuclear factor kappaB ligand (RANKL) and evaluated for osteoclast differentiation by marker protein analysis, tartrate-resistant acid phosphate (TRAP) and resorption assays. Our results showed that Ste20-like kinase 1 (MST1) underwent the maximum change after RANKL treatments and is negatively associated with osteoclast differentiation. Furthermore, proteomic approaches involving co-immunoprecipitation and mass spectrometry identified MST1 interaction with integrin-linked kinase (ILK) which is lost during RANKL induced differentiation. Finally, using RNAi and ectopic expression experiments we observed that MST1-ILK interaction negatively inhibits osteoclast differentiation at the level of actin ring structure formation, which is facilitated by ILK. Together, our data highlight a role for the Hippo pathway protein, MST1, in negatively regulating osteoclast differentiation through its interaction with integrin signaling. Given that integrin signaling is progressively implicated in pathological osteolysis, augmenting this pathway could have therapeutic implications.