Extracellular matrix alters PDGF regulation of fibroblast integrins.

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Abstract
Extracellular matrix (ECM) and growth factors are potent regulators of cell phenotype. These biological mediators of cellular responses are potentially interactive and as such could drive cells through progressive phenotypes to create new tissue as in morphogenesis and wound repair. In fact, ECM composition changes during tissue formation accompanied by alterations in cell growth and migration. How alterations in the ECM regulate cell activities is poorly defined. To address this question in wound repair, we cultured normal human dermal skin fibroblasts in relaxed collagen gels, fibronectin-rich cultures or stressed fibrin gels, and stressed collagen gels to model normal dermis, early wound provisional matrix, and late granulation tissue, respectively. Integrin subunits, alpha 2, alpha 3, and alpha 5, that define receptor specificity for collagen and provisional matrix, respectively, were measured at mRNA steady-state level before and after stimulation with platelet-derived growth factor-BB (PDGF-BB), a potent mitogen and chemoattractant for fibroblasts. Fibronectin-rich cultures and fibrin gels supported PDGF-BB induction of alpha 3 and alpha 5 mRNA. In contrast, both stressed and relaxed collagen attenuated these responses while promoting maximal alpha 2 mRNA expression. Posttranscriptional regulation was an important mechanism in this differential response. Together PDGF-BB and collagen gels promoted alpha 2, but not alpha 3 and alpha 5, mRNA stability. Conversely, when fibroblasts were in fibronectin-rich cultures, PDGF-BB promoted alpha 3 and alpha 5, but not alpha 2, mRNA stability. We suggest that ECM alterations during wound healing or any new tissue formation cause cells to respond differently to repeated growth factor stimuli. An ordered progression of cell phenotypes results, ultimately consummating tissue repair or morphogenesis.

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