1,25-(OH)(2)D-3 modulates growth plate chondrocytes via membrane receptor-mediated protein kinase C by a mechanism that involves changes in phospholipid metabolism and the action of arachidonic acid and PGE(2)

1,25-(OH)(2)D-3 (1,25) exerts its effects on growth plate chondrocytes thro ugh classical vitamin D (VDR) receptor-dependent mechanisms, resulting in m ineralization of the extracellular matrix. Recent studies have shown that m embrane-mediated mechanisms are involved as well. 1,25 targets cells in the prehypertrophic and upper hypertrophic zones of the costochondral cartilag e growth plate (GC cells), resulting in increased specific activity of alka line phosphatase (ALP), phospholipase A(2) (PLA(2)), and matrix metalloprot einases (MMPs). At the cellular level, 1,25 action results in rapid changes in arachidonic acid (AA) release and re-incorporation, alterations in memb rane fluidity and Ca ion flux, and increased prostaglandin E-1, and E-2, (P GE(2)) production. Protein kinase C (PKC) is activated in a phospholipase C (PLC) dependent-mechanism, due in part to the increased production of diac ylglycerol (DAG). In addition, AA acts directly on the cell to increase PKC specific activity. AA also provides a substrate for cyclooxygenase (COX), resulting in PGE(2) production. 1,25 mediates its effects through COX-1, th e constitutive enzyme, but not COX-2, the inducible enzyme. Time course stu dies using specific inhibitors of COX-1 show that AA stimulates PKC activit y and PKC then stimulates PGE(2) production. PGE(2) acts as a mediator of 1 ,25 action on the cells, also stimulating PKC activity. The rapid effects o f 1,25 on PKC are nongenomic, occurring within 3 min and reaching maximal a ctivation by 9 min. It promotes translocation of PKC to the plasma membrane . When 1,25 is incubated directly with isolated plasma membranes, PKC alpha is stimulated although PKC zeta is also present. In contrast, when isolate d matrix vesicles (MVs) are incubated with 1,25, PKC zeta is inhibited and PKC alpha is unaffected. These membrane-mediated effects are due to the pre sence of a specific membrane vitamin D receptor (mVDR) that is distinct fro m the classical cytosolic VDR. Studies using 1,25 analogs with reduced bind ing affinity for the classical VDR, confirm that rapid activation of PKC by 1,25 is not VDR dependent. The membrane-mediated effects of 1,25 are criti cal to the regulation of events in the extracellular matrix produced by the chondrocytes. MVs are extracellular organelles associated with maturation of the matrix, preparing it for mineralization. MV composition is under gen omic control, involving VDR-mechanisms. In the matrix, no new gene expressi on or protein synthesis can occur, however. Differential distribution of PK C isoforms and their nongenomic regulation by 1,25 is one way for the chond rocyte to control events at sites distant from the cell. GC cells contain 1 alpha-hydroxylase and produce 1,25; this production is regulated by 1,25, 24,25, and dexamethasone. 1,25 stimulates MMPs in the MVs, resulting in inc reased proteoglycan degradation in mineralization gels, and increased activ ation of latent transforming growth factor-beta 1 (TGF-beta 1). (C) 1999 El sevier Science Inc. All rights reserved.