Single cell analysis of changes in cytosolic calcium induced by vitamin D3 metabolites in cultured rat mesangial cells

Vitamin D and the brain: Genomic and non-genomic actions

1,25(OH)₂D₃ (vitamin D) is well-recognized as a neurosteroid that modulates multiple brain functions. A growing body of evidence indicates that vitamin D plays a pivotal role in brain development, neurotransmission, neuroprotection and immunomodulation. However, the precise molecular mechanisms by which vitamin D exerts these functions in the brain are still unclear. Vitamin D signalling occurs via the vitamin D receptor (VDR), a zinc-finger protein in the nuclear receptor superfamily. Like other nuclear steroids, vitamin D has both genomic and non-genomic actions. The transcriptional activity of vitamin D occurs via the nuclear VDR. Its faster, non-genomic actions can occur when the VDR is distributed outside the nucleus. The VDR is present in the developing and adult brain where it mediates the effects of vitamin D on brain development and function. The purpose of this review is to summarise the in vitro and in vivo work that has been conducted to characterise the genomic and non-genomic actions of vitamin D in the brain. Additionally we link these processes to functional neurochemical and behavioural outcomes. Elucidation of the precise molecular mechanisms underpinning vitamin D signalling in the brain may prove useful in understanding the role this steroid plays in brain ontogeny and function.

Effects of 1,25(OH)2D3 in experimental mesangial proliferative nephritis in rats

BACKGROUND

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3], the active form of vitamin D3, is a potent immunomodulatory agent on several cell types such as monocytes and mesangial cells. Recruitment of inflammatory cells, as well as stimulation of resident cells and mesangial matrix accumulation are key features of various experimental and human glomerular diseases. Here we show that 1,25(OH)2D3 attenuates the morphologic and functional alterations in anti-Thy-1.1. nephritis, an experimental model of mesangial proliferative glomerulonephritis.

METHODS

The anti-Thy-1.1 group (group I) comprised 24 rats that at day 0 received 0.5 mL of saline containing 400 microg of monoclonal antibodies (mAb) anti-Thy-1.1 IgG. The anti-Thy-1.1 treated with 1,25(OH)2D3 group (group II) were 24 rats given 1,25(OH)2D3 at the dose of 25 ng/100 g body wt/day, from day -3 to day 14. At day 0, the rats received 400 microg of anti-Thy-1.1 monoclonal IgG. The control group (group III) comprised 12 rats injected with vehicle alone; the control group treated with 1,25(OH)2D3 (group IV)-12 rats were given 1,25(OH)2D3 as in group II without mAb administration. Proteinuria and urinary interleukin-6 excretion were measured daily. Blood urea nitrogen and creatinine, creatinine clearance, calcium, and phosphate were measured at days 0, 4, 7, and 14. In addition to conventional periodic acid-Schiff staining, binding of anti-Thy-1.1 IgG and C3b complement fraction, His48- and ED1-positive cells were studied by immunofluorescence. Mesangial proliferation was studied by the proliferating cell nuclear antigen (PCNA) technique. Apoptosis was evaluated by the TUNEL assay.

RESULTS

The anti-Thy-1.1 treated with 1,25(OH)2D3 group versus the anti-Thy-1.1 alone group showed a significant reduction in urinary protein (at day 7, 424 +/- 228 vs. 66 +/- 30 mg/mg urinary creatinine, P = 0.02) and interleukin-6 excretion (at day 3, 537 +/- 360 pg/mL vs. 110 +/- 34 pg/mg urinary creatinine, P = 0.015), reduced glomerular diameters (at day 7, 283 +/- 38 vs. 261 +/- 48 microm, P < 0.01), decreased neutrophil (at day 4, 20 +/- 12 His48-positive cells/glomerulus vs. 3.7 +/- 1.3 His48-positive cells/glomerulus, P < 0.001), and monocyte accumulation (day 7, 4.9 +/- 2.9 ED1-positive cells/glomerulus vs. 2.8 +/- 2.9 ED1-positive cells/glomerulus, P < 0.05), and attenuated glomerular cells proliferation (day 7, 13 +/- 3.2 PCNA-positive cells/glomerulus vs. 9.4 +/- 3 PCNA-positive cells/glomerulus, P < 0.01). Apoptosis showed a biphasic behavior with an early peak at day 4 in the anti-Thy-1.1 group (2.3 +/- 2.2 TUNEL-positive cells/glom) related to cellular lysis and a late peak at day 14 related to the recovery phase.

CONCLUSIONS

1,25(OH)2D3 can reduce glomerular hypercellularity, inflammatory infiltration in anti-Thy-1.1 nephritis, preserving the apoptotic response of the reparative phase.

Inhibitory effect of calcium channel blockers on human mesangial cell growth: evidence for actions independent of L-type Ca2+ channels

Calcium channel blockers (CCB) are known to affect the outcome of glomerulosclerosis in vivo and to suppress mesangial cell proliferation and cytokine production in vitro. It is uncertain, however, whether (i) human adult mesangial cells (HMC) express L-type Ca2+ channels and (ii) whether the effect of CCB on HMC is mediated by inhibition of L-type Ca2+ channels. In single cell preparations of HMC, the L-type Ca2+ channel agonist Bay K 8644 and K+-depolarization of the cell membrane caused a transient increase of cytosolic free Ca2+ ([Ca2+]i) in 60 to 80% of the cells. The CCB verapamil and nifedipine partially inhibited the effect of Bay K 8644 and K+-depolarization on [Ca2+]i. Binding experiments confirmed these functional studies by showing specific binding at the phenylalkylamine binding site of L-Type Ca2+ channels. Quiescent HMC were stimulated with fetal calf serum (FCS) or growth factors (platelet derived growth factor A/B, epidermal growth factor, angiotensin II, endothelin 1) in the presence of various concentrations (10(-10) to 10(-5) M) of different CCB: either (R)-verapamil, (S)-verapamil or the raceme of verapamil, and nifedipine or diltiazem, respectively. In addition, the enantiomers of devapamil were studied, because their action on the L-type Ca2+ channel is more stereoselective than that of the enantiomers of verapamil. At high concentrations (10(-6) to 10(-5) M) (R,S)-verapamil decreased cell numbers in cultures of quiescent HMC, increased LDH in the supernatant, and caused loss of trypan blue exclusion (cytotoxicity). At lower concentrations (R,S)-verapamil showed no cytotoxicity, but had two effects: (1.) concentration dependent (down to 10(-8) M) inhibition of indices of cell proliferation, that is, (i) stimulated (FCS or growth factor) 3H-thymidine incorporation and (ii) increment in cell number; and (2.) inhibition of indices of cell or matrix protein synthesis, that is, (i) stimulated 3H-methionine incorporation and (ii) 3H-proline incorporation. At equimolar concentrations the dihydropyridine nifedipine was equipotent with verapamil, whereas the benzothiazepine diltiazem was conspicuously less effective. Even at the lowest effective concentration (10(-8) M) comparison of (R)- and (S)-verapamil showed no significant difference between the enantiomer with weak or with strong effect on L-type Ca2+ channels, and this was true even when the more stereoselective enantiomers of devapamil were tested. These observations argue against the notion that effects of CCB result from specific interaction with L-type Ca2+ channels. The data are more consistent with the idea that interactions with targets other than L-type Ca2+ channels are involved.

Direct activation of protein kinase C by 1 alpha,25-dihydroxyvitamin D3

The key metabolite of vitamin D3, 1 alpha,25-dihydroxyvitamin D3 (1,25-D3), induces rapid cellular responses that constitute a so-called "non-genomic" response. This effect is distinguished from its "classic" genomic role in calcium homeostasis involving the nuclear 1,25-D3 receptor. Evidence is presented that protein kinase C (PKC) is directly activated by 1,25-D3 at physiological concentrations (EC50 = 16 +/- 1 nM). The effect was demonstrable with single PKC-alpha, -gamma, and -epsilon isoform preparations, assayed in a system containing only purified enzyme, substrate, co-factors, and lipid vesicles, from which it is inferred that a direct interaction with the enzyme is involved. The finding that calcium-independent isoform PKC-epsilon was also activated by 1,25-D3 shows that the calcium binding C2 domain is not required. The level of 1,25-D3-induced activation, paired with either diacylglycerol or 4 beta-12-O-tetradecanoylphorbol-13-acetate, was greater than that achievable by any individual activator alone, each at a saturating concentration, a result that implies two distinct activator sites on the PKC molecule. Phosphatidylethanolamine present in the lipid vesicles potentiated 4 beta-12-O-tetradecanoylphorbol-13-acetate- and diacylglycerol-induced PKC activities, whereas 1,25-D3-induced activity decreased, consistent with 1,25-D3-activated PKC possessing a distinct conformation. The results suggest that PKC is a "membrane-bound receptor" for 1,25-D3 and that it could be important in the control of non-genomic cellular responses to the hormone.

Non-genomic signal transduction pathway of vitamin D in muscle

The secosteroid hormone 1,25(OH)2-vitamin D3 rapidly activates voltage-dependent Ca2+ channels of the L-type in skeletal and cardiac muscle cells by a non-genomic mechanism which involves guanine nucleotide binding (G) protein-medicated stimulation of the adenylate cyclase/cAMP/protein kinase A messenger system. Modifications in calmodulin intracellular distribution induced by PKA-dependent membrane protein phosphorylation may participate in the fast regulation of muscle Ca2+ influx by 1,25(OH)2D3. The protein kinase C pathway also plays a role modulating 1,25(OH)2D3 signal transduction in muscle by cross-talk with the PKA system. The hormone sequentially activates phospholipases C and D providing diacylglycerol for PKC activation and inositol triphosphate for intracellular Ca2+ mobilization. In addition, 1,25(OH)2D3 rapidly stimulates phospholipase A2 generating arachidonic acid for the eicosanoid pathway. Specificity of hormone effects suggests that binding to a muscle membrane-bound receptor mediates these events.

Rapid non-genomic actions of progesterone stimulate Ca2+ influx and the acrosome reaction in human sperm

This review summarizes some recent findings in human sperm which show that progesterone and 17 alpha hydroxyprogesterone are able rapidly (within seconds) to elevate [Ca2+]i and elicit the acrosome reaction (AR) via a non-genomic cell surface receptor. Progesterone promotes a transient elevation in [Ca2+]i which is blocked by extracellular La3+ and Ni2+ and removal of extracellular Ca2+ following chelation with EGTA. Some studies suggest that polyamines, trypsin-like proteases, and progesterone receptor aggregation are involved in progesterone-induced Ca2+ influx and AR. The receptor is not stimulated by the potent synthetic progestigins (e.g. promegestone, norethynodrel, megestrol acetate, cyproterone acetate) and is weakly antagonized by the genomic anti-progestins RU 486 and ZK 98.299. The sedative-hypnotic 3 alpha hydroxyl A-ring reduced pregnane steroids, which are powerful activators of the GABAA Cl- channel, are weak activators of Ca2+ influx and the AR. These data suggest that human sperm have a cell surface steroid receptor which is unlike the genomic progesterone receptor and the GABAA Cl- channel steroid receptor.

Potassium currents and effects of vitamin D-3 metabolites and cyclic GMP in rat osteoblastic cells

A K+ current (IK1), activated by depolarization above -20 mV, showing voltage-dependent inactivation within a few seconds and reduced by 40% by 1 mM TEA, was observed in all cells. In a few cells, we also observed a progressive K(+)-current increase during cell dialysis. The developing current (IK2) was not sensitive to 1 mM TEA and did not inactivate. It was detectable over the whole voltage range and slowly increased during 10 s depolarizations. 1,25-(OH)2D3 and 24,25-(OH)2D3 did not affect IK1, but induced a small K(+)-current increase in some cells showing no IK2. This effect was not mimicked by cyclic GMP analogs which, on the contrary, induced a K(+)-current decrease. 24,25-(OH)2D3 (even at 10(-11)M, but not 1,25-(OH)2D3, strongly reduced IK2. The results further document the diversity of voltage-gated currents of osteoblastic cells, confirm the existence of immediate effects of vitamin D-3 metabolites, which are independent of classical 1,25-(OH)2D3 receptors.