Vitamin D(3) auto-/paracrine system in rat brain relating to vitamin D(3) status in experimental type 2 diabetes mellitus

Growing evidence suggests that vitamin D3 (D3, cholecalciferol) deficiency and impaired signaling of the hormonally active form of D3, 1α,25(OH)2D3 (1,25D3), through its cellular receptor (VDR) can be significant risk factors for the development of numerous multifactorial diseases, including diabetes. Our investigation was aimed at researching the D3 status in relation to the state of the D3 auto-/paracrine system in the brain and clarifying the effectiveness of the therapeutic use of D3 as a neuroprotective agent in experimental type 2 diabetes mellitus (T2DM). T2DM was induced in male Wistar rats by a combination of a high fat diet and a low dose of streptozotocin (25 mg/kg BW). Diabetic animals were treated with or without cholecalciferol (1,000 IU/kg BW, 30 days). The content of 25-hydroxyvitamin D3 (25D3) in blood serum and brain tissue was determined by ELISA. Analysis of mRNA expression of CYP24A1 and CYP27B1 genes was performed by RT-PCR. Protein levels of VDR, vitamin D3 binding protein (VDBP), CYP27B1 and CYP24A1 were investigated by Western blotting. A significant T2DM-associated decrease in the content of 25D3 in the blood serum was revealed, which correlated with a reduced content of this metabolite in the brain tissue. Impaired D3 status in animals with T2DM was accompanied by an increase in the levels of mRNA and protein of both 25D3 lα-hydroxylase (CYP27B1) and 1,25-hydroxyvitamin D3-24-hydroxylase (CYP24A1), which, respectively, provide local formation and degradation in the nervous tissue of the hormonally active form of D3 – 1,25D3. At the same time, a significant T2DM-induced down-regulation of the brain content of VDBP was shown. In addition, diabetes caused a slight increase in the protein expression of the VDR, through which the auto-/paracrine effects of 1,25D3 are realized in the brain. We have established a complete or partial corrective effect of cholecalciferol on D3 status, its bioavailability in the CNS and the level of protein expression of CYP27B1 and CYP24A1 in the brain of rats with T2DM. Abnormal D3 status in animals with T2DM was accompanied by compensatory changes in the expression of key components of the auto-/paracrine vitamin D3 system. Cholecalciferol was demonstrated to be partially effective in counteracting the impairments caused by T2DM. Keywords: 25-hydroxyvitamin D3, brain, type 2 diabetes, vitamin D3, vitamin D3 auto-/paracrine system