Spatial organization of the extracellular matrix modulates the expression of PDGF-receptor subunits in mesangial cells

Spatial organization of the extracellular matrix modulates the expression of PDGF-receptor subunits in mesangial cells. The aim of this study was to test the hypothesis that changes in the extracellular matrix environment regulate rat mesangial cell growth by modulation of the expression of both PDGF-receptor α- and β-subunits. We investigated the mitogenic effects of the PDGF isoforms AA, AB and BB in conventional two-dimensional (2D) culture on laminin, fibronectin, type I, IV and V collagen and in the different spatial organization of matrix in type I collagen gels in three-dimensional culture (3D). In 2D culture PDGF BB was a potent mitogen, AB elicited an intermediate response while AA had no effect on cell proliferation. Extracellular matrix did not modify the PDGF responsiveness in 2D-culture. The different effects of the three PDGF isoforms were due to differential expression and isoform specific association of the PDGF-receptor subunits. Specifically, the β-receptor was strongly expressed, whereas the α-receptor was only barely detectable on the cell surface. Metabolic labeling revealed synthesis and intracellular accumulation of the complete α-receptor protein, and treatment with suramin increased its surface expression, suggesting continuous receptor down-regulation by endogenous PDGF. Morphological and ultrastructural analysis in 3D culture revealed a change in mesangial cell phenotype, forming a branching network of multicellular structures. Assessment of proliferation in 3D culture showed quiescent cells and PDGF unresponsiveness. Investigation of the PDGF β-receptors revealed a rapid down-regulation in 3D culture; both receptor subunits were not detectable on the cell surface. We conclude that 3D culture promotes the induction of a different mesangial cell phenotype. The lack of a mitogenic response to PDGF in 3D culture is due to the down-regulation of the PDGF β-receptors driven by the surrounding matrix. The nonproliferating but metabolically active cells in 3D culture appear to be more closely related to the in vivo phenotype than mesangial cells in conventional 2D culture.