Cullin 4B-RING E3 ligase negatively regulates the immunosuppressive capacity of mesenchymal stem cells by suppressing iNOS.

Mesenchymal stem cells (MSCs) are multipotent stem cells that can exert immunomodulatory capacity upon stimulation with pro-inflammatory cytokines. Our previous work has identified Cullin 4B (CUL4B), a scaffold protein in the CUL4B-RING E3 ligase (CRL4B) complex, as a key regulator in the differentiation of MSCs. Here, we demonstrate the critical role of CUL4B in regulating the immunosuppressive function of MSCs. When stimulated with pro-inflammatory cytokines, MSCs lacking CUL4B display enhanced immunosuppressive capacity, which is mediated by the elevated inducible nitric oxide synthase (iNOS). TGF-ß signaling can suppress iNOS by inhibiting its transcription as well as promoting its protein degradation. We show that the CRL4B complex cooperates with PRC2 complex and HDACs to repress transcription of Dlx1 and Pmepa1, two inhibitors of TGF-ß signaling, leading to decreased expression and accelerated degradation of iNOS. Our study unveils the CRL4B complex as a potential therapeutic target in promoting the immunosuppressive capacity of MSCs.

CUL4B orchestrates mesenchymal stem cell commitment by epigenetically repressing KLF4 and C/EBPδ.

Dysregulated lineage commitment of mesenchymal stem cells (MSCs) contributes to impaired bone formation and an imbalance between adipogenesis and osteogenesis during skeletal aging and osteoporosis. The intrinsic cellular mechanism that regulates MSC commitment remains unclear. Here, we identified Cullin 4B (CUL4B) as a critical regulator of MSC commitment. CUL4B is expressed in bone marrow MSCs (BMSCs) and downregulated with aging in mice and humans. Conditional knockout of Cul4b in MSCs resulted in impaired postnatal skeletal development with low bone mass and reduced bone formation. Moreover, depletion of CUL4B in MSCs aggravated bone loss and marrow adipose accumulation during natural aging or after ovariectomy. In addition, CUL4B deficiency in MSCs reduced bone strength. Mechanistically, CUL4B promoted osteogenesis and inhibited adipogenesis of MSCs by repressing KLF4 and C/EBPδ expression, respectively. The CUL4B complex directly bound to Klf4 and Cebpd and epigenetically repressed their transcription. Collectively, this study reveals CUL4B-mediated epigenetic regulation of the osteogenic or adipogenic commitment of MSCs, which has therapeutic implications in osteoporosis.