Insulin-Like Growth Factor Binding Protein 5: an Important Regulator of Early Osteogenic Differentiation of hMSCs.

Insulin-like growth factor binding protein 5 (IGFBP5) is broadly bioactive, but its role in osteogenic differentiation of human bone marrow-derived mesenchymal stem cells (hMSCs) remains to be clarified. Here, we demonstrated that IGFBP5 expression was markedly increased during the early osteogenic differentiation of hMSCs. We then over-expressed and knocked down this gene in hMSCs and evaluated the impact of manipulation of IGFBP5 expression on osteogenic differentiation based upon functional assays, ALP staining, and expression of osteogenic markers. Together, these analyses revealed that IGFBP5 over-expression enhanced early osteogenic differentiation, as evidenced by increased ALP staining and osteogenic marker induction, whereas knocking down this gene impaired the osteogenic process. Over-expression of IGFBP5 also markedly bolstered the extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation level, while IGFBP5 knockdown suppressed this signalling activity. We additionally compared the impact of simultaneous IGFBP5 overexpression and ERK1/2 inhibitor treatment to the effect of IGFBP5 over-expression alone in these hMSCs, revealing that small molecule-mediated EKR1/2 inhibition was sufficient to impair osteogenic differentiation in the context of elevated IGFBP5 levels. These findings indicated that IGFBP5 drives the early osteogenic differentiation of hMSCs via the ERK1/2 signalling pathway. Our results offer value as a foundation for future efforts to study and treat serious bone-related diseases including osteoporosis.