Vitamin D₃ regulation of stromelysin-1 (MMP-3) in chondrocyte cultures is mediated by protein kinase C

Matrix metalloproteinases (MMPs) are a group of enzymes with the potential to degrade extracellular matrix proteins. One of the MMPs, stromelysin-1 (MMP-3) has been localized to extracellular matrix vesicles in growth plate chondrocyte cultures, suggesting involvement of this enzyme in remodeling of the extracellular matrix during endochondral development, a process which is regulated by the vitamin D metabolites, 1,25-(OH)₂D₃ and 24,25-(OH)₂D₃. To determine whether stromelysin-1 is regulated by vitamin D as well, confluent cultures of cells derived from growth zone (GC) and resting zone (RC) rat costochondral cartilage were treated with 1α,25-(OH)₂D₃ (1,25) and 24R,25-(OH)₂D₃ (24,25), respectively, and the effect on stromelysin-1 assessed by casein gel zymography and Western blots. Although stromelysin-1 activity was enriched in the matrix vesicle fraction, only the plasma membrane enzyme was affected by the treatment; 1,25 and 24,25 caused a marked decrease in plasma membrane stromelysin-1 activity in their target cells. Since plasma membrane protein kinase C (PKC) activity is stimulated by 1,25 and 24,25, we hypothesized that stromelysin-1 activity was regulated by the vitamin D metabolites via PKC-dependent phosphorylation. To test this, membrane fractions (containing endogenous PKCα and ζ as well as stromelysin-1) were incubated in the presence of purified rat brain PKC and/ or recombinant human (rh) stromelysin-1 and [γ³²P]-ATP and anti-stromelysin-1 immunoprecipitates were analyzed by autoradiography and Western blots. Immuno-phospho-stromelysin-1 was localized to a 52-kDa band in the plasma membrane fraction only; no phosphorylation was observed in the matrix vesicle fraction. Selective inhibitors of PKC activity demonstrated that phosphorylation was inhibited by H7 and low concentrations of H8, but not by HA1004, indicating that PKC, not PKA, was responsible. Protein phosphatase 2A₁ (PP2A), a serine/threonine-specific phosphatase, selectively removed the radiolabel in a time-dependent manner, providing further support for a PKC-dependent phosphorylation mechanism. Incubation of resting zone cell plasma membranes with 24,25, but not 1,25, resulted in phosphorylation of stromelysin-1, demonstrating that the nongenomic effect was metabolite-specific. This suggests that this may be one mechanism by which vitamin D metabolites regulate stromelysin-1 activity and that PKC-dependent phosphorylation inhibits the metalloproteinase.