Christina H. Stuelten

Extracellular matrix glycoproteins and proteoglycans bind a variety of growth factors and cytokines thereby regulating matrix assembly as well as bone formation. However, little is known about the mechanisms by which extracellular matrix molecules modulate osteogenic stem cells and bone formation. Using mice deficient in two members of the small leucine-rich proteoglycans, biglycan and decorin, we uncovered a role for these two extracellular matrix proteoglycans in modulating bone formation from bone marrow stromal cells. Our studies showed that the absence of the critical transforming growth factor-beta (TGF-beta)-binding proteoglycans, biglycan and decorin, prevents TGF-beta from proper sequestration within the extracellular matrix. The excess TGF-beta directly binds to its receptors on bone marrow stromal cells and overactivates its signaling transduction pathway. Overall, the predominant effect of the increased TGF-beta signaling in bgn/dcn-deficient bone marrow stromal cells is a "switch in fate" from growth to apoptosis, leading to decreased numbers of osteoprogenitor cells and subsequently reduced bone formation. Thus, biglycan and decorin appear to be essential for maintaining an appropriate number of mature osteoblasts by modulating the proliferation and survival of bone marrow stromal cells. These findings underscore the importance of the micro-environment in controlling the fate of adult stem cells and reveal a novel cellular and molecular basis for the physiological and pathological control of bone mass.

We used 2D-cocultures employing fibroblasts of different genetic backgrounds and MCF10A-derived human breast epithelial cells of increasingly malignant potential to investigate tumor-stroma interactions in breast cancer and to identify possible signaling pathways involved. Tumor cells induced expression of matrix-metalloproteinase 9 (MMP-9) in fibroblasts in a pattern dependent on the degree of their malignancy. In-situ zymography localized the main gelatinolytic activity around stromal cells in cocultures and xenografted tumors. Use of Smad3 knockout fibroblasts, small molecule inhibitors, and neutralizing antibodies showed that MMP-9 expression was induced by tumor cell-derived TNF-alpha and TGF-beta, dependent on Smad-, Ras-, and PI3-kinase-signaling, and likewise modulated by subsequent HGF- and EGF-signaling. Together, our results indicate that MMP-9 levels in tumor fibroblasts are regulated by a complex tumor-stroma cross-talk, involving multiple ligands and cellular signaling pathways.