Rho-related GTP-binding protein RhoE (RND3) regulates multiple myeloma bone disease

Multiple Myeloma bone disease (MMBD) is highly prevalent in the multiple myeloma (MM) patients. Using the patients’ gene expression profile, we identified Rho Family GTPase 3 (RND3) as a gene whose expression in MM negatively associated with MMBD severity. Using MM patients’ biopsies, we validated that the patients with MMBD had decreased RND3 expression, compared with the patients without MMBD. To investigate the function of RND3 in MMBD, we established RND3 knocking down (RN-KD) MM cells, as well as control knock down (CT-KD). In a human MM xenograft mouse model, RN-KD MM caused severer MMBD than CT-KD. RN-KD MM promoted osteoclast (OC) differentiation, while inhibited osteoblast (OB) maturation. Mechanistically, RND3 interacted with ROCKs, Rho-associated kinases, in MM cells. ROCK family kinases included ROCK1 and ROCK2. In OC regulation, RND3 interacted with ROCK1 and ROCK2, exerted influence on Smad2/3 activation, which in turn controlled the downstream MMP-9, a known MM-secreted OC regulatory factor. In OB regulation, RND3 interacted with ROCK2, and regulated STAT3, which had downstream factor DKK1, a pivotal MM-secreted OB regulator in context of MM. Pan-ROCK inhibitor Y-27,632, which inhibited both ROCK1 and 2, decreased MMP-9 and DKK1 expressions in MM, and showed effective in MMBD treatment in vivo in the MM mouse model. Overall, our data suggested that intra-MM RND3-ROCK1/2 axis affected both OC and OB regulatory cytokines’ expression from MM cells.