ABSTRACT
The matricellular protein thrombospondin-2 (TSP2) inhibits proliferation and enhances osteoblastogenesis of multipotent mesenchymal progenitor cells (MPC). Osteoblastogenesis and adipogenesis are reciprocally regulated, thus we hypothesized that TSP2 could be an inhibitor of adipogenesis. TSP2 gene expression is up-regulated during MPC osteoblast differentiation and down-regulated during adipogenic differentiation through a cAMP-PKA pathway, relative to control cells. Next, the importance of TSP2 in adipogenesis was studied by comparing gene-targeted knockout mice that lack TSP2 protein (TSP2-null) and control wild-type mice. TSP2-null marrow-derived MPC show 25% increased adipocytes. Similarly, TSP2-null adipose-derived MPC show increased adipocytes (25-50%) and proliferation (2-fold) relative to wild-type cells. However, the increase in TSP2-null adipocytes is not due to an increase in MPC number, because the percentage of adipocytes relative to total MPC is still significantly increased. Surprisingly, there are only minor alterations in the adipogenic transcriptional program (PPARγ and C/EBPα expression). Weight gain over time was evaluated in TSP2-null and control wild-type mice fed normal and high-fat diets. Female TSP2-null mice are 30% heavier than wild-type controls due to an increase in non-visceral adipose tissue, measured by dual-energy X-ray absorptiometry (DEXA) and direct weighing of fat pads. These results show that TSP2 is an endogenous matricellular protein produced by MPC that in addition to enhancing osteoblastogenesis, inhibits adipocytes and decreases subcutaneous adiposity.
