A dual role of cholesterol in osteogenic differentiation of bone marrow stromal cells
Osteoblasts, the main bone-developing cells, are differentiated from mesenchymal stromal/stem cells. Disruption of the differentiation process may cause brittle bones, a bone disease characterised by low bone mass and deteriorated bone structure. Cholesterol continues to be implicated in pathogenesis of brittle bones, and it was lately recognized as an endogenous activator of Hedgehog (Hh) signaling. However, its pathological and physiological roles in osteoblast differentiation continue to be poorly understood. Furthermore, it’s unclear whether these potential roles performed by cholesterol are based on its capacity to modulate Hh path. Within this study, we investigated the function of exogenous versus endogenous cholesterol in osteogenesis and Hh path activation using ST2 cells, a bone marrow stromal cell line. We discovered that exogenous cholesterol considerably inhibited alkaline phosphatase (ALP) activity and messenger RNA expression of osteoblast markers genes (Alpl, Sp7, and Ibsp) while modestly activating expression of Gli1 (a readout of Hh signaling) under both basal osteogenic culture condition and Wnt3a treatment.
Similarly, exogenous cholesterol covered up osteogenic Purmorphamine response of ST2 cells to sonic Hh (Shh) or purmorphamine (Purmo) treatment, which, however, was supported by reduced induction of Gli1, indicating the participation of the Hh-dependent mechanism. Interestingly, depletion of endogenous cholesterol also reduced Shh-caused ALP activity and Gli1 expression. Likewise, cholesterol depletion inhibited osteogenic reaction to Purmo, although it didn’t affect Gli1 induction. Taken together, our findings have shown that cholesterol plays a dual role in osteoblast differentiation likely through both Hh-dependent and -independent mechanisms.