Splicing factor arginine/serine‐rich 8 promotes multiple myeloma malignancy and bone lesion through alternative splicing of CACYBP and exosome‐based cellular communication

Background Multiple myeloma (MM) is a distinctive malignancy of plasma cell within the bone marrow (BM), of which alternative splicing factors play vital roles in the progression. Splicing factor arginine/serine‐rich 8 (SFRS8) is the exclusive factor associated with MM prognosis, however its role in MM remains undefined. Methods The analyses of 3‐(4,5)‐dimethylthiahiazo (‐z‐y1)‐3,5‐di‐ phenytetrazoliumromide (MTT) assay, immunohistochemistry, flow cytometry and xenograft model were performed to examine cell proliferation, cell cycle and apoptosis in SFRS8 overexpression or knockdown MM cells in vitro and in vivo. The SFRS8‐regulated alternative splicing events were identified by RNA immunoprecipitation sequencing (RIP‐seq) and validated by RIP‐qPCR and Co‐IP methods. Exosomes were extracted from the supernatant of myeloma cells by ultracentrifugation. Bone lesion was evaluated by TRAP staining in vitro and SCID/NOD‐TIBIA mouse model. A neon electroporation system was utilised to deliver siRNA through exosomes. The effect of siRNA‐loaded exosomes in vivo was evaluated by using a patient‐derived tumor xenograft (PDX) model and SCID/NOD‐TIBIA mouse model. Results SFRS8 was significantly upregulated in MM samples and positively associated with poor overall survival (OS) in MM patients. SFRS8 promoted MM cell proliferation in vitro and in vivo. Furthermore, calcyclin binding protein (CACYBP) was identified as the downstream target of SFRS8. Particularly, SFRS8 could reduce CACYBP isoform1 (NM_014412.3) and increase CACYBP isoform2 (NM_001007214.1) by mediating the alternative splicing of CACYBP, thereby altering the ubiquitination degradation of β‐catenin to promote MM progression. In addition, SFRS8 promoted osteoclast differentiation through exosomes in vitro and in vivo. More importantly, exosomal siRNA targeting CACYBP isoform2 inhibited tumour growth in PDX and SCID/NOD‐TIBIA mouse models. Conclusion Our findings demonstrate that targeting the SFRS8/CACYBP/β‐catenin axis may be a promising strategy for MM diagnosis and treatment. Graphic working model illustrates that SFRS8 reduces CACYBP isoform1 (NM_014412.3) and increases CACYBP isoform2 (NM_001007214.1) by mediating the alternative splicing of CACYBP, thereby altering the ubiquitination degradation of β‐catenin and exosome‐based cellular communication to promote MM malignancy and bone lesion.