Cation-dependent vitamin D activation of human renal cortical guanylate cyclase

Combination treatment with progesterone and vitamin D hormone may be more effective than monotherapy for nervous system injury and disease

More than two decades of pre-clinical research and two recent clinical trials have shown that progesterone (PROG) and its metabolites exert beneficial effects after traumatic brain injury (TBI) through a number of metabolic and physiological pathways that can reduce damage in many different tissues and organ systems. Emerging data on 1,25-dihydroxyvitamin D(3) (VDH), itself a steroid hormone, have begun to provide evidence that, like PROG, it too is neuroprotective, although some of its actions may involve different pathways. Both agents have high safety profiles, act on many different injury and pathological mechanisms, and are clinically relevant, easy to administer, and inexpensive. Furthermore, vitamin D deficiency is prevalent in a large segment of the population, especially the elderly and institutionalized, and can significantly affect recovery after CNS injury. The combination of PROG and VDH in pre-clinical and clinical studies is a novel and compelling approach to TBI treatment.

Vitamin D

The vitamin D endocrine system plays an essential role in calcium homeostasis and bone metabolism, but research during the past two decades has revealed a diverse range of biological actions that include induction of cell differentiation, inhibition of cell growth, immunomodulation, and control of other hormonal systems. Vitamin D itself is a prohormone that is metabolically converted to the active metabolite, 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. This vitamin D hormone activates its cellular receptor (vitamin D receptor or VDR), which alters the transcription rates of target genes responsible for the biological responses. This review focuses on several recent developments that extend our understanding of the complexities of vitamin D metabolism and actions: the final step in the activation of vitamin D, conversion of 25-hydroxyvitamin D to 1,25(OH)(2)D in renal proximal tubules, is now known to involve facilitated uptake and intracellular delivery of the precursor to 1alpha-hydroxylase. Emerging evidence using mice lacking the VDR and/or 1alpha-hydroxylase indicates both 1,25(OH)(2)D(3)-dependent and -independent actions of the VDR as well as VDR-dependent and -independent actions of 1,25(OH)(2)D(3). Thus the vitamin D system may involve more than a single receptor and ligand. The presence of 1alpha-hydroxylase in many target cells indicates autocrine/paracrine functions for 1,25(OH)(2)D(3) in the control of cell proliferation and differentiation. This local production of 1,25(OH)(2)D(3) is dependent on circulating precursor levels, providing a potential explanation for the association of vitamin D deficiency with various cancers and autoimmune diseases.

Generation of inositol phosphates, diacylglycerol and calcium fluxes in myoblasts treated with 1,25-dihydroxyvitamin D3

We have examined the effects of the seco-steroid hormone 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on membrane phosphoinositide metabolism, protein kinase C (PKC) activation and influx of extracellular Ca2+ in chick-embryo muscle-cell (myoblast) cultures. At physiological concentrations, the hormone induces a rapid (15 s) and transient release of inositol triphosphate (InsP3) and diacylglycerol (DAG). InsP3 release is maximal at 60 s (80% above controls) and then declines. The effects of 1,25(OH)2D3 on InsP3 production exhibited specificity, as 25-hydroxy-vitamin D3 and 24,25-dihydroxy-vitamin D3 did not alter myoblast InsP3 levels. The stimulation of DAG is biphasic, with peaks at 60 s (+105%) and 5 min (+700%). The second phase of DAG release is not associated with changes in InsP3. 1,25(OH)2D3 induces a rapid (within 60 s) accumulation of InsP2, and its effect on InsP is delayed (120 s). The hormone rapidly activates myoblast PKC, with maximal translocation of activity from the cytosol to the cell membrane occurring at 60 s. Myoblast 45Ca uptake significantly increases within 30 s of exposure to 1,25(OH)2D3. The response is time- (0.5-10 min) and dose- (1 pM-10 nM) dependent. The effects of the hormone are mimicked by the Ca(2+)-channel agonist Bay K 8644 and are effectively suppressed by nifedipine and extracellular EGTA. The results suggest that the rapid non-genomic actions of 1,25(OH)2D3 in myoblasts involve second-messenger systems associated with the generation of InsP3 and DAG and regulation of Ca2+ fluxes through voltage-operated channels.

Receptor-mediated rapid action of 1 alpha,25-dihydroxycholecalciferol: increase of intracellular cGMP in human skin fibroblasts

The intracellular cGMP concentration in normal human cultured fibroblasts was increased 2- to 3-fold by 1 alpha,25-dihydroxycholecalciferol [1 alpha,25-(OH)2D3] in a dose-dependent manner between 0.01 nM and 1 microM. The response was detectable within 1 min, reached a maximum (225% +/- 8% of baseline) at 6-8 min, and was no longer detectable at 30 min. The half-maximal effect of 1 alpha,25-(OH)2D3 was at 1.8 nM, and 24,25-dihydroxycholecalciferol showed an estimated EC50 100-fold higher. 1 beta,25-Dihydroxycholecalciferol, 25-hydroxycholecalciferol, and cholecalciferol had no detectable effect. Human skin fibroblasts with three different types of 1 alpha,25-(OH)2D3 receptor defect did not respond to 1 alpha,25-(OH)2D3 exposure with cGMP increase; however, the same cells (like normal cells) responded to testosterone or sodium nitroprusside with a rapid rise of cGMP. We conclude that the rapid rise of cGMP in response to calciferols shows an EC50 for 1 alpha,25-(OH)2D3, a cholecalciferol analog specificity, and a cell line dependency that are all suggestive of mediation through a specific 1 alpha,25-(OH)2D3 receptor.

B complex vitamins activate rat guanylate cyclase and increase cyclic GMP levels

Since B complex vitamins and the intracellular messenger cyclic guanosine monophosphate (GMP) have similar effects of promoting growth, DNA and protein synthesis, the present investigation was designed to determine if the B complex vitamins' mechanism of action might involve cyclic GMP. All of the B complex vitamins increased rat cyclic GMP tissue levels. The cause of these increased cyclic GMP levels was activation of the enzyme guanylate cyclase [E.C.4.6.1.2.] which was increased significantly (P less than 0.001) in a variety of tissues at the l nmol l-1 concentration of these vitamins. The maximal activation of this enzyme required the presence of manganese ion. The present investigation suggests that the guanylate cyclase-cyclic GMP system may play a role in the mechanism of action of B complex vitamins at the cellular level.