The 25(OH)D(3)/1alpha,25(OH)(2)D(3)-24R-hydroxylase: a catabolic or biosynthetic enzyme?

The kidney is the major source of the circulating dihydroxylated metabolites of vitamin D, 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] and 24R,25-dihydroxyvitamin D(3) [24R,25(OH)(2)D(3)]. The enzymes which catalyze the production of these two dihydroxylated vitamin D metabolites are the 25(OH)D(3)-1alpha-hydroxylase (1alpha-hydroxylase) and -24R-hydroxylase (24R-hydroxylase), respectively. While there is no controversy regarding the fundamental importance of the 1alpha-hydroxylase in the production of the steroid hormone 1alpha,25(OH)(2)D(3), the biologic significance of the 24R-hydroxylase has been the subject of ongoing discussion. Some hold that it is strictly catabolic, leading to side chain oxidation and cleavage of 25-hydroxylated vitamin D sterols, and others hold that it plays a biosynthetic role in the production of 24R,25(OH)(2)D(3) which has biologic activities distinct from those of 1alpha,25(OH)(2)D(3). The 24R-hydroxylase has properties in common with other multicatalytic steroidogenic enzymes: (1) the enzyme carries out multiple oxidative and carbon-carbon bond cleavages; (2) it utilizes two natural substrates; (3) its regulation varies depending on the cell or tissue in which it occurs. The purpose of this paper is to review the current literature relevant to the characteristics of the 24R-hydroxylase and its regulation in the context of other multicatalytic steroid hydroxylases in order to provide a perspective regarding its possible function as both a catabolic and activating enzyme in the vitamin D endocrine system.

Different shapes of the steroid hormone 1alpha,25(OH)(2)-vitamin D(3) act as agonists for two different receptors in the vitamin D endocrine system to mediate genomic and rapid responses

Vitamin D(3) produces biologic responses as a consequence of its metabolism into 1alpha,25(OH)(2)-vitamin D(3) [1alpha,25(OH)(2)D(3)] and 24R,25(OH)(2)-vitamin D(3). The metabolic production of these two seco steroids and their generation of the plethora of biologic actions that are attributable to the parent vitamin D(3) are orchestrated via the integrated operation of the vitamin D endocrine system. This system is very similar in its organization to that of classic endocrine systems and is characterized by an endocrine gland (the kidney, the source of the two steroid hormones), target cells which possess receptors for the steroid hormones, and a feed-back loop involving changes in serum Ca(2+) that alter the secretion of parathyroid hormone (a stimulator of the renal 1-hydroxylase) which modulates the output by the kidney of the steroid hormones. There are, however, at least two unique aspects to the vitamin D endocrine system. (a) The chemical structures of vitamin D and its steroid hormones dictate that these be highly conformationally flexible molecules present a wide variety of shapes to their biologic environments. (b) It is now believed that 1alpha,25(OH)(2)D(3) produces biologic responses through two distinct receptors which recognize totally different shapes of the conformationally flexible 1alpha,25(OH)(2)D(3). Thus, the classic actions of 1alpha,25(OH)(2)D(3) to regulate gene transcription occur as a consequence of the stereospecific interaction of a modified 6-s-trans bowl-shape of 1alpha,25(OH)(2)D(3) with its nuclear receptor (VDR(nuc)). The ability of 1alpha,25(OH)(2)D(3) to generate a variety of rapid (seconds to minutes) biologic responses (opening of chloride channels, activation of PKC and MAP kinases) requires a planar 6-s-cis ligand shape which is recognized by a putative plasma membrane receptor (VDR(mem)) to initiate appropriate signal transduction pathways. This report summarizes the evidence for the specificity of different ligand shapes and the operation of the two receptor families for 1alpha,25(OH)(2)D(3).

Genomic cloning, structure, and regulatory elements of the 1 alpha, 25(OH)2D3 down-regulated gene for cyclic AMP-dependent protein kinase inhibitor

The cyclic AMP-dependent protein kinase inhibitor (PKI) mRNA and protein are negatively and tissue-specifically regulated in the kidney by 1 alpha, 25(OH)2D3. A 17-kb PKI clone, isolated from a chick genomic library, revealed that the PKI gene consists of two exons separated by a 4.5-kb intron. A 411-bp upstream region (constituting 93 bp upstream and 318 bp downstream from the transcriptional start site) containing a putative negative VDRE (nVDRE) fused to the luciferase gene was used for transient transfections of primary cultures of chick kidney cells. Luciferase activity was significantly down-regulated in response to 1 alpha, 25(OH)2D3. This result suggests that the promoter region containing the putative nVDRE plays a pivotal role in the negative regulation of PKI gene transcription.

Inhibitors of 25-hydroxyvitamin D3-1alpha-hydroxylase: thiavitamin D analogs and biological evaluation

Six A-ring analogs of 1alpha,25-dihydroxyvitamin D3 (1, 1alpha,25-(OH)2-D3) 3-deoxy-3-thia-1alpha,25-(OH)2-D3 (3), 3-deoxy-3-thia-1alpha,25-(OH)2-D3-3alpha-oxide (6), 3-deoxy-3-thia-1alpha,25-(OH)2-D3-3beta-oxide (7) and the 5,6-trans counterparts 5, 8, and 9, respectively--were tested for their ability to inhibit 25-hydroxy-D3-1alpha-hydroxylase (1-OH-ase) in vitro in mitochondria isolated from kidneys of vitamin D deficient chicks. The six analogs were also evaluated in terms of their ability to bind to the chicken intestinal nuclear receptor (VDR) in comparison to the natural hormone 1alpha,25-(OH)2-D3. Analog 7 is not only the best inhibitor of the 1-OH-ase but it also binds effectively to the chick intestinal receptor. It is established that vitamin D analogs must have a 1alpha oxygen group for effective inhibition of the 1-OH-ase. This functional group is also needed for effective binding to the chick intestinal VDR.

Tissue-specific expression and regulation by 1,25(OH)2D3 of chick protein kinase inhibitor (PKI) mRNA

The heat-stable protein kinase inhibitor (PKI) protein is a specific and potent competitive inhibitor of the catalytic subunit of cAMP-dependent protein kinase (PKA). Previously, it has been shown that vitamin D status affects chick kidney PKI activity: a 5- to 10-fold increase in PKI activity was observed in kidneys of chronically vitamin D-deficient chicks and treatment with 1,25-dihydroxyvitamin D3 (1,25[OH]2D3) in cultured kidney cells resulted in a 95% decrease in PKI activity. The authors have recently cloned the cDNA for chick kidney PKI and have used the coding sequence to study the regulation of PKI mRNA. Northern analysis showed the expression of two PKI messages, which are 2.7 and 3.3 kb in size. These mRNAs are expressed in brain, muscle, testis, and kidney, but not in pancreas, liver, or intestine. PKI mRNA steady-state levels are downregulated by 47% in kidneys from vitamin D-replete chicks as compared to vitamin D-deficient chicks. PKI mRNA levels in brain, muscle, and testis are not affected by vitamin D status. Treatment of primary chick kidney cultures treated with 10(-7) M 1,25(OH)2D3 for 24h resulted in a 20-30% decrease in PKI mRNA. 1,25(OH)2D3 treatment does not affect the stability of PKI mRNA as determined by treatment of cell cultures with actinomycin D. This study shows that 1,25(OH)2D3 directly and tissue-specifically downregulates PKI mRNA in the chick kidney.

Tissue-specific regulation by vitamin D3 of a novel protein containing ankyrin-like repeats

Vitamin D3 is the precursor of the steroid hormone 1,25-dihydroxyvitamin D3 which is involved in the regulation of calcium metabolism, growth and differentiation. We used differential display of mRNA populations from kidney and intestine of vitamin D3-deficient and -replete chicks to determine the steady-state abundance of approximately 5000 mRNAs. One of these sequences, whose differential expression in kidney and down-regulation by vitamin D3 was confirmed by Northern analysis, was used to screen a cDNA library from vitamin D3-deficient chick kidney in order to obtain a full length cDNA. Subcloning and sequencing revealed that this cDNA encodes a novel protein containing ankyrin-like repeats and a C-terminal Fe-S binding region signature. The encoded protein consists of 617 amino acids and contains two sets of four ankyrin-like repeats separated by 146 amino acids. This motif consists of approximately 33 amino acids containing a highly conserved central hydrophobic alpha helix and is abundant in a wide variety of proteins, particularly those participating in the protein-protein or protein-membrane interactions involved in signal transduction, regulation of the cell cycle and control of transcription. Outside of the ankyrin-like domains, no homologies with other proteins in existing data bases were found. Our results have revealed a novel protein containing ankyrin-like repeats tissue-specifically down-regulated by vitamin D3 in the kidney.

Chick kidney ferredoxin: complementary DNA cloning and vitamin D effects on mRNA levels

Vertebrate ferredoxin is non-heme iron-sulfur protein found in steroideogenic tissues that serves as an electron shuttle in mitochondrial mixed function oxidase systems such as the 25-hydroxyvitamin D3-1 alpha-hydroxylase. A 2530-bp chick kidney ferredoxin cDNA was cloned, and the association between ferredoxin mRNA levels and the regulation of 1 alpha-hydroxylase activity by vitamin D status was examined. The cDNA sequence indicates that the chick kidney mitochondrial mixed function oxidases use the same ferredoxin as do those in the chick testis and that the chick ferredoxin shares greater than 92% amino acid homology with mammalian ferredoxins. Southern blot analysis of genomic DNA indicates that there is a single copy of the ferredoxin gene present in the chick genome. Three species of mRNA, 1.8, 3.5 and 5.5 kb, were identified by Northern analysis. Slot blot analysis of poly A+ RNA from kidneys of vitamin D-deficient or replete chicks indicates a 40% induction of ferredoxin message levels in the vitamin D-deficient chick kidney. This suggests that gene regulation of ferredoxin may be part of the mechanism of regulation for 25-hydroxyvitamin D3-1 alpha-hydroxylase activity in the chick kidney.

Tissue-specific regulation by vitamin D status of nuclear and mitochondrial gene expression in kidney and intestine

Vitamin D is responsible, through the actions of its metabolite, 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25-(OH)2D3], for the generation of a wide array of biological responses, particularly in the intestine, kidney, and bone. 1 alpha,25-(OH)2D3 is known to interact with its nuclear receptor to mediate the regulation of gene transcription. Although many genes and gene products have been shown to be regulated by 1 alpha,25-(OH)2D3 (e.g. calbindin-D28K in the intestine and kidney; collagen, osteocalcin,and osteopontin in bone), their recognition has been largely the result of empirical testing. In this report we have used subtractive hybridization analysis of complementary DNA libraries prepared from messenger RNA (mRNA) isolated from the intestine and kidney of vitamin D-replete or vitamin D-deficient chicks to identify genes for novel proteins whose steady state mRNA levels are regulated by dietary vitamin D status. In the kidney we observed the down-regulated expression of at least seven mitochondrially encoded transcripts and the up-regulated expression of five nuclear encoded genes, two of which are metallothionein and the beta-subunit of aldolase. In the intestine, six mitochondrially encoded transcripts are up-regulated, and seven nuclear encoded transcripts were either up- or down-regulated. Thus, in addition to identifying new nuclear encoded genes whose mRNAs are regulated by vitamin D status, our approach has demonstrated the tissue-specific regulation of mitochondrial gene expression in the intestine and kidney.

Cloning and sequencing of the cDNA encoding the avian kidney cAMP-dependent protein kinase inhibitor protein

The endogenous inhibitor of cAMP-dependent protein kinase (PKA) is down-regulated in the kidneys from vitamin-D-replete chicks as compared to vitamin-D-deficient chicks. Screening of a vitamin-D-deficient chick kidney library resulted in the isolation of a 450-bp cDNA clone encoding the 76-amino acid (aa) protein kinase inhibitor (PKI). The deduced aa sequence of avian PKI shares 80 and 41% identity with the mammalian PKI alpha and PKI beta 1 isoforms, respectively. The chick and mammalian PKI contain conserved N-terminal sequences, including the pseudo-substrate site (18GRRNA22), which are required for potent inhibition of the catalytic subunit of PKA. Chick kidney PKI contains ten unique aa in the C-terminal portion of the protein that are not shared with the mammalian PKI alpha or beta isoforms.

Regulation of the ferredoxin component of renal hydroxylases at transcriptional and postranslational levels and of the protein inhibitor of cyclic AMP-dependent kinase

We have studied two proteins potentially involved in the regulation of the 25-OH-D-1-hydroxylase, which is located in the renal mitochondria and which is responsible for the production of the steroid hormone 1,25(OH)2D3. The endogenous inhibitor of cyclic AMP-dependent protein kinase, PKI, is down regulated by 1,25(OH)2D3. Having cloned and sequenced PKI cDNA, we studied its message levels and found them to be regulated by 1,25(OH)2D3 tissue specifically in the kidney and in kidney cell culture. In other experiments we over expressed the ferredoxin component of the 1-hydroxylase and found it to be physically and chemically indistinguishable from those of classic steroidogenic tissues. The mRNA encoding the ferredoxin component is up-regulated by chronic vitamin D deficiency, which at the same time leads to sustained elevation in 1-hydroxylase activity; no short term effect of 1,25(OH)2D3 on ferredoxin mRNA in kidney cell culture could be demonstrated. Finally, there was an association between decreased phosphorylation of ferredoxin and decreased 1-hydroxylase activity brought about by treatment of cultured kidney cells with TPA. Control of the renal signaling events involved in the production of 1,25(OH)2D3 remains a fruitful area of investigation in the field of the metabolism and actions of vitamin D and its metabolites.

The phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate stimulates the dephosphorylation of mitochondrial ferredoxin in cultured chick kidney cells

The tumor promoting phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA), presumably through activation of protein kinase C, decreases the production of 1 alpha, 25-dihydroxyvitamin D3 [1,25(OH)2D3] and increases that of 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3] by primary cultures of chick kidney cells. We have previously shown that the regulation of the cellular output of 1,25(OH)2D3 and 24,25(OH)2D3 by PTH and 1,25(OH)2D3 can be quantitatively accounted for by altered hydroxylase activities within isolated mitochondria. In the present paper, we examined the effects of TPA and 1-oleoyl-2-acetyl-glycerol (OAG) on the state of mitochondrial protein phosphorylation and on 25-hydroxyvitamin D3 [25(OH)D3] metabolism. There was a good correlation between 25(OH)D3- 1 alpha- and 24-hydroxylase activities in mitochondria isolated from cells pretreated with either TPA or OAG and the pattern of 1- and 24-hydroxylation of 25(OH)D3. The most notable change in protein phosphorylation in the molecular mass range of 10-60 kilodaltons (kDa) was a dramatic decrease in the phosphorylation of a 12.5-kDa mitochondrial matrix protein after treatment of kidney cells with TPA or OAG. The amino acid composition of the 12.5-kDa protein was similar to bovine and human ferredoxins and it comigrated with bovine and human ferredoxins in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 12.5-kDa phosphoprotein was immunoprecipitated specifically by an antipeptide polyclonal antibody for chick ferredoxin. The dephosphorylation of ferredoxin in response to TPA was both rapid and transient, with the phosphate content of the 12.5-kDa protein reduced by 70% after a 5-min exposure and returning to control levels by 20 min. A similar transience was observed with regard to the rapid effects of TPA on 1 alpha-hydroxylase activity, again showing maximal inhibition at 5 min. The results of our studies are consistent with the idea that ferredoxin phosphorylation plays a role in the regulation of steroid hydroxylation.

Overexpression in Escherichia coli and affinity purification of chick kidney ferredoxin

Vertebrate ferredoxins are 12-14-kDa iron-sulfur proteins, some of which transfer electrons to mitochondrial cytochrome P450s. The function of many of these cytochrome P450s is to catalyze stereospecific hydroxylation of endogenous steroids. As part of our interest in the kidney mitochondrial 1 alpha-hydroxylation of 25-hydroxyvitamin D3, we have constructed an expression plasmid coding for a fusion protein containing the chick kidney ferredoxin. We subcloned chick kidney ferredoxin cDNA, obtained from our vitamin D-deficient chick kidney library by polymerase chain reaction (Brandt, M. E., Gabrik, A. H., and Vickery, L. E. (1991) Gene (Amst.) 97, 113-117) into Qiagen's pQE9, which contains an N-terminal 6xHis tag (peptide sequence for 6 adjacent histidines present in the recombinant proteins). The coding sequence was preceded by a factor Xa cleavage site. The resulting plasmid, pQTcFdx, was overexpressed in Escherichia coli, and the soluble fusion protein was purified from the cell lysate in one step by Ni(II)-nitrilotriacetic acid-agarose chromatography. We obtained 7-10 mg of greater than 99% homogeneous fusion protein from a 1-liter culture and 4-6 mg of mature ferredoxin cleaved by factor Xa. The fusion protein possessed an absorption spectrum and an electron paramagnetic resonance spectrum quantitatively indistinguishable from those published for ferredoxin purified from adrenal glands and placenta or expressed in E. coli with another vector. The fusion protein was active in supporting the 1 alpha-hydroxylation of 25-hydroxyvitamin D3 in a reconstitution assay of a solubilized, partially purified preparation of cytochrome P450 from vitamin D-deficient chick kidney. We conclude that the procedure described here is an efficient way to produce and purify vertebrate ferredoxin; the [2Fe-2S] cofactor is assembled in vivo and effectively incorporated into the fusion protein in E. coli; slight alterations at the N terminus do not alter incorporation of the [2Fe-2S] cofactor or the biological activity of ferredoxin, and post-translational modifications, such as phosphorylation, are not an absolute requirement for ferredoxin electron transporting activity. The recombinant ferredoxin can be used for physical studies and other structure-function studies.

Vitamin D hydroxylases

There are three mixed function oxidases which catalyze hydroxylations of vitamin D and its derivatives. These include the hepatic mitochondrial or microsomal vitamin D3-25-hydroxylase and the two renal mitochondrial enzymes which further hydroxylate 25-hydroxyvitamin-D3 (25-OH-D3) to form 24R,25-dihydroxyvitamin D3 (24,25(OH)2D3) and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], the primary steroid hormonal derivative of vitamin D3. All three enzymes are cytochrome P450 dependent. The two renal mitochondrial enzymes are regulated, usually in a reciprocal fashion. The intracellular signalling systems involved in this regulation include 1,25(OH)2D3 itself and both protein kinases A and C. Recent progress has been made in the purification and cloning of the vitamin D3-25-hydroxylase and the 25-OH-D3-24-hydroxylase. When the 25-OH-D3-1-hydroxylase is purified and cloned, efforts which have thus far been frustrated by its low abundance, fertile new ground for the study of the regulation of vitamin D metabolism at the molecular level will be opened up.

The cellular and molecular regulation of 1,25(OH)2D3 production

The synthesis of 1,25(OH)2D3 is a critical control point in the regulation of calcium metabolism, and possibly in the growth and differentiation of a number of cell types. This paper reviews our current understanding of the regulation of this process at the cellular and molecular levels, with the emphasis on the mechanisms of feedback control 1,25(OH)2D3 itself, control of parathyroid hormone, the roles of cyclic AMP dependent protein kinase and protein kinase C, and the interaction between the various intracellular regulators of 1,25(OH)2D3 production.

Arocalciferols: synthesis and biological evaluation of aromatic side-chain analogues of 1 alpha,25-dihydroxyvitamin D3(1a)

Aromatic side-chain analogues (arocalciferols 6-9) of the steroid hormone 1 alpha,25-dihydroxyvitamin D3 (1) were synthesized and biologically evaluated. The analogues were prepared by coupling the vitamin D A-ring enyne 14 with the appropriate enol triflate of a modified CD steroid fragment of the type 22. The resulting dienyne 23 was then transformed in three steps to the vitamin D analogues 6-9. Biological evaluation of these analogues have provided information concerning side-chain topographical effects on in vivo and in vitro activity.

Effect of three A-ring analogs of 1 alpha,25-dihydroxyvitamin D3 on 25-OH-D3-1 alpha-hydroxylase in isolated mitochondria and on 25-hydroxyvitamin D3 metabolism in cultured kidney cells

Three A-ring analogs of 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3)--2-nor-1,3-seco-1,25(OH)2D3 (2-nor analog), 2-oxa-3-deoxy-25-OH-D3 (2-oxa analog), and A-homo-3-deoxy-3,3-dimethyl-2,4-dioxa-25-OH-D3 (A-homo analog)--were tested for their ability to inhibit 25-OH-D3-1 alpha-hydroxylase (1 alpha-hydroxylase) in isolated mitochondria and to alter 25-OH-D3 metabolism in cultured chick kidney cells. The 2-nor and 2-oxa analogs were relatively potent (Kis of 60 and 30 nM, respectively, compared with 170 nM for 1,25(OH)2D3), whereas the A-homo analog was completely ineffective in inhibiting 1 alpha-hydroxylase activity. In contrast, all three analogs were able to repress 1 alpha-hydroxylase and induce 24-hydroxylase activity in cultured chick kidney cells, suggesting that this process is not one of direct action in the mitochondria, but is more likely to be a receptor-mediated one.

Structure-function studies on analogues of 1 alpha,25-dihydroxyvitamin D3: differential effects on leukemic cell growth, differentiation, and intestinal calcium absorption

The hormonally active form of vitamin D, 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3], is an efficient stimulator of intestinal calcium absorption (ICA) and bone calcium mobilization (BCM) in humans and experimental animals and, as well, has been shown to be effective in inducing differentiation and inhibiting proliferation of leukemia cells. Thus, it has been proposed that analogues of 1,25(OH)2D3 could be synthesized which might allow for separation of biological functions, i.e., promote a differentiation of leukemia cells without a significant stimulation of ICA or BCM, both biological effects which can cause hypercalcemia in humans. Here we report the results of an evaluation of four analogues of the previously studied (Zhou et al., Blood, 74:82-92, 1989) 1 alpha,25-dihydroxy-16-ene-23-yne-vitamin D3 [1,25(OH)2-16-ene-23-yne-D3]; these analogues allowed evaluation of the consequences of (a) the presence or absence of six deuterium atoms on carbons 26 and 27 of the side chain and (b) the deletion or substitution by a fluorine atom of the 1 alpha-hydroxyl group on the A-ring. The 1,25(OH)2-16-ene-23-yne-D3 analogue was found to be 7-fold more potent than the parent 1,25(OH)2D3 with respect to (a) inhibition of clonal proliferation of HL-60 cells as well as (b) induction of differentiation of HL-60 promyelocytes. Variants of this analogue which possessed the six deuterium atoms on carbons 26 and 27 were slightly less active than the 1,25(OH)2-16-ene-yne-D3. However, replacement of the 1 alpha-hydroxyl group by a 1-fluoro group, or the absence of the 1-hydroxyl group, resulted in analogues that were somewhat less effective than the parent 1,25(OH)2D3 in achieving these biological responses but more potent as inhibitors of the renal mitochondrial 25-OH-D3-1 alpha-hydroxylase, the site of endogenous production of 1,25(OH)2D3. ICA and BCM were assessed in vivo in vitamin D-deficient chickens, and each of the analogues was markedly less potent than the standard 1,25(OH)2D3. The analogue 1,25(OH)2-16-ene-23-yne-D3 had 2% of the ICA and 3% of the BCM activity of the parent 1,25(OH)2D3. Absence of the 1 alpha-hydroxyl group or substitution of the 1-fluoro group for the 1-hydroxyl group significantly diminished both the ICA and BCM activity in comparison to 1,25(OH)2-16-ene-23-yne-D3. Receptor binding studies indicated that 1,25(OH)2-16-ene-23-yne-D3 competed about 75% as effectively as 1,25(OH)2D3 for 1,25(OH)2D3 receptors present in both chick intestinal cells and HL-60 cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitation of proteins bound to polyvinylidene difluoride membranes by elution of coomassie brilliant blue R-250

A rapid and simple method for the quantitation of stained proteins bound to polyvinylidene difluoride (PVDF) membranes via the elution of Coomassie brilliant blue R-250 is described. A mixture of standard proteins was resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electroblotted onto PVDF membranes. Spectrophotometric analysis of dye eluted from protein bands in the range of 0.5-10 micrograms gave a linear change in the absorbance at 595 nm. Maximal absorbance readings were attained following 5 min of dye elution, and the readings remained unchanged for elution times up to 60 min. The method requires no unusual reagents or equipment, is suitable for the analysis of multiple samples, and does not consume the protein in the process of quantitation. This technique provides a useful means for the quantitation of proteins bound to PVDF membranes prior to amino acid sequence determination, immunological analysis, or other biochemical characterizations.

Detection of hemoprotein peroxidase activity on polyvinylidene difluoride membrane

The feasibility of detecting hemoproteins after electroblotting was examined. Hemoproteins were subjected to lithium dodecyl sulfate-polyacrylamide gel electrophoresis (LDS-PAGE) and then electroblotted and peroxidase activity was detected with 3,3',5,5'-tetramethylbenzidine The sensitivity and specificity of tetramethylbenzidine staining of LDS-PAGE gels was retained when proteins were electroblotted. Subsequent staining of the membrane with Coomassie blue R-250 revealed a protein pattern similar to that in the polyacrylamide gel. Thus electroblotting of hemoproteins does not affect resolution of the electrophoretic pattern and heme-associated peroxidase activity. Additionally, the ability to stain hemoproteins on polyvinylidene difluoride membranes offers the advantage of utilizing the same membrane for further biochemical and immunological characterizations.

Characterization of 19-nor-10-oxo-25-hydroxyvitamin D3 production by solubilized chick kidney mitochondria and bovine serum albumin

The vitamin D3 metabolite obtained from the incubation of 3-[(cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO)-solubilized chick kidney mitochondria with 25-hydroxyvitamin D3 (25-OH-D3) was identified to be 5(E)-19-nor-10-oxo-25-hydroxyvitamin D3 (5(E)-19-nor). The production of 19-nor was dependent on time and on protein concentration, but was not dependent on the pH of the incubation. 19-Nor was not formed in the absence of protein or when protein had been heat-treated following detergent solubilization. 19-Nor was not further metabolized to any other product upon incubation with the CHAPSO-solubilized proteins. No 19-nor-10-oxo derivative of 1,25(OH)2D3 was formed when 1,25(OH)2D3 was used as substrate in the incubation. Kinetic analysis showed a substrate saturation with an apparent Vmax of about 4.1 pmol/min.mg and S0.5 of approximately 1.3 x 10(-6) M. The production of 19-nor was not restricted to the CHAPSO-soluble protein fraction of kidney mitochondria but was also found in both the CHAPSO-soluble and -insoluble fractions of chick liver mitochondria and CHAPSO-treated bovine serum albumin (BSA). 19-Nor production by detergent-treated BSA also showed saturation kinetics with a similar S0.5 and an apparent Vmax which was about 5-fold higher than that obtained with CHAPSO-solubilized mitochondria. The evidence suggests that the formation of 19-nor is not mediated by a traditional enzyme, but does require protein. A mechanism for the conversion of 25-OH-E3 to 19-nor is proposed, in which the naturally-occurring 5(Z)-25-OH-D3 substrate binds to protein, isomerizes to 5(E)-25-OH-D3 and is oxidized by hydrogen peroxide to 5(E)-19-nor via a dioxetane intermediate.

Hormonal regulation of chick kidney inhibitor of adenosine 3',5'-monophosphate-dependent protein kinase

The endogenous inhibitor of cAMP-dependent protein kinase (PKI) in chick kidney is regulated by the vitamin D status of the animal. To determine the specific factors that are involved in the regulation of chick kidney PKI, chicks were raised on a low (0.05%), normal (1%), or high (3%) calcium diet and given vitamin D3 or vehicle three times a week orally. The results from this experimental protocol show that vitamin D3 or one or more of its metabolites and serum calcium levels are both involved in the regulation of chick kidney PKI in vivo. Measurement of PKI activity in primary cultures of chick kidney cells revealed treatment with 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3) led to a 90-95% decrease in PKI activity. This effect of 1,25-(OH)2D3 was dose dependent, and neither PTH nor insulin was able to reverse it completely. Treatment with PTH caused 30-60% increase in PKI activity, and cell cultures that were grown in medium containing either 0.5 or 2 mM calcium chloride had similar PKI activities. Taken together, these results indicate that 1,25-(OH)2D3, the most physiologically active form of vitamin D3, is the predominant regulator of PKI, but serum calcium, indirectly through the regulation of PTH secretion, is also involved.

25-Hydroxyvitamin D3 metabolism in mitochondria from primary renal cultures

Previous in vitro studies concerning the renal metabolism of 25-hydroxyvitamin D3 (25(OH)D3) to form 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and 24,25R-dihydroxyvitamin D3 (24,25(OH)2D3) have utilized intact cell systems. In reflecting upon the possible mechanisms by which hormonally induced changes in the production of 1,25(OH)2D3 and 24,25(OH)2D3 may be brought about, we asked whether altered mitochondrial hydroxylase activities can quantitatively account for changes in the total cellular output of these steroids. Our objective was to delineate between extramitochondrial processes (e.g. altered substrate delivery), and those events restricted to the renal mitochondria (altered hydroxylase activities). We have examined the effect of pretreating primary cultures of chick kidney cells with either 1,25(OH)2D3 or parathyroid hormone (PTH) on 25(OH)D3-hydroxylase activities present in subsequently isolated mitochondria. Pretreatment with 10(-7) M 1,25(OH)2D3 reduced 1 alpha-hydroxylase activity in both cells and mitochondria to approximately 60% of control values by 1 h, and to 25-30% by 2 h. The effect of PTH (10 ng/ml) in both mitochondrial and whole cell preparations was an approximate 40% increase in measured 1 alpha-hydroxylase activity. 10 microM forskolin (FSK) elicited an approximate 2-fold increase in 1,25(OH)2D3 production. Reciprocal effects were observed with respect to 24-hydroxylase activity in both whole cell and mitochondrial preparations in response to exogenous 1,25(OH)2D3, PTH, and FSK. The findings demonstrate that these hormones initiate intracellular events which lead directly to altered 25(OH)D3 1 alpha- and 24-hydroxylase activities within the renal mitochondria.

Interactions between intracellular signals involved in the regulation of 25-hydroxyvitamin D3 metabolism

The tumor-promoting phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) increases 25-hydroxyvitamin D3 (25OHD3)-24-hydroxylase and decreases 25OHD3-1-hydroxylase activity in cultured kidney cells, effects similar to those exerted by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and opposite those of PTH, forskolin, and cAMP. In this paper it is shown that the effects of TPA and 1,25-(OH)2D3 are additive, suggesting that they operate through distinct mechanisms. TPA did not alter cAMP metabolism by cultured chick kidney cells, not did it alter their response, in terms of 25OHD3 metabolism, to cAMP, suggesting that these two regulators of 25OHD3 metabolism also operate through distinct pathways. Another presumed activator of protein kinase-C 1,oleoyl-2-acetyl-glycerol, was tested and found to have the same effect as TPA in decreasing 1-hydroxylase activity, but it does not increase 24-hydroxylase activity. In addition, 1-oleoyl-2-acetyl-glycerol increases intracellular cAMP levels to approximately 25% of those attained by stimulation with PTH. None of the treatments resulted in altered [3H]PDBu binding by the cells. The results, taken together, suggest that 25OHD3-1-hydroxylase and the 25OHD3-24-hydroxylase are subject to multifactorial regulation and can be regulated independently of one another.

The number of 1,25-dihydroxyvitamin D3 receptors is decreased in both intestine and kidney of genetically diabetic db/db mice

In previous studies on calcium homeostasis in diabetes, drug-induced diabetic rats have generally been used, and various alterations have been demonstrated in several parameters of the vitamin D-endocrine system. It is, however, still questionable whether the drug-induced diabetic rat is the most appropriate animal model for the investigation of calcium and vitamin D metabolism because of the toxicity of diabetogenic agents toward the principle organs of vitamin D metabolism, such as liver and kidney. Therefore, in the present study, we examined the strain of genetically diabetic mice, C57BL/KsJ db/db, to evaluate 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptors in intestine and kidney and to investigate the alterations of calcium and vitamin D metabolism. In both control and diabetic mice, intestinal and renal 1,25-(OH)2D3 receptors were demonstrated; they had a sedimentation coefficient of 3.4S. The number of specific 1,25-(OH)2D3-binding sites in intestine was 118 +/- 11 fmol/mg protein in diabetic mice, significantly lower than the value of 199 +/- 11 fmol/mg protein in controls (P less than 0.01). Moreover, the renal concentration of specific 1,25-(OH)2D3-binding sites of 34.6 +/- 7.1 fmol/mg protein in diabetic mice was also significantly reduced compared to the value of 63.3 +/- 5.7 fmol/mg protein in controls (P less than 0.01). There were no significant differences in the equilibrium dissociation constants (Kd) of intestinal and renal receptors between control and diabetic mice. Significant hypocalcemia was demonstrated in the diabetic mice (P less than 0.01), suggesting the development of a negative calcium balance. Diabetic mice showed a significant decrease in renal 24,25-(OH)2D3 production (P less than 0.02), whereas renal 1,25-(OH)2D3 production was significantly increased in the diabetic group (P less than 0.05) compared to the control value. It is probable from these results that the genetic/endogenous diabetes may be directly associated with the alterations of mineral homeostasis. The altered calcium and vitamin D metabolism in diabetic mice is suggested to be derived, at least in part, from the decreased number of the 1,25-(OH)2D3 receptors in both intestine and kidney in the diabetic state.

Effect of vitamin D status on chick kidney proteins: detection of a 45-kilodalton mitochondrial protein suppressed by vitamin D

Two-dimensional polyacrylamide gel electrophoresis along with L-[35S]methionine radiolabeling studies were used to examine the effect of chronic vitamin D status on the composition and relative abundance of chick kidney proteins. Comparison of silver-stained gels revealed no extensive differences in either the electrophoretic mobility or the amounts of kidney proteins present in the mitochondrial fraction from vitamin D-replete and vitamin D-deficient chicks. A similar result was obtained in studies with L-[35S]methionine-labeled proteins. Vitamin D deficiency specifically elevated levels of a 45-kilodalton mitochondrial protein (pI 5.0 to 5.5) by approximately 5- to 12-fold relative to amounts present in vitamin D-replete tissue. This protein could not be detected in postmitochondrial supernatant fractions and was only faintly visible in crude kidney homogenates. The specificity of the observed suppression of this 45-kilodalton protein by vitamin D suggests that it may play an important role in renal functions influenced by the vitamin D endocrine system.

Evidence that stimulation of 1,25(OH)2D3 production in primary cultures of mouse kidney cells by cyclic AMP requires new protein synthesis

When primary culture of C75BL6 mouse cortical kidney cells in serum-free medium were incubated with unlabeled 25(OH)D3, they produced a metabolite which co-migrated with authentic 1,25(OH)2D3 and which could be measured by competitive receptor assay. A metabolite co-migrating with authentic 10-oxo-19-nor-25-OH-D3 was also produced. However, when cultures were incubated with 25(OH)D3 for 1 hour or longer, 10-oxo-19-nor-25-OH-D accounted for less than 15% of the total 3H-1,25(OH)2D3 displacement activity. Production of 1,25(OH)2D3 increased with increasing content of the culture, with time of incubation, and with substrate concentration. The apparent Km was 1.4 +/- 0.6 microM and Vmax 2.6 +/- 0.4 pM/mg protein/hr. These cultures possessed a very high level of phosphodiesterase activity, as indicated by their high cyclic AMP (cAMP) response to IBMX. This high phosphodiesterase activity may have been responsible for the lack of stimulation of 1,25(OH)2D3 production by physiologic or near physiologic concentrations of parathyroid hormone (PTH) in the absence of IBMX. However, when IBMX 10(-6) M was present, bPTH 10(-9) M significantly increased production of both cAMP and 1,25(OH)2D3. There was a close correlation between 1,25(OH)2D3 production and cAMP content of the cultures (basal or stimulated). An incubation time of at least 4 hours was required for cAMP to increase 1,25(OH)2D3 production and was inhibited in the presence of cycloheximide and actinomycin D. This study further documents the regulation of renal 1,25(OH)2D3 synthesis by PTH in mammalian kidney and provides evidence for cAMP as a possibly important second messenger in this effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of a tumor promoting phorbol ester on the metabolism of 25-hydroxyvitamin D3

When added to primary cultures of chick kidney cells, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) decreased the basal rate of production of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and increased that of 24,25-dihydroxyvitamin D3 (24,25(OH)2D3). The normal stimulatory effect of parathyroid hormone and forskolin on 1,25(OH)2D3 production was abolished or blunted by the presence of TPA and TPA overcame the inhibitory effect of PTH and forskolin on 24,25(OH)2D3 production. The evidence suggests that protein kinase C may be involved in the regulation of 25(OH)D3 metabolism by chick kidney cells.

The renal mitochondrial metabolism of 25-hydroxyvitamin D-3: a possible role for phospholipids

The effect of exogenous phospholipids on chick kidney mitochondrial 25-hydroxyvitamin D-3 metabolism was examined. Phosphatidylserine, phosphatidylcholine and phosphatidylinositol had no effect on either the 1- or 24-hydroxylation of 25-hydroxyvitamin D-3. Phosphatidylethanolamine and cardiolipin both brought about a dose-dependent decrease in the 1-hydroxylase activity in mitochondria from vitamin D-deficient chicks but not from vitamin D-replete chicks. There were no major differences in the phospholipid composition of mitochondria from vitamin D-deficient and -replete chicks nor in the fatty acid composition of these phospholipids. Preliminary kinetic studies suggest that cardiolipin acts as a noncompetitive inhibitor of the 1-hydroxylase in mitochondria isolated from vitamin D-deficient chicks. It does not appear to exert its effect by virtue of altering the distribution of substrate or products. Investigation of the effect of fatty acid methyl esters on the hydroxylase activities suggests that it may be the fatty acid moiety of the phospholipid, rather than the phosphate moiety in the polar head group, that is involved in the phospholipid effect on the hydroxylation of 25-hydroxyvitamin D-3.

Effect of dexamethasone on 25-hydroxyvitamin D3 metabolism by chick kidney cell cultures

The ability of dexamethasone to alter the metabolism of [3H]25-hydroxyvitamin D3 ([3H]25OHD3) metabolism by primary cultures of chick kidney cells was tested. Dexamethasone, present for 24 or 48 h at 10(-8)-10(-6) M, decreased production of [3H]1,25-dihydroxyvitamin D3 to approximately 60% of control levels. If cultures were pretreated with 1,25-dihydroxyvitamin D3 to reduce 25OHD3-1-hydroxylase activity and induce 25OHD3-24-hydroxylase activity, no effect of dexamethasone on either of the enzymes was observed. When the substrate concentration was varied, analysis of the data revealed that dexamethasone decreases both the maximal velocity of the rate of 1-hydroxylation to 25OHD3 and the half-maximal substrate concentration for 25OHD3. Dexamethasone had no effect on the cell number of the cultures, as assessed by DNA content, but did reduce the total protein content to approximately 70% of control values. Dexamethasone did not alter the response of chick kidney cells to PTH in terms of cAMP production or the metabolism of 25OHD3. The results suggest that dexamethasone has the potential to alter 25OHD3 metabolism through a direct effect on the renal cell.

Effect of ketoconazole and miconazole on 25-hydroxyvitamin D3 metabolism by cultured chick kidney cells

The antifungal imidazoles, ketoconazole and miconazole, were tested for effects on 25-hydroxyvitamin D3 metabolism in primary cultures of chick kidney cells. Both behave as competitive inhibitors of 1-hydroxylation of 25-OH-D3 with approximate Ki's of 0.8 and 5.0 microM for ketoconazole and miconazole, respectively. Ketoconazole was as effective when added at the same time as the substrate as when the cells were preincubated with the compound. Ketoconazole also inhibited the production of 24,25(OH)2D3 in cells in which this activity was induced by 1,24(OH)2D3. The data suggest that therapeutic doses of these antifungal imidazoles could affect vitamin D status and calcium metabolism.

Protein kinase activity and protein kinase inhibitor in mouse kidney: effect of the X-linked Hyp mutation and vitamin D status

cAMP-dependent protein kinase, Ca+2- and phospholipid-dependent protein kinase, and protein kinase inhibitor activity were examined in renal homogenates and 20,000 X g supernatant fractions of normal and Hyp mice. In both genotypes, 70% of total renal cAMP-dependent protein kinase activity was recovered in the soluble fraction in which the activity ratio (without cAMP to with cAMP) of the enzyme was 0.35. The requirement for cAMP was not different for protein kinase of normal and mutant littermates, with an apparent Km for cAMP of 0.05 microM in both genotypes. Furthermore, vitamin D and calcium deficiencies did not significantly affect cAMP-dependent protein kinase activity in normal and Hyp mouse kidney. The concentration of the heat-stable protein kinase inhibitor protein in the 20,000 X g supernatant fraction was identical in normal and Hyp kidney. Whereas protein kinase inhibitor levels were increased 1.8-fold by vitamin D and calcium deficiencies in normal mice (P less than 0.001), no such increase was detectable in Hyp mice. Ca+2- and phospholipid-dependent-protein kinase (protein kinase C) activity in the 20,000 X g supernatant fraction comprised 50% of the total activity of kidney homogenates of both normal and mutant mice. The initial rate of protein kinase C was increased 1.5-fold in kidney supernatants of Hyp mice (P less than 0.001). In contrast, protein kinase C was not significantly different from normal in supernatant fractions of heart, spleen, and liver prepared from Hyp mice. The present demonstration of abnormally high renal protein kinase C activity in Hyp mice may serve to explain the relationship between the previously reported renal defects in brush border membrane phosphate transport and vitamin D metabolism in the mutant strain and elucidate the nature of the primary defect in the Hyp mouse.

Interactions between aluminum and the actions and metabolism of vitamin D3 in the chick

The effects of intraperitoneal injections of aluminum chloride were tested on the intestinal calcium absorption and bone calcium mobilization responses to vitamin D3 and 1,25(OH)2D3, as measured by bioassay in chicks. Aluminum at 5 mg/kg given 5 days before the bioassay in vitamin D-deficient chicks, partially blocked the intestinal calcium absorption response to low (0.65 and 3.2 nmol), but not to higher (32 nmol) doses of vitamin D3. The responses to all doses (0.32-2.1 nmol) of 1,25(OH)2D3 were partially blocked by aluminum treatment. Serum calcium values were elevated in vitamin D-deficient chicks by aluminum administration, but no consistent effects of the treatment on bone calcium mobilization in response to vitamin D3 or 1,25(OH)2D3 were noted. Aluminum treatment in vivo led to decreased 25-OH-D3-1-hydroxylase activity subsequently measured in renal homogenates; under a variety of conditions, no direct effect of aluminum on 25-OH-D3 metabolism by primary cultures of chick kidney cells was observed. The results suggest that the ability of the intestine to respond normally to 1,25(OH)2D3 may be compromised by exposure to high levels of aluminum and that the effect of this element on 25-OH-D3 metabolism observed in vivo may not be exerted by direct action on the renal cell.

Parathyroid hormone modulation of 25-hydroxyvitamin D3 metabolism by cultured chick kidney cells is mimicked and enhanced by forskolin

In order to determine whether cAMP mediates the effects of PTH on the metabolism of 25-hydroxyvitamin D3 (25-OH-D3) on chick kidney cells in primary culture, the effect of forskolin on the production of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3] was assessed. In 4-h incubations with [3H]25-OH-D3 and forskolin, (1-10 microM) [3H]1,25-(OH)2D3 accumulation was increased 50-100%, and that of [3H]24,25-(OH)2D3 was decreased 30-60%. PTH (1-10 ng/ml) brought about identical changes. Similar results were observed when cultures were preincubated with nonradioactive 25-OH-D3 for 4 h in the presence of PTH and forskolin, followed by a 30-min incubation with radioactive substrate. At a low concentration (0.05 microM), forskolin alone had no effect on the metabolism of [3H]25-OH-D3 but markedly enhanced that of PTH. At maximal concentrations of PTH (10 ng/ml) and forskolin (10 microM), the effects of the two on 25-OH-D3 metabolism were not additive. Both PTH and forskolin decreased the further metabolism of [3H]1,25-(OH)2D3, probably by inhibiting its 24-hydroxylation, but there are also cycloheximide-sensitive steps in the metabolism of 1,25-(OH)2D3 that are not affected by PTH and forskolin. In time course experiments, increased [3H]1,25-(OH)2D3 accumulation could be observed before the detection of 24-hydroxylase activity suggesting that the primary effect of PTH and forskolin is on the production of [3H] 1,25-(OH)2D3 rather than its catabolism. Raising the calcium concentration of the medium to 2.5 mM from the normal 1.8 mM or lowering it to 0.5 mM for 24 h in serum-free medium did not alter the response of 25-OH-D3 metabolism to these agents. The results of these studies indicate that the effects of PTH on the metabolism of 25-OH-D3 by chick kidney cells are mediated by cAMP, since they can be enhanced and mimicked by forskolin, that they are exerted at the level of both 1- and 24-hydroxylase activity, and that they are not dependent on the calcium concentration of the medium.

Vitamin D: metabolism and biological actions

In the last few years, it has become clear that the vitamin D endocrine system is comprised of many more target cells and tissues than were imagined a decade ago; in addition to the intestine, kidney, and bone, the vitamin D endocrine system now includes the beta cells of the pancreas, breast tissue, placenta, the pituitary gland, cells of the reticuloendothelial system, and several other cells and tissues. The complexity of the metabolic pathway by which the active metabolite(s) of vitamin D are produced and further metabolized has emerged, as has the complexity of its regulation.

Enhancement of the production of 1,25-dihydroxyvitamin D3, in chick kidney mitochondria by an extramitochondrial factor

The 1 alpha-hydroxylation of 25-hydroxyvitamin D3 (25-hydroxycholecalciferol) by isolated chick kidney mitochondria is stimulated 1.5-4.0-fold by a factor or factors in postmitochondrial and postmicrosomal supernatants of homogenates of chick kidney. The stimulatory factor is heat-stable, dialyzable, and trypsin-sensitive and does not appear in lipid extracts of cytosol. The stimulatory effect of cytosol was quantitatively similar over a 4-fold range in substrate concentration and a 5-fold enzyme concentration range. Cytosol did not appear to increase substrate availability to the mitochondria as determined by measurement of substrate and products in mitochondria following incubation with [3H]25-hydroxyvitamin D3. The stimulatory activity is equivalent in cytosolic fractions from kidneys of vitamin D-deficient and replete chicks and is also present in brain and liver tissue. These latter observations suggest that the stimulatory factor (or factors) is not involved in the regulation of the 25-hydroxy-vitamin-D3-1-hydroxylase.

Homologous desensitization of cultured chick kidney cells to parathyroid hormone

The development of refractoriness of the cAMP response to PTH in primary cultures of chick kidney cells and recovery from the refractory state was investigated. When cells were preincubated with bovine PTH1-34, complete refractoriness to a subsequent challenge with the hormone developed within 2 h and at hormone concentrations as low as 5 ng/ml. The ability of PTH to stimulate activation of cAMP-dependent protein kinase was also abolished by preincubation with the hormone. When cells were desensitized and then incubated in hormone-free medium, recovery of the cAMP response began within an hour and was maximal, but not complete (80%) after 16 h. Cycloheximide did not affect either desensitization or the rate or extent of recovery from the refractory state. Low concentrations of forskolin (2.5 X 10(-7) M) greatly enhanced cAMP production stimulated by PTH and higher concentrations (10(-6) - 10(-4) M) stimulated rates of cAMP production 50 times those obtained with PTH alone. Preincubation with forskolin did not bring about desensitization to PTH nor did preincubation with PTH affect the subsequent response to forskolin. The half-life of biologically active bovine PTH1-34 in chick kidney cell culture was approximately 12 h and the rate of its removal was not significantly altered during a 20-h incubation period. The results suggest that desensitization of chick kidney cells to PTH is not suggest that desensitization of chick kidney cells to PTH is not brought about by cAMP generation itself, is not primarily dependent on protein synthesis, and does not involve a change in the rate of removal of biologically active hormone from the medium. In addition, recovery of the cAMP response to PTH also does not require new protein synthesis. These results are compatible with a mechanism of desensitization which occurs at the level of the receptor or hormone-receptor coupling to adenyl cyclase.

Protein phosphorylation in chick kidney. Response to parathyroid hormone, cyclic AMP, calcium, and phosphatidylserine

The regulation of endogenous protein phosphorylation by parathyroid hormone (PTH) was investigated using confluent monolayer cultures of chick kidney cells. Homogenates and subcellular fractions of PTH (bovine 1-34)-treated cells were subjected to an endogenous protein phosphorylation assay using ((gamma- 32P]ATP in the presence or absence of 2.0 microM cAMP or 0.5 mM Ca2+ with 25 micrograms/ml of phosphatidylserine and reactions terminated with sodium dodecyl sulfate. In other experiments, cultures were incubated in a phosphate-free 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid-buffered saline containing 50 muCi/ml of [32P]PO4 and incubations were terminated with sodium dodecyl sulfate. Protein phosphorylation was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Cyclic AMP stimulated 32P incorporation into proteins having molecular weights of 17,000, 22,000, 35,000, 42,000, 54,000, 75,000, 80,000, 120,000, and 143,000. Calcium-phosphatidylserine stimulated the phosphorylation of proteins of 20,000, 52,000, 58,000, 60,000, and 143,000. The protein phosphorylation patterns in cultured kidney cells and freshly isolated kidney tissue were quite similar. Treatment of cultured cells with 5-50 ng/ml of PTH resulted in stimulated phosphorylation of the 35,000 and 42,000 dalton proteins as assessed by endogenous phosphorylation in homogenates. In intact cells incubated with [32P]PO4, PTH stimulated most noticeably the phosphorylation of the 35,000-dalton protein. Based on studies with cultured and fresh kidney cells, the majority of the substrate proteins for cAMP and calcium-dependent protein kinases were located in the cytoplasm with the exception of the 42,000-dalton protein which was located in the brush-border-plasma membrane fraction. The cytoplasmic cAMP-dependent protein kinase activity was responsible for the majority of PTH-stimulated protein phosphorylation.

Cyclic AMP dependent protein kinase and its endogenous inhibitor protein: tissue distribution and effect of vitamin D status in the chick

1. Vitamin D deficiency in the chick leads to decreased (to 55% of normal) cyclic AMP-dependent protein kinase activity in the kidney but does not alter calcium-dependent phospholipid-sensitive protein kinase activity. 2. Decreased cyclic AMP-dependent protein kinase activity in response to vitamin D deficiency was not observed in other tissues including pancreas, brain, liver, intestinal mucosa, or heart. 3. Vitamin D deficiency leads to elevated levels of the endogenous inhibitor protein of cyclic AMP-dependent protein kinase in kidney, but not heart, muscle, pancreas, or brain.

Effect of vitamin D status on cyclic AMP-dependent protein kinase activity and its heat-stable inhibitor in chick kidney

Cyclic AMP-dependent protein kinase activity in supernatants of homogenates of kidneys from vitamin D-deficient chicks is decreased to 70% of the level measured in kidneys from normal chicks. Activity was restored to normal by oral administration of vitamin D or 1,25-dihydroxyvitamin D3 for 1 or 2 weeks. Both isozymes of cAMP-dependent protein kinase were reduced to the same extent by vitamin D deficiency. The decreased enzyme activity could not be accounted for by a shift to the particulate fraction nor by an increased requirement for cyclic AMP. A heat stable, trichloroacetic acid-precipitable, trypsin-labile inhibitor of protein kinase activity was identified and quantitated in kidneys from vitamin D-deficient chicks (16 to 26 units/mg of protein) and from those given vitamin D (2 to 6 units/mg of protein). The measured difference in inhibitor levels could not be attributed to differential stability in kidney homogenates from vitamin D-deficient or -repleted chicks. The observed increase in inhibitor level with vitamin D deficiency is not sufficient to account for the decrease in cyclic AMP-dependent protein kinase activity, suggesting that the total amount of this enzyme activity is reduced in vitamin D deficiency.

25(OH)D3 metabolism in kidney cell cultures: lack of a direct effect of estradiol

There are several reports of increased production of 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] by the kidney of birds in response to estrogen treatment. To determine whether estradiol influences the renal cell directly, primary cultures of chick kidney cells were exposed to the steroid under a variety of conditions. In the absence of 1,25(OH)2D3, treatment of cultures for 20-24 h with 10(-5) and 10(-6) M estradiol led to inhibition of 25-hydroxyvitamin D3 [25(OH)D3]-1-hydroxylase activity. When the 1-hydroxylase was suppressed and 25(OH)2D3-24-hydroxylase was induced by treatment with 1,25(OH)2D3, estradiol in concentrations of 10(-9) and 10(-5) M either had no effect or slightly inhibited 1,25(OH)2D3 production. Similarly, 24R,25-dihydroxyvitamin D3[24,25(OH)2D3] production was not affected consistently by estradiol. These results were unaltered when either testosterone (10(-6) M) or insulin (5 micrograms/ml) was present in the medium. Shorter treatments (0.5, 2, 4, and 8 h) with estradiol resulted in a transient decrease in both 1,25(OH)2D3 and 24,25(OH)2D3 production, but at no time was stimulation observed. These results suggest that the effects of estrogens on 25(OH)D3 metabolism observed in vivo are exerted elsewhere than directly at the renal cell.

Insulin permits parathyroid hormone stimulation of 1,25-dihydroxyvitamin D3 production in cultured kidney cells

Primary cultures of chick kidney cells in serum free medium respond to PTH with increased production of 1,25(OH)2D3 only when exposed to insulin. The response of 1,25(OH)2D3 is maximal at 5 ng bPTH (1-34) per ml and decreases at higher hormone concentrations. Increased 1,25(OH)2D3 synthesis is not evident after 30 minutes exposure to bPTH and is maximal at 4-6 hours of treatment. Insulin does not increase the cyclic AMP response to PTH suggesting that whatever permissive role it is playing occurs beyond the generation of cyclic AMP.

Recent advances in the understanding of the metabolism and functions of vitamin D

Many advances have been made in the past several years in our understanding of the metabolism and mechanism of action of vitamin D. Recognition of the clinical implications of this knowledge continues to grow. Despite these gains, however, many questions remain unanswered. These include the role of 24,25(OH)2D3 in physiologic processes, the nature of the contribution of vitamin D metabolism to bone growth and development, the responses of other possible target tissues such as the pancreas and parathyroid gland, and the further elucidation of interactions between vitamin D metabolites and parathyroid hormone in the maintenance of calcium and phosphorus homeostasis. The next decade of research is bound to bring insight into these and other questions.

Effects of vitamin D metabolites on bovine parathyroid hormone release in vitro

We evaluated the effects of 1 alpha,25-dihydroxycholecalciferol (1,25(OH)2D3), 24R,25-dihydroxycholecalciferol (24,25(OH)2D3), and 25-hydroxycholecalciferol (25(OH)D3) on the release of parathyroid hormone (PTH). Bovine parathyroid tissues were incubated in vitro for 4 h in low-calcium (1.0 mM) medium. 1,25(OH)2D3 ((10(-9)-10(-12)M), 24,25(OH)2D3 (10(-6)-10(-8)M), and 25(OH)D3 (5 X 10(-7)-5 X 10(-9)M) inhibited PTH release. Inhibition by all metabolites was concentration and time dependent. On a molar basis, 1,25(OH)2D3 was the most potent metabolite, being at least 100 times more potent than 24,25(OH)2D3 and 25(OH)D3; 24,25(OH)2D3 was about 5 times more potent than 25(OH)D3 at concentrations producing 65% inhibition. Inhibition by high concentrations of metabolites was evident by 1 h of incubation; inhibition was progressive throughout incubation, and maximal suppression to 30-40% of control occurred during the fourth and final hour of incubation. 1,25(OH)2D3 (10(-11) M), a low concentration that did not inhibit secretion, transiently stimulated release. In conclusion, under conditions of low-calcium-stimulated PTH release, 1,25(OH)2D3, 24,25(OH)2D3, and 25(OH)D3 inhibited PTH release, 1,25(OH)2D3 was the most potent inhibitor.