1,25-DIHYDROXYVITAMIN D-3 REGULATES THE SYNTHESIS OF NERVE GROWTH-FACTOR IN PRIMARY CULTURES OF GLIAL-CELLS

The effect of 1,25-dihydroxyvitamin D-3 (1,25-(OH)(2) D-3) On nerve growth factor (NGF) synthesis was investigated in primary cultures of astrocytes prepared from brain of neonatal rats. 1,25-(OH)(2) D-3 elicited a dose-dependent increase of NGF mRNA with a maximal effect at 10(-7) M, which persisted for at least 48 h. Northern blot analysis revealed an expression of the vitamin D-3 receptor (VDR) gene in primary glial cells. Treatment of cells with 1,25-(OH)(2) D-3 led to an increase in the VDR mRNA levels. Similar results were obtained in C6 glioma cells. Exposure of primary glial cells to 10(-8) M 1,25-(OH)(2) D-3 caused only a 2-fold increase of the levels of cell-secreted NGF after 3 days of treatment. However, a 5-fold increase was observed three days after a second addition of vitamin D-3. Likewise, a pretreatment with lower doses of hormone such as 10-(10) M or 10(-9) M enhanced the responsiveness of the cells to a 24 h treatment with 10(-8) M hormone. It appears, therefore, that the duration of the treatment influences the level of synthesis of NGF, possibly as a consequence of the increase of the VDR gene expression. The specificity of 1,25-(OH)(2) D-3 is supported by the fact that a concentration of 10(-7) M of an another vitamin D-3 metabolite, 24,25-(OH)(2) D-3, had no effect on NGF synthesis. Several lines of evidence indicate that astrocytes constitute the major cell type responsive to 1,25-(OH)(2) D-3 in primary cultures of glial cells. Firstly, the removal of microglial cells or oligodendrocytes did not modify the time-course and amplitude of the response to 1,25-(OH)(2) D-3. Secondly, 1,25-(OH)(2) D-3 had no effect on the synthesis of NGF in meninge-derived fibroblasts. Thirdly, an 1,25-(OH)(2) D-3-dependent increase of NGF mRNA was also found in C6 glioma cells. In conclusion, the results suggest that 1,25-(OH)(2) D-3 may control the synthesis of NGF in the central nervous system, and strongly support the involvement of 1,25-(OH)(2) D-3 as a mediator of astrocytic plasticity.