MURINE OSTEOBLAST INTERLEUKIN-4 RECEPTOR EXPRESSION - UP-REGULATION BY 1,25-DIHYDROXYVITAMIN-D(3)

The immune cytokine interleukin 4 has newly recognized effects on skel etal metabolism. While the interaction of many cells ultimately determ ines bone mass, we have examined the possibility that the osteoblast m ay be an IL-4 target in bone by characterizing IL-4 receptor (IL-4R) e xpression by MC3T3-El (MC3T3) murine osteoblastic cells. Based on I-12 5-IL-4 binding, MC3T3 cells express large numbers of IL-4 receptors (I -125-IL-4 Bmax = 3,000-7,500 sites/cell, I-125-IL-4 K = 13-40 pM) with an affinity similar to the IL-4 receptor expressed by an IL-4-respons ive T cell line. Monoclonal anti-IL-4R antibodies (Ml) blocked specifi c MC3T3 I-125-IL-4 binding and MC3T3 total cell RNA contained full-len gth IL-4R mRNA as detected by reverse transcription DNA amplification utilizing IL-4R primers and Northern blot analysis. Functionally, IL-4 treatment of MC3T3 cells resulted in increased cellular proliferation (10-20%) and inhibition of alkaline phosphatase levels (20-40%). Whil e parathyroid hormone (PTH) exposure did not influence IL-4R levels, v itamin D3 treatment augmented MC3T3 I-125-IL-4 binding, in a time-depe ndent manner, up to threefold after a 24 h exposure with a metabolite specificity indicating the involvement of the vitamin D receptor. Equi librium binding studies showed that the impact of 1,25 (OH)2 D3 on MC3 T3 I-125-IL-4 binding was due to an increased IL-4R Bmax. Cycloheximid e treatment inhibited 1,25 (OH)2 D3-induced IL-4R upregulation, sugges ting that protein synthesis was required. Furthermore, the steroid inc reased steady-state IL-4R mRNA levels in both a time- and concentratio n-dependent manner. The IL-4R message half-life was not altered by 1,2 5 (OH)2 D3, suggesting that increased IL-4R mRNA expression resulted f rom increased IL-4R gene transcription. Taken together, these findings raise the possibility that IL-4's influence on mineral metabolism cou ld be mediated by osteoblasts and that the effectiveness of this cytok ine may be influenced by vitamin D3'S impact on IL-4R expression. (C) 1993 Wiley-Liss, Inc.