Extracellular matrix regulates induction of alkaline phosphatase expression by ascorbic acid in human fibroblasts

During wound healing and inflammation, fibroblasts express elevated alkaline phosphatase (ALP), but are not in contact with collagen fibrils in the fibronectin (FN)-rich granulation tissue. We hypothesized that the extracellular matrix (ECM) environment might influence the induction of ALP in fibroblasts. Here we tested this hypothesis by studying the ALP-inductive response of normal human gingival fibroblasts to ascorbic acid (AsA). AsA induced ALP activity and protein in cells in conventional monolayer culture. This induction was inhibited by blocking-antibodies to the FN receptor alpha 5 beta 1 integrin and by the proline analog 3,4-dehydroproline (DHP). DHP prevented cells from arranging FN fibrils into a pericellular network and reduced the activity of cell spreading on FN. Plating of cells on FN facilitated the up-regulation by AsA of ALP expression, but did not substitute for AsA. In contrast, AsA did not cause ALP induction in cells cultured on and in polymerized type I collagen gels. Collagen fibrils inhibited the up-regulation by AsA of ALP expression in cells plated on FN. These results indicate that the ECM regulates the induction of ALP expression by AsA in fibroblasts: FN enables them to express ALP in response to AsA through interaction with integrin alpha 5 beta 1, whereas type I collagen fibrils cause the suppression of ALP expression and overcome FN.