Modulation of growth factor responsiveness of murine mammary carcinoma cells by cell matrix interactions: Correlation of cell proliferation and spreading

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Abstract
We have examined the role of growth factors and extracellular matrix in the proliferation and cell adhesion of a murine mammary carcinoma, SP1, and a stable highly metastatic variant, SP1‐3M. On fibronectin, both cell types proliferated strongly in response to basic fibroblast growth factor (bFGF) and platelet‐derived growth factor BB (PDGF‐BB) after culture for 24 h and 72 h. In contrast, on collagen type I, SP1 cells proliferated only weakly to PDGF‐BB at either time, and SP1‐3M cells showed a response to PDGF‐BB only at 72 h. The proliferative response to bFGF was also consistently lower when the cells were cultured on collagen than on fibronectin. No significant proliferative responses were detected to epithelial growth factor (EGF), transforming growth factor‐β (TGF‐β), or estrogen on any substratum. The lack of responsiveness to PDGF‐BB of cells cultured on collagen type I was not due to differences in numbers or affinity of PDGF receptors. We therefore examined the adhesion and spreading properties of SP1 and SP1‐3M cells. Without exogenous growth factors, both cell lines adhered to fibronectin and laminin. SP1‐3M cells did not bind to collagen type I, whereas SP1 cells did. Attachment to all three substrata was inhibited by anti‐β1 integrin IgG, suggesting that the primary adhesion to these substrata is mediated by β1 integrins. SP1 and SP1‐3M cells showed similar integrin patterns following immunoprecipitation by anti‐β1 integrin IgG. bFGF stimulated increased adhesion and spreading of both SP1 and SP1‐3M cells to collagen type I within 24 h, whereas PDGF‐BB was less capable of this effect. Our results suggest that the proliferative response of SP1 and SP1‐3M cells to PDGF‐BB and bFGF is dependent on the extracellular matrix environment, and imply that modification of extracellular matrix and/or surface integrin receptors may regulate responsiveness to these growth factors in the SP1 tumor model.