Complement C3a promotes the formation of osteoclasts by inhibiting Sirt1 to activate the PI3K/PDK1/SGK3 pathway in patients with multiple myeloma.

BACKGROUND

Myeloma bone disease (MBD) is the most common complication of multiple myeloma (MM). Our previous study showed that the complement C3a activates osteoclasts to participate in the pathogenesis of MBD; however, its mechanism of action is diverse and complex. Studies have shown that the Sirtuin (Sirt) family of proteins (i.e., Sirt1-7) are expressed in human bone and cartilage, and participate in bone metabolic balance.

METHODS AND RESULTS

We measured the levels of complement C3a, Sirt1, osteoclast-related genes, and bone disease-related biological indicators using enzyme-linked immunosorbent assay (ELISA), quantitative real-time PCR and western blotting. Sirt1 expression in osteoclasts was observed to be lower in patients with MM compared to healthy donors and negatively correlated with complement C3a levels, osteoclast-related gene expression, and osteolysis-related markers. Co-immunoprecipitation (Co-IP) and immunostaining were used to verify the interaction between C3a and Sirt1 in RAW264.7 cells. Osteoclasts were then induced from bone marrow mononuclear cells (BMMCs) in patients with MM or cultured RAW264.7 cells, using C3a and/or Sirt1 activator (SRT1720)/inhibitors (EX527) in vitro. Sirt1 inhibits osteoclast formation and complement C3a reverses this inhibitory function of Sirt1 to activate osteoclasts. RAW264.7 cells with induced overexpression or knockdown Sirt1 were transfected with plasmid or shRNA, and RNA-seq analysis was performed. Increased Sirt1 expression resulted in the inhibition of the PI3K/PDK1/SGK3 pathway, which could be reactivated by complement C3a. Sirt1 knockdown activated the PI3K/PDK1/SGK3 pathway, which was further enhanced by complement C3a. A mouse model of MBD was successfully constructed. We injected this model with complement C3a or SRT1720, which further verified that complement C3a can significantly increase the degree of MBD bone damage, whereas SRT1720 can reduce the bone damage aggravated by C3a and treat MBD.

CONCLUSION

We demonstrated that complement C3a interacts with Sirt1 in osteoclasts to participate in the pathogenesis of MBD. Complement C3a promotes osteoclast formation by inhibiting Sirt1 to activate the PI3K/PDK1/SGK3 pathway in patients with MM, which is reduced by treatment with a Sirt1 activator. The application of a Sirt1 activator can reduce the formation of osteoclasts and reduce the severity of bone diseases in vivo and may be useful for the treatment of MBD. This study identified novel potential therapeutic targets and strategies for patients with MBD.