MLN64 deletion suppresses RANKL-induced osteoclastic differentiation and attenuates diabetic osteoporosis in streptozotocin (STZ)-induced mice

Diabetes mellitus is known as a main cause to induce osteoporosis. However, the pathogenesis of osteoporosis induced by diabetes has not been fully understood. MLN64 is highly homologous to the steroidogenic acute regulatory (STAR) protein, sharing the highly conserved START domain and exhibiting various biological activities. In the study, we attempted to explore the role of MLN64 in osteoporosis progression through the in vitro and in vivo studies. At first, the in vitro study suggested that MLN64 was over-expressed during the osteoclast differentiation induced by receptor activator of NF-kB ligand (RANKL). MLN64-knockdown markedly reduced the number of TRAP-positive multinucleated cells induced by RANKL, along with the down-regulation of specific genes related to osteoclastogenesis, including tartrate-resistant acid phosphatase (TRAP), nuclear factor of activated T cells (NFATc1), cathepsin K (CTSK), tartrate resistant (ACP5) and c-FOS and up-regulation of transcriptional receptor runt related transcription factor 2 (Runx2) and osteopontin (OPN). In contrast, over-expressing MLN64 significantly promoted the production of TRAP-positive multinucleated cells triggered by RANKL. Moreover, RANKL exposure led to remarkable increase in inflammatory factors, including tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), IL-6, and MMP-1, which was attenuated by the knockdown of MLN64. Additionally, the expression of phosphorylated (p)IKKα, p-IκBα and p-nuclear factor (NF)-κB stimulated by RANKL was clearly alleviated by MLN64 silence. However, RANKL-induced inflammation was accelerated by MLN64 over-expression. Further, the in vivo study suggested that streptozotocin (STZ)-caused reduction in body weight of mice was improved by the loss of MLN64. STZ-induced diabetic mice exhibited higher levels of blood alkaline phosphatase (ALP) and TRAP, while being down-regulated in MLN64-knockout mice. MLN64 deletion reversed STZ-induced trabecular deleterious effects and stimulated bone remodeling. What's more, MLN64 knockout inhibited the expression levels of TRAP, RANKL and RANK in femoral heads of STZ mice, accompanied with the repression of inflammatory response. In conclusion, the results in our study suggested that MLN64 played a critical role in the meditation of osteoclastic differentiation, and its suppression alleviated diabetic osteoporosis in STZ-induced mice. Thus, MLN64 could be served as an essential target for developing effective therapeutic strategy to prevent diabetic osteoporosis.