Primary mesenchymal cells isolated from SPARC-null mice exhibit altered morphology and rates of proliferation

SPARC (secreted protein acidic and rich in cysteine)/BM 40/osteonectin is a matricellular protein shown to function as a counteradhesive factor that i nduces cell rounding and as an inhibitor of cell proliferation. These activ ities have been defined in cell culture, in which interpretation has been c omplicated by the presence of endogenous SPARC. We therefore sought to dete rmine whether cell shape and proliferation would be affected by the absence of SPARC. Mesangial cells, fibroblasts, and aortic smooth muscle cells wer e isolated from SPARC-null and age-matched, wild-type mice. In contrast to wild-type cells, SPARC-null mesangial cells exhibited a flat morphology and an altered actin cytoskeleton. In addition, vinculin-containing focal adhe sions were distributed over the center of SPARC-null cells, whereas in wild -type cells, the number of focal adhesions was reduced, and these structure s were restricted largely to the cell periphery. Although the SPARC-null fi broblasts did not display overt differences in cell morphology, the cells r esponded to exogenous recombinant SPARC by rounding up in a manner similar to that of wild-type fibroblasts. Thus, the expression of endogenous SPARC is not required for the response of cells to SPARC. Additionally, SPARC-nul l mesangial cells, fibroblasts, and smooth muscle cells proliferated faster than their respective wild-type counterparts. Null cells also showed a gre ater sensitivity to the inhibition of cell cycle progression by the additio n of recombinant SPARC. The increased proliferation rate of SPARC-null cell s appeared to be mediated, at least in part, by an increase in the cell cyc le regulatory protein cyclin A. We conclude that the expression of SPARC in fluences the cellular architecture of mesangial cells and that SPARC plays a role in the regulation of cell cycle in mesangial cells, fibroblasts, and smooth muscle cells.