Molecular cloning of the rat thyroxine-binding globulin gene and analysis of its promoter activity

We cloned the rat T4-binding globulin (rTBG) gene, characterized its organization, and studied its promoter activity and regulation. A genomic DNA library was constructed and screened using an rTBG complementary DNA (cDNA) as a probe. An 8.6-kilobase pair (kbp) clone was partially sequenced and compared with the sequence of the previously cloned cDNA. It helped complete the cDNA sequence and identify the first noncoding exon (exon 0). The transcription start site was identified using an RNase protection assay. The rTBG genomic clone contained 1.2 kbp 5'-flanking and 1.7 kbp 3'-flanking regions. The sizes of exons and introns of the rTBG gene are similar to those of the human TBG gene, belonging to the serine protease inhibitor family. The 5'-flanking region contains a TATA box, a CAAT box, and a consensus sequence for the hepatocyte nuclear factor 1-binding site. We tested the promoter activity of the 1.2-kbp 5'-flanking region using a luciferase reporter plasmid. When transfected into a hepatocyte-derived cell line (HepG2), the plasmid construct containing the fragment -1227 to +11 (transcription start site, +1) showed a 9-fold increase in luciferase activity compared with that of a promoterless luciferase vector. No promoter activity was detected in a nonhepatocyte-derived cell line (COS1). Serial 5'-deletion revealed that the construct containing the fragment -180 to +11 had 40% of the maximal promoter-induced luciferase activity. And that containing the fragment -53 to +11 showed no significant increase of luciferase activity. These results suggest that the region -180 to -53, containing hepatocyte nuclear factor 1-binding site, is essential to the liver-specific expression. We previously reported down-regulation of rTBG messenger RNA by T3 in vivo. The present study failed to show T3's effect on the promoter activity of the 1.2-kbp 5'-flanking region of the rTBG gene.

Effect of glucocorticoid on prostaglandin F2 alpha-induced prostaglandin E2 synthesis in osteoblast-like cells: inhibition of phosphoinositide hydrolysis by phospholipase C as well as phospholipase A2

It is well known that osteoporosis is a common complication of patients with glucocorticoid excess. We showed previously that prostaglandin (PG) F2 alpha stimulates the synthesis of PGE2, a potent bone resorbing agent, and that the activation of protein kinase C amplifies the PGF2 alpha-induced PGE2 synthesis through the potentiation of phospholipase A2 activity in osteoblast-like MC3T3-E1 cells. In the present study, we examined the effect of dexamethasone on PGE2 synthesis induced by PGF2 alpha in MC3T3-E1 cells. The pretreatment with dexamethasone significantly inhibited the PGE2 synthesis in a dose-dependent manner in the range between 0.1 and 10 nmol/l in these cells. This effect of dexamethasone was dependent on the time of pretreatment up to 8 h. Dexamethasone also inhibited PGE2 synthesis induced by melittin, known as a phospholipase A2 activator. Furthermore, dexamethasone significantly inhibited the enhancement of PGF2 alpha- or melittin-induced PGE2 synthesis by 12-O-tetradecanoylphorbol-13-acetate, known as a protein kinase C activator. In addition, dexamethasone significantly inhibited PGF2 alpha-induced formation of inositol phosphates in a dose-dependent manner between 0.1 and 10 nmol/l in MC3T3-E1 cells. These results strongly suggest that glucocorticoid inhibits PGF2 alpha-induced PGE2 synthesis through the inhibition of phosphoinositide hydrolysis by phospholipase C as well as phospholipase A2 in osteoblast-like cells.

Effect of endothelin-1 on phospholipase D activity in osteoblast-like cells

We examined the effect of endothelin-1 (ET-1) on phosphatidylcholine-hydrolyzing phospholipase D activity in osteoblast-like MC3T3-E1 cells. ET-1 stimulated the formation of choline (EC50 10 nM) as well as that of inositol phosphates (EC50 1.2 nM). The effects of ET-1 and 12-O-tetradecanoylphorbol-13-acetate, a protein kinase C-activating phorbol ester, were additive. Staurosporine enhanced the ET-1-induced formation of choline. NaF or pertussis toxin were ineffective. The results indicate that ET-1 activates phospholipase D independent of protein kinase C in osteoblast-like cells.

Glucocorticoid inhibits cAMP production induced by vasoactive agents in aortic smooth muscle cells

It is well-known that atherosclerotic change and hypertension are common manifestations in patients with glucocorticoid excess. We previously reported that pituitary adenylate cyclase activating polypeptide (PACAP), prostaglandin E2 (PGE2) and carbacyclin, a stable analog of prostacyclin, have suppressive effects on vasopressin-induced DNA synthesis of rat aortic smooth muscle cells through cAMP production (Murase et al., J. Hypertens., 10 (1992) 1505; Oiso et al., Biochem. Cell. Biol., 71 (1993) 156). In the present study, we investigated the effect of glucocorticoid on cAMP production induced by PACAP, PGE2 and carbacyclin in aortic smooth muscle cells. The pretreatment with dexamethasone significantly inhibited cAMP accumulation induced by these vasoactive agents in a dose dependent manner in the range between 10 pM and 10 nM. These inhibitory effects of dexamethasone were dependent on the time of pretreatment up to 8 h. Dexamethasone inhibited cAMP accumulation induced by NaF, a GTP-binding protein activator, and forskolin which directly activates adenylate cyclase. Moreover, forskolin-induced adenylate cyclase activity was significantly reduced in membranes prepared from the cells treated with dexamethasone. These results strongly suggest that glucocorticoid inhibits cAMP production induced by vasoactive agents in primary cultured rat aortic smooth muscle cells and the inhibitory effect is exerted at the level of adenylate cyclase.

Impaired intracellular transport contributes to partial thyroxine-binding globulin deficiency in a Japanese family

We have previously reported a Japanese family manifesting partial TBG deficiency (TBG-PDJ). This variant was characterized by a decreased level of serum TBG concentration, heat lability, and normal isoelectric focussing pattern, but the affinity to iodothyronine is unknown. The TBG-PDJ gene possesses a single nucleotide substitution replacing the normal Pro363(CCT) with Leu(CTT); however, the precise mechanism that results in the reduction of the TBG concentration in the serum of the subjects harboring this mutation remains unknown. This was investigated in the current communication by expressing the complementary DNAs of TBG-PDJ and the common type TBG (TBG-C) in COS-1 cells. Pulse-chase experiments revealed impaired secretion of TBG-PDJ. TBG-C secretion into the medium was evident during 60 min of the pulse period and was almost completed by 12 h. On the other hand, TBG-PDJ was secreted slowly and continued to accumulate between 12-24 h of the chase period. The molecular mass of TBG-PDJ in the cell lysate was identical to that of TBG-C when estimated by gel electrophoresis (54 kilodaltons). The content of TBG-PDJ in the cell lysate decreased less rapidly than that of TBG-C, indicating that impaired TBG-PDJ secretion accounts for the partial TBG deficiency. Oligosaccharide units of intracellular TBG-C were resistant to endoglycosidase-H, but half of those of TBG-PDJ were sensitive to the enzyme digestion, suggesting partial retention of TBG-PDJ within the rough endoplasmic reticulum. Northern blot analysis revealed abundant messenger ribonucleic acid for the glucose-regulated protein-78, the level of which was 3.54-fold greater in the cells transfected with TBG-PDJ than in nontransfected COS-1 cells, whereas that in TBG-C-transfected cells was same as that in the nontransfected cells. Increased expression of glucose-regulated protein-78 together with the sensitivity to endoglycosidase-H suggests impairment of intracellular processing of TBG-PDJ. Our results indicate that the impaired intracellular transport of the TBG-PDJ molecule is the main cause of the reduced concentration of immunoreactive TBG in the serum of subjects harboring this TBG variant.

Novel mutations in the V2 vasopressin receptor gene in two pedigrees with congenital nephrogenic diabetes insipidus

Novel mutations in the V2 vasopressin receptor gene were identified in two Japanese pedigrees with X-linked congenital nephrogenic diabetes insipidus. The V2 receptor belongs to the family of G-protein-coupled receptors that contain seven distinct transmembrane domains, and the V2 receptor gene is encoded by three exons. The coding regions amplified by polymerase chain reaction were directly sequenced. In a pedigree, one of four consecutive guanine sequences (nucleotides 528-531) in the second exon was deleted (528delG). This deletion mutation results in a frame shift beginning at codon 154 in the second intracellular domain and a premature termination at codon 161. In another pedigree, a missense mutation (A-->G) was identified at nucleotide position 310 in the second exon. This point mutation, H80R, changes a histidine at codon 80 in the second transmembrane domain to an arginine that is more positively charged than histidine under the neutral environment. Each mutation cosegregated with the phenotype of diabetes insipidus and supposed to be a cause for resistance to arginine vasopressin.

Effect of retinoic acid on prostaglandin F2 alpha-induced prostaglandin E2 synthesis in osteoblast-like cells

We previously reported that prostaglandin F2 alpha (PGF2 alpha) stimulates phosphoinositide hydrolysis via pertussis toxin-sensitive GTP-binding protein in osteoblast-like MC3T3-E1 cells (Miwa, Tokuda, Tsushita, Kotoyori, Takahashi, Ozaki, Kozawa and Oiso 1990) and that PGF2 alpha stimulates arachidonic acid release and prostaglandin E2 (PGE2) synthesis, and the activation of protein kinase C (PKC) amplifies the effect of PGF2 alpha in MC3T3-E1 cells (Tokuda, Oiso and Kozawa 1992). In the present study, we investigated the effect of retinoic acid (RA), a vitamin A (retinol) metabolite, on PGF2 alpha-induced PGE2 synthesis in MC3T3-E1 cells. The pretreatment with RA, which by itself had little effect on synthesis, significantly inhibited PGE2 synthesis induced by PGF2 alpha in a dose-dependent manner in the range between 1 nM and 0.1 microM. This effect of RA was dependent on the time of pretreatment up to 8 h. In addition, RA inhibited the amplification of PGF2 alpha-induced PGE2 synthesis by 12-O-tetradecanoylphorbol-13-acetate, known to be a PKC activator. However, RA had little effect on PGE2 synthesis induced by melittin, known as a phospholipase A2 activator. Moreover, pertussis toxin had little effect on arachidonic acid release induced by PGF2 alpha. These results strongly suggest that RA inhibits PGE2 synthesis induced by PGF2 alpha in osteoblast-like cells and the inhibitory effect is exerted at the point prior to the activation of phospholipase A2.

Prostaglandin F2 alpha activates phospholipase D independently from activation of protein kinase C in osteoblast-like cells

We previously reported that prostaglandin F2 alpha (PGF2 alpha) receptor is coupled to pertussis toxin (PTX)-sensitive GTP-binding protein (G protein) in osteoblast-like MC3T3-E1 cells [Miwa et al. (1990): Biochem Biophys Res Commun 171:1229-1235]. In the present study, we examined the effect of PGF2 alpha on the activation of phosphatidylcholine-hydrolyzing phospholipase D in MC3T3-E1 cells. PGF2 alpha stimulated the formation of choline in a dose-dependent manner in the range between 10 nM and 10 microM. The formation of choline was stimulated by 12-O-tetradecanoylphorbol-13-acetate (TPA), a protein kinase C (PKC)-activating phorbol ester. 4 alpha-Phorbol 12, 13-didecanoate, a PKC-nonactivating phorbol ester, had little effect on choline formation. The formation of choline stimulated by a combination of PGF2 alpha and TPA was additive. Staurosporine, an inhibitor for protein kinases, which inhibited the effect of TPA on choline formation, dose-dependently enhanced the formation of choline induced by PGF2 alpha. NaF, an activator of G protein, stimulated the formation of choline. The formation of choline stimulated by a combination of PGF2 alpha and NaF was not additive. NaF-induced formation of choline was dose-dependently enhanced by staurosporine. PTX dose-dependently inhibited the PGF2 alpha-induced formation of choline. These results strongly suggest that PGF 2 alpha activates phospholipase D independently from the activation of PKC in osteoblast-like cells and PTX-sensitive G protein is involved in the PGF2 alpha-induced phospholipase D activation.

Effect of vitamin D3 on prostaglandin E2 synthesis in osteoblast-like cells

We previously showed that prostaglandin (PG) F2 alpha stimulates PGE2 synthesis in osteoblast-like MC3T3-E1 cells, and that the activation of protein kinase C (PKC) amplifies the effect of PGF2 alpha through the potentiation of phospholipase A2 activity (H. Tokuda, Y. Oiso and O. Kozawa, J Cell Biochem. 48: 262-268, 1992). In the present study, we investigated the effects of vitamin D3 on PGF2 alpha-induced PGE2 synthesis in MC3T3-E1 cells. The pretreatment with 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], an active form of vitamin D3, significantly inhibited the PGF2 alpha-induced PGE2 synthesis in a dose-dependent manner in the range between 1 pM and 1 nM. This inhibitory effect of 1,25-(OH)2D3 was dependent on the time of pretreatment up to 8 h. On the contrary, the pretreatment with 24,25-dihydroxyvitamin D3, an inactive form of vitamin D3, had little effect on the PGF2 alpha-induced PGE2 synthesis in these cells. However, the pretreatment with 1,25-(OH)2D3 no longer affected the amplification of PGF2 alpha-induced PGE2 synthesis by 12-O-tetradecanoylphorbol-13-acetate, a PKC activator. These results strongly suggest that 1,25-(OH)2D3 inhibits PGF2 alpha-induced PGE2 synthesis in osteoblast-like cells, however, the activation of PKC reverses this inhibitory effect of 1,25-(OH)2D3.

Okadaic acid reverses the inhibitory effect of protein kinase C on alkaline phosphatase activity in osteoblast-like cells

We previously reported that fetal calf serum-induced alkaline phosphatase activity is suppressed due to the activation of protein kinase C in osteoblast-like MC3T3-E1 cells (Miwa et al. (1991) Bone Miner. 14, 15-25; Kotoyori et al. (1994) Horm. Metab. Res. 26, 116-118). In the present study, we examined the effect of okadaic acid, a potent and specific inhibitor of protein phosphatase type 1 and 2A, on fetal calf serum-induced alkaline phosphatase activity in MC3T3-E1 cells. The pretreatment with okadaic acid enhanced the fetal calf serum-induced alkaline phosphatase activity in a dose-dependent manner in the range between 0.1 and 5 nM. 1-Norokadaone, a less potent analogue of okadaic acid, had little effect on the fetal calf serum-induced alkaline phosphatase activity. Okadaic acid partially reversed the suppression of fetal calf serum-induced alkaline phosphatase activity by 12-O-tetradecanoylphorbol-13-acetate, a protein kinase C activator. The effect of okadaic acid was dose-dependent in the range between 0.1 and 5 nM. The patterns of the dose-dependency of both okadaic acid effects on fetal calf serum-induced alkaline phosphatase activity and on the suppression by 12-O-tetradecanoylphorbol-13-acetate were similar. These results strongly suggest that protein phosphatase type 1 and/or 2A act as a regulator of alkaline phosphatase activity at a point downstream from protein kinase C in osteoblast-like cells.

Intracellular signaling mechanism of bradykinin in osteoblast-like cells: comparison with prostaglandin E2

Bradykinin is recognized to be involved in the process of bone resorption in chronic inflammatory diseases. We previously reported that prostaglandin E2 (PGE2), known as a potent bone resorbing agent, induces phosphoinositide hydrolysis, cAMP production and Ca2+ influx in osteoblast-like MC3T3-E1 cells, and these dose-dependencies are different to one another. To clarify the signaling mechanism of bradykinin, we compared the intracellular signaling system of bradykinin with that of PGE2 in these cells. Bradykinin stimulated Ca2+ influx dose-dependently in the range between 0.1 nM and 0.1 microM even in the presence of nifedipine, a Ca2+ antagonist that inhibits the voltage-dependent L-type Ca2+ channel. The maximum effect of bradykinin (0.1 microM) on Ca2+ influx was almost as great as that of PGE2 (0.5 microM). Bradykinin had little effect on cAMP accumulation, while PGE2 significantly stimulated it. Bradykinin stimulated the formation of inositol phosphates much less strongly than PGE2. Bradykinin stimulated inositol 1, 4, 5-trisphosphate [Ins(1, 4, 5)P3] formation dose-dependently between 0.1 nM and 0.1 microM, and the dose-dependent curves of bradykinin-induced Ca2+ influx and Ins(1, 4, 5)P3 were similar. However, the maximum effect of PGE2 (10 microM) on Ins (1, 4, 5) P3 formation was about 2-fold higher than that of bradykinin (0.1 microM). These results suggest that bradykinin induces Ca2+ influx independent of the voltage-dependent L-type Ca2+ channel and phosphoinositide hydrolysis in a similar dose-dependent manner in osteoblast-like cells.

Effect of prostaglandin F2 alpha on Ca2+ influx in osteoblast-like cells: function of tyrosine kinase

We previously reported that pertussis toxin-sensitive GTP-binding protein is involved in prostaglandin F2 alpha (PGF2 alpha)-induced phosphoinositide (PI) hydrolysis in osteoblast-like MC3T3-E1 cells [Miwa et al. (1990): Biochem Biophys Res Commun 171:1229-1235]. In the present study, we investigated the mechanism of PGF2 alpha-induced Ca2+ influx in MC3T3-E1 cells. PGF2 alpha-induced formation of total inositol phosphates (IPs) was markedly reduced by the depletion of extracellular Ca2+ with EGTA. On the other hand, the depletion of extracellular Ca2+ had little effect on PGF2 alpha-induced inositol 1,4,5-trisphosphate formation. PGF2 alpha stimulated 45Ca2+ influx dose dependently, attaining a maximum effect at 10 nM. Dose of PGF2 alpha above 10 nM caused less than maximal stimulation. Genistein, an inhibitor of protein tyrosine kinase, which by itself had little effect on 45Ca2+ influx, significantly suppressed the PGF2 alpha-induced 45Ca2+ influx in a dose-dependent manner in the range between 1 micrograms/ml and 0.1 mg/ml. Sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, enhanced the PGF2 alpha-induced 45Ca2+ influx. Genistein also suppressed the PGF2 alpha-induced total IPs formation dose dependently in the range between 1 micrograms/ml and 0.1 mg/ml. However, it had little effect on the PGF2 alpha-induced inositol 1,4,5-trisphosphate formation. The pretreatment with pertussis toxin had little effect on the PGF2 alpha-induced 45Ca2+ influx. These results strongly suggest that PGF2 alpha stimulates Ca2+ mobilization from extracellular space and PI hydrolysis via independent pathways in osteoblast-like cells, and the PGF2 alpha-induced Ca2+ influx is regulated by protein tyrosine kinase, resulting in the promotion of PI hydrolysis.

A truncated thyroxine-binding globulin due to a frameshift mutation is retained within the rough endoplasmic reticulum: a possible mechanism of complete thyroxine-binding globulin deficiency in Japanese

We have previously reported six unrelated Japanese families having the same mutation in the TBG gene and manifesting complete TBG deficiency (TBG-CDJ). The deficiency consists of a single nucleotide deletion resulting in the production of C-terminal truncation due to a frameshift and premature termination. However, the reason for the failure to detect TBG in the serum of subjects harboring this mutation remains unknown. In this communication we investigated the mechanism of the TBG deficiency associated with TBG-CDJ. The complementary DNAs of TBG-CDJ and normal TBG (TBG-N) were expressed in COS-1 cells. Pulse-chase experiments revealed a complete absence of secretion of TBG-CDJ, whereas TBG-N was already present in the medium at time 0 and was almost entirely secreted by 3 h. In cell lysates, TBG-CDJ had a smaller molecular mass [52 kilodalton (kDa)] than TBG-N (54 kDa) and gradually decreased during the chase. Thus, failure of TBG-CDJ secretion accounts for the complete TBG deficiency. The molecular mass of TBG-CDJ (42 kDa) was also smaller than that of TBG-N (44 kDa) when synthesized in the presence of tunicamycin. These findings are consistent with the premature termination of the TBG-CDJ molecule deduced from the nucleotide sequence analysis. Intracellular TBG-N was resistant to endoglycosidase H but not TBG-CDJ, suggesting the retention of TBG-CDJ within the rough endoplasmic reticulum. Indeed, subcellular fractionation revealed that most of TBG-CDJ was located in the rough endoplasmic reticulum compartment, and TBG-N was distributed in the Golgi fractions. Our results suggested that the lack of intracellular transport of the truncated TBG molecule is the cause for the absence of immunoreactive TBG in the serum of subjects harboring the TBG-CDJ variant.

Humoral hypercalcemia of malignancy associated with parathyroid hormone-related protein producing transitional cell carcinoma of the ureter

A 78-year old male with ureteral carcinoma manifesting hypercalcemia is reported. He was diagnosed as having ureteral carcinoma of the left side 2 years previously and was treated by nephrectomy with ureterovesicostomy. In October 1991, he was admitted for anorexia. A clinical examination revealed recurrence of the ureteral carcinoma with metastasis to the rectum and liver. His serum calcium level was elevated (13.9 mg/dl). In addition to rehydration and furosemide, treatment with eel-calcitonin and prednisolone failed to decrease his serum calcium level. Finally, he was administered mithramycin but he died 13 days later. He had no evidence of bone metastasis or hyperparathyroidism. Nephrogenic cAMP and urinary parathyroid hormone-related protein (PTHrP) were markedly elevated. Immunohistochemical study demonstrated expression of PTHrP in the tumor cells. Thus, the hypercalcemia was thought to be mediated by PTHrP secreted from the neoplastic tumor. Although there have been several reports of ureteral carcinoma associated with humoral hypercalcemia of malignancy, this is considered to be the first case associated with elevation of PTHrP.

Pituitary adenylate cyclase-activating polypeptide induces cAMP production independently from vasoactive intestinal polypeptide in osteoblast-like cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) isolated from ovine hypothalamic tissue is a novel neuropeptide which stimulates adenylate cyclase in rat anterior pituitary cell cultures. In osteoblasts, the detail of intracellular signalling systems of PACAP has not yet been clarified. In this study, we investigated the effects of PACAP on cAMP accumulation, phosphoinositide hydrolysis and Ca2+ influx in osteoblast-like MC3T3-E1 cells, compared with those of vasoactive intestinal polypeptide (VIP), which shows a considerable homology with PACAP in the N-terminal sequence. PACAP stimulated cAMP accumulation in a dose-dependent manner in the range between 0.1 nM and 0.1 microM in these cells. VIP also stimulated cAMP accumulation dose-dependently between 1 nM and 0.1 microM. The effect of PACAP on cAMP accumulation (EC50 = 3 nM) was more potent than that of VIP (EC50 = 30 nM). The cAMP accumulation stimulated by a combination of PACAP (3 nM) and VIP (30 nM) was additive. [Lys1, Pro2,5, Arg3,4, Tyr6]-VIP, and antagonist for the VIP receptor which markedly inhibited the VIP-induced cAMP accumulation, had little effect on the PACAP-induced cAMP accumulation. Either PACAP or VIP had little effect on the formation of inositol phosphates and Ca2+ influx in these cells. These results strongly suggest that PACAP stimulates cAMP production via an independent binding site from VIP in osteoblast-like MC3T3-E1 cells and that PACAP has no effect on the activation of protein kinase C nor the intracellular Ca2+ mobilization in these cells.

Effects of glucocorticoid on signalling by prostaglandin E2 in osteoblast-like cells

It is well-known that osteoporosis is a common complication of patients with glucocorticoid excess. We previously reported that dexamethasone inhibits Ca2+ influx induced by prostaglandin E2 (PGE2), a potent bone resorbing agent, in osteoblast-like MC3T3-E1 cells (O. Kozawa, H. Tokuda, J. Kotoyori, A. Suzuki, Y. Ito and Y. Oiso, Prostagland Leuk Essent Fatty Acids, in press). In the present study, we examined the effects of dexamethasone on cyclic adenosine monophosphate (cAMP) accumulation and phosphoinositide hydrolysis by PGE2 in MC3T3-E1 cells. The pretreatment with dexamethasone significantly inhibited cAMP accumulation stimulated by PGE2 in a dose-dependent manner in the range between 10 pM and 1 nM in MC3T3-E1 cells. This effect of dexamethasone was dependent on the time of pretreatment up to 8 h. Dexamethasone also inhibited the cAMP accumulation induced by NaF, an activator of guanosine triphosphate (GTP)-binding protein, or forskolin which directly activates adenylate cyclase. In contrast, dexamethasone had little effect on the formation of inositol phosphates stimulated by PGE2 in MC3T3-E1 cells. These results strongly suggest that glucocorticoid modulates the signal transduction by PGE2 in osteoblast-like cells and that it inhibits PGE2-induced cAMP production without affecting PGE2-induced phosphoinositide hydrolysis.

Gene screening of 23 Japanese families with complete thyroxine-binding globulin deficiency: identification of a nucleotide deletion at codon 352 as a common cause

Thyroxine-binding globulin (TBG) is a major thyroid hormone transport protein in human serum. Its complete deficiency (TBG-CD) is one of inherited TBG abnormalities that transmit on X-chromosome. We previously reported a nucleotide deletion at codon 352 of the TBG gene (TBG-CDJ) in Japanese families with TBG-CD. To determine the prevalence of this mutation in Japanese with TBG-CD, 23 affected subjects (19 males and 4 females) belonging to unrelated families living in 4 major islands of Japan were analyzed with regard to the mutation at codon 352. Their genomic DNAs were amplified by the polymerase chain reaction with allele specific primers. Nineteen male and four female subjects were shown to have the mutation as hemizygotes and heterozygotes, respectively. It is concluded that TBG-CDJ may be a common cause of TBG-CD in Japanese and might have appeared in the ancestors of the Japanese after the human race divergence.

Vasopressin induces arachidonic acid release through pertussis toxin-sensitive GTP-binding protein in aortic smooth muscle cells: independence from phosphoinositide hydrolysis

We previously reported that pertussis toxin (PTX) had little effect on arginine vasopressin-induced formation of inositol trisphosphate (IP3) in rat aortic smooth muscle cells [Kondo et al.: Biochemical and Biophysical Research Communications 161:677-682, 1989]. In the present study, we investigated the mechanism of vasopressin-induced arachidonic acid release in rat aortic smooth muscle cells. Vasopressin stimulated both the release of arachidonic acid and the formation of IP3 dose dependently in the range between 10 pM and 1 microM. The effect of vasopressin on arachidonic acid release was more potent than that on the formation of IP3. Quinacrine, a phospholipase A2 inhibitor, significantly suppressed the vasopressin-induced arachidonic acid release but had little effect on the formation of inositol phosphates. NaF, a GTP-binding protein activator, mimicked vasopressin by stimulating the arachidonic acid release. The arachidonic acid release stimulated by a combination of vasopressin and NaF was not additive. PTX partially but significantly suppressed the vasopressin-induced arachidonic acid release. In the cell membranes, PTX catalyzed ADP-ribosylation of a protein with an M(r) of about 40,000. Pretreatment of membranes with 0.1 microM vasopressin in the presence of 2.5 mM MgCl2 and 100 microM GTP markedly attenuated this PTX-catalyzed ADP-ribosylation of the protein in a time-dependent manner. These results strongly suggest that PTX-sensitive GTP-binding protein is involved in the coupling of vasopressin receptor to phospholipase A2 in primary cultured rat aortic smooth muscle cells.

Glu-47, which forms a salt bridge between neurophysin-II and arginine vasopressin, is deleted in patients with familial central diabetes insipidus

The arginine vasopressin (AVP) gene was sequenced in a pedigree with familial central diabetes insipidus (DI). When polymerase chain reaction-amplified DNAs from affected subjects were subjected to polyacrylamide gel electrophoresis, fragments including exon 2 displayed two additional, slower migrating bands. These extra bands represented DNA heteroduplexes, indicating that there was a deletion or insertion mutation in exon 2. As the region with such a mutation was identified by direct sequence analysis, polymerase chain reaction-amplified fragments including the region were subcloned and sequenced. A 3-basepair deletion (AGG) out of two consecutive AGG sequences (nucleotides 1824-1829) was identified in one of two alleles. The cosegregation of the mutation with the DI phenotype in the family was confirmed by restriction enzyme analyses. This mutation should yield an abnormal AVP precursor lacking Glu47 in its neurophysin-II (NP) moiety. Since Glu47 is essential for NP molecules to form a salt bridge with AVP, it is very likely that the function of NP as a carrier protein for AVP would be impaired. We suggest that AVP would undergo accelerated proteolytic degradation, and this mechanism would be involved in the pathogenesis of DI in this pedigree.

Modulation of prostaglandin E2-induced Ca2+ influx by steroid hormones in osteoblast-like cells

In osteoblast-like MC3T3-E1 cells, we previously reported that prostaglandin E2 (PGE2), a potent bone resorbing agent, stimulates Ca2+ influx (H. Tokuda, M. Miwa, Y. Oiso and O. Kozawa, Cell Signal 1992; 4: 261-266). In this study, we examined the effects of various hormones belonging to the steroid hormone superfamily on PGE2-induced Ca2+ influx in MC3T-E1 cells. 1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3], an active form of vitamin D3, dexamethasone and retinoic acid significantly inhibited the PGE2-induced Ca2+ influx in a dose-dependent manner in these cells. The effects of these hormones were dependent on the time of pretreatment and submaximum inhibitions were observed at 6 h. In contrast, 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3], an inactive form of vitamin D3, 17 beta-estradiol, progesterone, testosterone and triiodothyronine had little effect on the PGE2-induced Ca2+ influx in these cells. These results suggest that, in the steroid hormone superfamily, 1,25-dihydroxyvitamin D3, glucocorticoid and retinoic acid modulate bone metabolism through the inhibition of Ca2+ influx induced by PGE2 in osteoblast-like cells.

Prostaglandin E2 is a potential mediator of extracellular ATP action in osteoblast-like cells

Extracellular ATP dose dependently stimulated 45Ca2+ influx even in the presence of nifedipine, a Ca2+ antagonist that inhibits voltage-dependent Ca2+ channel, in osteoblast-like MC3T3-E1 cells. ATP stimulated arachidonic acid release and the synthesis of prostaglandin E2 (PGE2). However, the ATP-induced arachidonic acid release was significantly reduced by chelating extracellular Ca2+ with EGTA. On the other hand, ATP induced DNA synthesis of these cells in a dose-dependent manner in the range between 1 microM and 1 mM. The pretreatment with indomethacin, a cyclooxygenase inhibitor, suppressed both ATP-induced PGE2 synthesis and DNA synthesis in these cells. The inhibitory effect by 50 microM indomethacin on the DNA synthesis was reversed by adding 10 microM PGE2. These results strongly suggest that extracellular ATP stimulates Ca2+ influx resulting in the release of arachidonic acid in osteoblast-like cells and that extracellular ATP-induced proliferative effect is mediated, at least in part, by ATP-stimulated PGE2 synthesis.

Effects of retinoic acid on signalling by prostaglandin E2 in osteoblast-like cells

We investigated the effects of retinoic acid (RA) on the signalling pathways by prostaglandin E2 (PGE2) in osteoblast-like MC3T3-E1 cells. The pretreatment with RA significantly inhibited the formation of inositol phosphates induced by 10 microM PGE2 in a dose-dependent manner in the range between 0.1 nM and 0.1 microM, without affecting protein contents in the cultured cells. This effect of RA was dependent on the time of pretreatment up to 8 h. However, RA had little effect on the formation of inositol phosphates induced by NaF, a GTP-binding protein activator. On the other hand, RA significantly inhibited PGE2-induced cAMP accumulation in a dose-dependent manner between 0.1 nM and 0.1 microM. This effect of RA was dependent on the time of pretreatment up to 8 h. RA also inhibited the cAMP accumulation induced by NaF or forskolin which directly activates adenylate cyclase. These results strongly suggest that RA modulates the signalling by PGE2 in osteoblast-like cells as follows: the inhibitory effect on the phosphoinositide hydrolysis is exerted at the point between PGE2 receptor and GTP-binding protein, and the inhibitory effect on the cAMP production is exerted at a point downstream from adenylate cyclase.

Effects of vitamin D3 on signaling by prostaglandin E2 in osteoblast-like cells

We investigated the effects of vitamin D3 on the signaling pathways by prostaglandin E2 (PGE2) in osteoblast-like MC3T3-E1 cells. The pretreatment with 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), an active form of vitamin D3, significantly inhibited cAMP accumulation induced by 10 microM PGE2 in a dose-dependent manner in the range between 1 pM and 1 nM. This effect of 1,25-(OH)2D3 was dependent on the time of pretreatment up to 8h. 1,25-(OH)2D3 also inhibited the cAMP accumulation induced by NaF, a GTP-binding protein activator, or forskolin which directly activates adenylate cyclase. On the other hand, 1,25-(OH)2D3 significantly inhibited PGE2-induced IP3 formation in a dose-dependent manner between 10 pM and 1 nM. However, 1,25-(OH)2D3 had little effect on NaF-induced IP3 formation. The pretreatment with 24,25-dihydroxyvitamin D3, an inactive form of vitamin D3, affected neither cAMP accumulation nor IP3 formation induced by PGE2. These results strongly suggest that 1,25-(OH)2D3 modulates the signaling by PGE2 in osteoblast-like cells as follows: the inhibitory effect on the cAMP production is exerted at a point downstream from adenylate cyclase and the inhibitory effect on the phosphoinositide hydrolysis is exerted at the point between the PGE2 receptor and GTP-binding protein, probably Gi2.

Pituitary adenylate cyclase-activating polypeptide stimulates arginine vasopressin release in conscious rats

The effect of pituitary adenylate cyclase-activating polypeptide (PACAP) on arginine vasopressin (AVP) release was investigated in conscious rats. Intracerebroventricular (i.c.v.) administration of PACAP raised the plasma AVP concentration in a dose-dependent manner (50-500 pmol/rat), and the maximum effect was obtained at 5 min after the administration. This AVP-releasing effect was not due to a fall of blood pressure, increase of plasma Na or decrease of plasma volume, all of which are known to stimulate AVP release. PACAP had little effect on blood pressure at a low dose, but at higher doses increased it. Vasoactive intestinal peptide (VIP), which is homologous to PACAP, also raised the plasma AVP concentration by i.c.v. injection. An antagonist for VIP receptor, [Lys, Pro, Arg, Tyr]-VIP inhibited the VIP-induced increase of plasma AVP, but had little effect on PACAP-induced increase of plasma AVP. These results suggest that PACAP stimulates AVP release, via specific receptors which are distinct from VIP receptors.

Possible involvement of inefficient cleavage of preprovasopressin by signal peptidase as a cause for familial central diabetes insipidus

A transition of G to A at nucleotide position 279 in exon 1 of the vasopressin gene has been identified in patients with familial central diabetes insipidus. The mutation predicts an amino acid substitution of Thr (ACG) for Ala (GCG) at the COOH terminus of the signal peptide in preprovasopression (preproVP). Translation in vitro of wild-type and mutant mRNAs produced 19-kD preproVPs. When translated in the presence of canine pancreatic rough microsomes, wild-type preproVP was converted to a 21-kD protein, whereas the mutant mRNA produced proteins of 21 kD and 23 kD. NH2-terminal amino acid sequence analysis revealed that the 21-kD proteins from the wild-type and the mutants were proVPs generated by the proteolytic cleavage of the 19-residue signal peptide and the addition of carbohydrate. Accordingly, mutant preproVP was cleaved at the correct site after Thr-19, but the efficiency of cleavage by signal peptidase was < 25% that observed for the wild-type preproVP, resulting in the formation of a predominant glycosylated but uncleaved 23-kD product. These data suggest that inefficient processing of preproVP produced by the mutant allele is possibly involved in the pathogenesis of diabetes insipidus in the affected individuals.

Effect of pituitary adenylate cyclase activating polypeptide on vasopressin-induced proliferation of aortic smooth muscle cells: comparison with vasoactive intestinal polypeptide

Pituitary adenylate cyclase activating polypeptide (PACAP) inhibited dose dependently the DNA synthesis stimulated by arginine vasopressin (AVP) in cultured rat aortic smooth muscle cells (SMC). The inhibition was cell cycle dependent and the maximum inhibition was observed when added at the late G1 phase of the cell cycle. Vasoactive intestinal polypeptide (VIP), which shows a considerable homology with PACAP, also inhibited dose dependently the AVP-induced DNA synthesis in a cell cycle dependent manner. The maximum inhibition was also observed at the late G1 phase. The patterns of both the dose-dependent inhibitions were similar, and the inhibition by a combination of PACAP and VIP was not additive. PACAP stimulated dose dependently cAMP accumulation in aortic SMC. VIP also stimulated cAMP accumulation, and the accumulation by a combination of PACAP and VIP was not additive. Both PACAP and VIP had little effect on phosphoinositide hydrolysis in these cells. The suppression of the AVP-induced DNA synthesis by PACAP or VIP was enhanced by 3-isobutyl-1-methylxanthine, an inhibitor for phosphodiesterases. Dibutyryl cAMP, but not 8-bromo-cGMP, inhibited the AVP-induced DNA synthesis, and a combination of PACAP and dibutyryl cAMP was not additive. [Ac-Tyr1,D-Phe2]growth hormone-releasing factor, an antagonist for VIP receptor, reversed the inhibitory effect of PACAP on the AVP-induced DNA synthesis. These results suggest that PACAP has an antiproliferative effect on aortic SMC at the late G1 phase of the cell cycle through cAMP production, and that PACAP and VIP inhibit the AVP-induced DNA synthesis by a common mechanism.

Sequence of a variant thyroxine-binding globulin (TBG) in a family with partial TBG deficiency in Japanese (TBG-PDJ)

Thyroxine-binding globulin (TBG) is the major transport protein of thyroid hormones in human serum. In this communication, we present a sequence abnormality of the TBG-gene in a Japanese family manifesting partial TBG deficiency (TBG-PDJ). The propositus was a male with a reduced concentration of TBG (3.2 micrograms/ml). Thyroid function tests suggested that the inheritance of this TBG abnormality was X-linked. The TBG exhibited increased heat-lability compared with the common type TBG (TBG-C). The isoelectric focusing pattern of this TBG molecule was indistinguishable from TBG-C. Genomic DNAs from white blood cells of four members of a TBG-PDJ family were subjected to polymerase chain reaction (PCR), and the products were sequenced. The sequencing of the entire coding exons and exon/intron junctions of TBG allele of the propositus revealed a single nucleotide substitution: CCT (proline) to CTT (leucine) at amino acid 363 of the TBG-C. The heterozygosity as revealed by the direct sequencing of the PCR product correlated with the TBG concentration in serum. The proline to leucine substitution may cause a change in the TBG tertiary structure and result in decreased heat stability, resulting in decreased TBG levels in the affected subjects.

Galanin as a physiological neurotransmitter in hemodynamic control of arginine vasopressin release in rats

Previous neuroanatomical studies have revealed a localization of galanin in several nuclei in the brain stem which are involved in the hemodynamic control of arginine vasopressin (AVP) release. The present study, therefore, investigates the contribution of endogenous galanin to the plasma volume-mediated control of AVP release in conscious rats. Injection of synthetic rat galanin (12.5-50 pmol/rat) into the cisterna magna (i.c.s.) suppressed plasma AVP increased by polyethylene glycol-induced hypovolemia (2.45 +/- 0.24 pg/ml at 50 pmol/rat vs. the vehicle group 5.72 +/- 0.69 pg/ml, p < 0.01). In contrast, when plasma AVP was suppressed by isotonic plasma volume expansion, immunoneutralization of endogenous galanin by antigalanin-antibody i.c.s. significantly reversed the suppression (1.02 +/- 0.07 pg/ml vs. vehicle group 0.63 +/- 0.05 pg/ml, p < 0.01) without altering the mean arterial blood pressure. These results suggest that endogenous galanin is physiologically involved in the plasma volume-mediated control of AVP release through an inhibitory action on this pathway.

Activation of protein kinase C inhibits 45Ca-accumulation in cultures of osteoblast-like cells: possible involvement of insulin-like growth factor-I

In a previous report, we have demonstrated that exogenous insulin-like growth factor-I (IGF-I) stimulates 45Ca-accumulation into extracellular matrix in long-term cultures of osteoblast-like MC3T3-E1 cells and that 45Ca-accumulation occurs even in the cultures without exogenous IGF-I. In this study, effects of protein kinase C (PKC) on IGF-I secretion and 45Ca-accumulation into extracellular matrix were examined in 6-week cultured MC3T3-E1 cells. The MC3T3-E1 cells secreted IGF-I spontaneously. The PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA) suppressed IGF-I secretion in a dose-dependent manner. 4 alpha-Phorbol 12,13-didecanoate (4 alpha-PDD), which is inactive for PKC, had little effect on the secretion. 1-Oleoyl-2-acetylglycerol, a specific activator for PKC, also suppressed the IGF-I secretion dose dependently. H-7, a PKC inhibitor, recovered the inhibitory effect of TPA. On the other hand, TPA inhibited the 45Ca-accumulation into extracellular matrix in cultures of these cells dose dependently, whereas 4 alpha-PDD was ineffective in this capacity. The TPA-induced inhibition of 45Ca-accumulation was recovered almost to the control level by H-7. Exogenous IGF-I recovered the inhibitory effect of TPA on 45Ca-accumulation. In spite of the inhibitory effects of TPA as above, TPA had little effect on DNA synthesis in these cells. These results suggest that the activation of PKC inhibits calcification via suppression of IGF-I secretion in osteoblast-like cells.

Regulation of proliferation by vasopressin in aortic smooth muscle cells: function of protein kinase C

AIM

To investigate the effect of arginine vasopressin-stimulated prostaglandin synthesis and the activation of protein kinase C on DNA synthesis in rat aortic smooth muscle cells.

METHODS

The effects of arginine vasopressin on the release of arachidonic acid and the synthesis of prostaglandin (PG) E2 and prostacyclin (PGI2) were determined. The effects of 12-o-tetradecanoylphorbol-13-acetate (TPA), a protein kinase C-activating phorbol ester, and of 1-oleoyl-2-acetylglycerol, a specific activator of protein kinase C, were evaluated in cultured rat aortic smooth muscle cells. The effects of arginine vasopressin and prostaglandins on the progression from the late G1 to the S phase of the cell cycle were evaluated by measuring the DNA synthesis, and the effects of TPA on them were evaluated.

RESULTS

Arginine vasopressin dose-dependently stimulated arachidonic acid release. TPA and 1-oleoyl-2-acetylglycerol dose-dependently increased the vasopressin-induced arachidonic acid release. Vasopressin stimulated the synthesis of both PGE2 and PGI2. TPA increased the vasopressin-stimulated prostaglandin synthesis as well as the arachidonic acid release. Vasopressin, added at the G0/G1 phase of the cell cycle, stimulated DNA synthesis of aortic smooth muscle cells. Exogenous PGE2 and PGI2 inhibited the DNA synthesis and showed maximum inhibition when added at the late G1 phase. TPA alone, added at the late G1 phase, reduced the DNA synthesis stimulated by vasopressin at the G0/G1 phase to about 45%, but vasopressin alone, added at the late G1 phase, had little effect. However, with TPA pretreatment, vasopressin significantly suppressed the DNA synthesis by about 70%. Staurosporine, a protein kinase C inhibitor, reduced the suppression by TPA alone or by vasopressin with TPA pretreatment almost to the control level. Indomethacin, a cyclo-oxygenase inhibitor, reduced the suppression by vasopressin with TPA pretreatment almost to the level of TPA alone.

CONCLUSIONS

These results suggest that arginine vasopressin has a suppressive effect on DNA synthesis in rat aortic smooth muscle cells by inhibiting progression from the late G1 into the S phase of the cell cycle through the synthesis of PGE2 and PGI2, and that protein kinase C acts as an amplifier of this mechanism.

Mechanism of prostaglandin E2-induced arachidonic acid release in osteoblast-like cells: independence from phosphoinositide hydrolysis

We previously reported that pertussis toxin (PTX)-sensitive GTP-binding protein is involved in the coupling of prostaglandin E2 (PGE2) receptor to phospholipase C in osteoblast-like MC3T3-E1 cells (1). In the present study, we analyzed the mechanism of PGE2-induced arachidonic acid (AA) release in MC3T3-E1 cells. PGE2 stimulated the release of AA and the formation of inositol trisphosphate (IP3) dose dependently in the range between 1 nM and 10 microM. The effect of PGE2 on AA release (ED50 was 80 nM) was more potent than that on IP3 formation (ED50 was 0.8 microM). Quinacrine, a phospholipase A2 inhibitor, suppressed the PGE2-induced AA release but had little effect on the IP3 formation. NaF, a GTP-binding protein activator, mimicked PGE2 by stimulating the AA release. The AA release stimulated by a combination of PGE2 and NaF was not additive. PTX had little effect on the PGE2-induced AA release. These results strongly suggest that the AA release and the phosphoinositide hydrolysis are separately stimulated by PGE2 in osteoblast-like cells, and the PGE2-induced AA release is mediated by PTX-insensitive GTP-binding protein.

Autoregulation of prostaglandin E2-induced Ca2+ influx in osteoblast-like cells: inhibition by self-induced activation of protein kinase C

In cloned osteoblast-like MC3T3-E1 cells, prostaglandin E2 (PGE2) stimulated 45Ca2+ influx even in the presence of nifedipine, a Ca2+ antagonist that inhibits voltage-dependent Ca2+ channel, in a dose-dependent manner, attaining a maximum at 0.5 microM. Dose of PGE2 above 0.5 microM caused less than maximal stimulation. While PGE2 stimulated the formation of inositol trisphosphate dose dependently in the range between 1 nM and 10 microM. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a protein kinase C (PKC)-activating phorbol ester, which by itself had little effect on 45Ca2+ influx, significantly suppressed the 45Ca2+ influx induced by PGE2 in a dose-dependent manner between 1 nM and 1 microM. 4 alpha-Phorbol 12,13-didecanoate, a phorbol ester which is inactive for PKC, showed little effect in this capacity. Staurosporine, a PKC inhibitor, enhanced the PGE2-induced 45Ca2+ influx. On the other hand, dibutyryl cAMP had little effect on the 45Ca2+ influx induced by PGE2. Our data suggest that PGE2 regulates Ca2+ influx through self-induced activation of PKC. These results indicate that there is an autoregulatory mechanism in signal transduction by PGE2, and PGE2 modulates osteoblast functions through the interaction between Ca2+ influx and phosphoinositide hydrolysis in osteoblast-like cells.

Proliferative effect of PGD2 on osteoblast-like cells; independent activation of pertussis toxin-sensitive GTP-binding protein from PGE2 or PGF2 alpha

PGD2 stimulated DNA synthesis and decreased alkaline phosphatase activity dose-dependently between 10 nM and 10 microM in osteoblast-like MC3T3-E1 cells. PGD2 had little effect on cAMP production, but caused very rapid enhancement of phosphoinositide (PI) hydrolysis dose-dependently between 10 nM and 10 microM. The formation of inositol trisphosphate (IP3) induced by PGD2 reached the peak within 1 min and decreased thereafter, which is more rapid than that induced by PGE2 or PGF2 alpha and both PGE2 and PGF2 alpha affected PGD2-induced IP3 formation additively. Pertussis toxin (PTX) inhibited both PGD2-induced formation of inositol phosphates and DNA synthesis. The degree of these PTX (1 micrograms/ml)-induced inhibitions was similar. In addition, neomycin, a phospholipase C inhibitor, inhibited PGD2-induced DNA synthesis as well as the formation of IP3, and the patterns of both inhibitions were similar. In the cell membranes, PTX-catalyzed ADP-ribosylation of a 40-kDa protein was significantly attenuated by pretreatment of PGD2. Time course of the attenuation of PTX-catalyzed ADP-ribosylation by PGD2 was apparently different from that by PGE2 or PGF2 alpha. These results indicate that PGD2 activates PTX-sensitive GTP-binding protein independently from PGE2 or PGF2 alpha and stimulates PI hydrolysis resulting in proliferation of osteoblast-like cells.

Protein kinase C activation amplifies prostaglandin F2 alpha-induced prostaglandin E2 synthesis in osteoblast-like cells

In cloned osteoblast-like cells, MC3T3-E1, prostaglandin F2 alpha (PGF2 alpha) stimulated arachidonic acid (AA) release in a dose-dependent manner in the range between 1 nM and 10 microM. 12-O-tetradecanoylphorbol-13-acetate (TPA), a protein kinase C (PKC) activator, which by itself had little effect on AA release, markedly amplified the release of AA stimulated by PGF2 alpha in a dose-dependent manner. 4 alpha-phorbol 12,13-didecanoate, a phorbol ester which is inactive for PKC, showed little effect on the PGF2 alpha-induced AA release. 1-oleoyl-2-acetylglycerol (OAG), a specific activator for PKC, mimicked TPA by enhancement of the AA release induced by PGF2 alpha. H-7, a PKC inhibitor, markedly suppressed the effect of OAG on PGF2 alpha-induced AA release. Quinacrine, a phospholipase A2 inhibitor, showed partial inhibitory effect on PGF2 alpha-induced AA release, while it suppressed the amplification by OAG of PGF2 alpha-induced AA release almost to the control level. Furthermore, TPA enhanced the AA release induced by melittin, known as a phospholipase A2 activator. On the other hand, TPA inhibited the formation of inositol trisphosphate stimulated by PGF2 alpha. Under the same condition, PGF2 alpha indeed stimulated prostaglandin E2 (PGE2) synthesis and TPA markedly amplified the PGF2 alpha-induced PGE2 synthesis as well as AA release. These results indicate that the activation of PKC amplifies PGF2 alpha-induced both AA release and PGE2 synthesis through the potentiation of phospholipase A2 activity in osteoblast-like cells.

Inhibitory effect of 17 beta -estradiol on prostaglandin E2-induced phosphoinositide hydrolysis in osteoblast-like cells

We examined the effect of estradiol on PGE2-induced phosphoinositide hydrolysis and cAMP production in cloned osteoblast-like MC3T3-E1 cells. 17 beta -Estradiol pretreatment significantly inhibited the formation of inositol phosphates induced by 10 microM PGE2 in a dose-dependent manner between 1 pM and 10 nM. This effect of 17 beta -estradiol was dependent on the time of pretreatment and submaximum inhibition was observed at 4 h. However, 17 beta -estradiol had little effect on the formation of inositol phosphates induced by 20 mM NaF, a GTP-binding protein activator. The cAMP production induced by PGE2 was not influenced by 17 beta -estradiol. These results suggest that 17 beta -estradiol modulates the signal transduction by PGE2 and that the effect seems to be exerted between PGE2 receptor and the GTP-binding protein coupled to phospholipase C in osteoblast-like MC3T3-E1 cells.

Cross-talk regulation between cyclic AMP production and phosphoinositide hydrolysis induced by prostaglandin E2 in osteoblast-like cells

In cloned osteoblast-like MC3T3-E1 cells, PGE2 stimulated both cAMP accumulation and the formation of inositol trisphosphate (IP3) dose dependently. The cAMP accumulation showed the peak value at 5 min and decreased thereafter, whereas the IP3 formation reached a plateau almost within 10 min and sustained it up to 30 min. The effect of PGE2 on cAMP accumulation (EC50 was 80 nM) was more potent than that on IP3 formation (EC50 was 0.8 microM). 12-O-Tetradecanoyl-phorbol-13-acetate (TPA), a protein kinase C (PKC)-activating phorbol ester, reduced the PGE2-induced cAMP accumulation, whereas 4 alpha-phorbol 12,13-didecanoate, a PKC-nonactivating phorbol ester, had little effect on the cAMP accumulation. 1-Oleoyl-2-acetyl-glycerol, a specific activator for PKC, inhibited PGE2-induced cAMP accumulation. TPA had little effect on cAMP accumulation induced by forskolin or NaF, a GTP-binding protein activator. So, the effect of TPA is presumed to be exerted at the point between the PGE2 receptor and Gs. On the other hand, forskolin and dibutyryl cAMP had little effect on the IP3 formation stimulated by PGE2. H-7, a PKC inhibitor, enhanced the PGE2-induced cAMP accumulation in comparison with HA1004, a control for H-7. Our data suggest that PGE2 regulates cAMP production through self-induced activation of PKC. These results strongly suggest that there is an autoregulatory mechanism in PGE2 signaling, and PGE2 modulates osteoblast functions through a cross-talk interaction between cAMP production and phosphoinositide hydrolysis in osteoblast-like cells.

[Clinical assessment of posterior pituitary function by measurement of unextracted random urine]

To evaluate posterior pituitary function without any provocative examination, vasopressin (AVP) concentrations of random urine were measured by high-sensitive radioimmunoassay (AVP-RIA Kit, Mitsubishi Petrochemical Co., Ltd.). No apparent interference for the AVP measurement in unextracted urine was seen after appropriate dilution of urine sample. Urinary AVP did not degenerate at least for 24 hr at room temperature. AVP concentration of random urine was significantly correlated with AVP excretion in 24hr-urine in normal subjects. In 25 patients with neurogenic diabetes insipidus diagnosed by hypertonic saline infusion test, the AVP concentration in random urine was less than 13 pg/mg Cr. In approximately 1% of the normal subjects the urinary AVP level was below this range. Therefore, more intensive examinations should be planned to rule out the failure of AVP secretion in the case of such low AVP level in random urine. We also investigated the physiological changes in AVP secretion in 815 children and 352 pregnant women by measurement of urinary AVP. Even a large number of samples could be measured using this simple procedure.

Nucleotide deletion resulting in frameshift as a possible cause of complete thyroxine-binding globulin deficiency in six Japanese families

Complete T4-binding globulin deficiency (TBG-CD) is inherited in an X-linked fashion. A nucleotide substitution has been shown to cause this hereditary condition in caucasians of French Canadian origin. Heterogeneity in molecular mechanisms for TBG-CD has also been reported. Genomic DNA from a Japanese male exhibiting TBG-CD was subjected to polymerase chain reaction, and the generated DNA fragments were sequenced. A single nucleotide deletion was found in the first base of the codon for amino acid 352 of the common-type TBG molecule. This mutation causes a frameshift in translation and premature termination. Compared with common-type TBG, the mutated polypeptide results in 1) 22 different amino acids on its carboxy-terminus, 2) a 22-amino acid truncation, and 3) the absence of a potential N-linked glycosylation site. These alterations may lead to profound changes in the secondary and tertiary structures of the molecule. To ascertain the presence of this nucleotide deletion in the genomic DNA of affected subjects, a mutated primer was designed which together with the nucleotide deletion produced a new endonuclease restriction site in the polymerase chain reaction fragment. Results revealed the presence of the mutation in genomic DNA of the subject, and his mother was shown to have both mutant and normal alleles. The same mutation was also detected in five other unrelated families carrying TBG-CD. This mutation may be frequent in Japanese subjects with TBG-CD.

Possible involvement of endogenous opioid peptides in the inhibition of arginine vasopressin release by gamma-aminobutyric acid in conscious rats

We examined the effects of gamma-aminobutyric acid (GABA) and naloxone, a potent opioid antagonist, on arginine vasopressin (AVP) secretion in conscious rats in order to study the relationship of GABA and endogenous opioid peptides in the regulation of AVP secretion. Intracerebroventricular administration of GABA caused a time- and dose-dependent decrease in the plasma concentration of AVP that was elevated by hypertonic saline injection, whereas it did not affect the basal AVP. Pretreatment with naloxone (10 mg/kg) significantly attenuated the inhibitory effect of GABA (100 micrograms) on AVP release. These results suggest that GABA produces an inhibition of AVP release stimulated by hypertonic saline, and that this inhibitory effect may be mediated at least in part by the endogenous opioid systems.

Centrally administered galanin inhibits osmotically stimulated arginine vasopressin release in conscious rats

The effect of centrally administered galanin on arginine vasopressin (AVP) release was investigated in conscious rats. Intracerebroventricular injection of porcine galanin suppressed hypertonic saline-induced increase in plasma AVP in a dose-dependent manner (12.5-100 pmol/rat) at 10 min after the injection. Pretreatment with subcutaneous injection of naloxone (1 mg/100 g b.wt.) partially blocked the galanin-induced effect on plasma AVP. These results suggest that central galanin inhibits osmotically stimulated AVP release and endogenous opioids are, at least in part, involved in the mechanism.

Effects of hypergravity on proliferation and differentiation of osteoblast-like cells

We investigated the effects of hypergravity on DNA synthesis and alkaline phosphatase (ALP) activity in cloned osteoblast-like cells, MC3T3-E1. Hypergravity (5 x g) stimulated DNA synthesis in these cells in a time-dependent manner and increased it approximately up to 150% of that of the control (1 x g). 12-O-Tetra-decanoylphorbol-13-acetate (TPA), a protein kinase C activator, and insulin-like growth factor I (IGF-I) enhanced DNA synthesis additively with hypergravity (5 x g). An increase in ALP activity induced by 10% fetal calf serum (FCS) was suppressed by hypergravity (2 x g, 5 x g). Five x g completely suppressed the increase in ALP activity. TPA and hypergravity (2 x g) suppressed the increase in ALP activity induced by FCS additively. Hypergravity (5 x g) showed no significant effect on cAMP nor cGMP production in these cells, but increased prostaglandin E2 (PGE2) production. Exogenous PGE2 stimulated DNA synthesis in these cells but had little effect on 10% FCS-induced ALP activity. These results suggest that hypergravity stimulates proliferation but suppresses differentiation of osteoblast-like cells through a pathway independent of the activation of protein kinase C and the production of cyclic nucleotides, and that hypergravity and IGF-I stimulate proliferation of these cells through an independent signal transduction pathway. Moreover, our data strongly suggest that PGE2 mediates the signalling of hypergravity on the proliferation of osteoblast-like cells.

Molecular cloning and primary structure of rat thyroxine-binding globulin

Rat thyroxine-binding globulin (TBG) cDNAs were isolated from a rat liver cDNA library by using a human TBG cDNA as a probe. From two overlapping cDNA inserts, an aligned cDNA sequence of 1714 nucleotides was obtained. There was 70% homology with human TBG cDNA over the span of 1526 nucleotides. In order to confirm that the cloned cDNA encodes rat TBG and to localize the NH2-terminal amino acid of the mature molecule, the protein was purified by affinity chromatography and subjected to direct protein microsequencing. The NH2-terminal amino acid sequence was identical with that deduced from the nucleotide sequence. The rat TBG cDNA sequenced consisted of a truncated leader sequence (35 nucleotides), the complete sequence encoding the mature protein (1194 nucleotides) and the 3'-untranslated region (485 nucleotides), containing two polyadenylation signals. It was deduced that rat TBG consists of 398 amino acids (Mr = 44,607), three NH2-terminal residues more than human TBG, with which it shares 76% homology in primary structure. Of the six potential N-glycosylation sites, four are located in conserved positions compared to human TBG. Northern blot analysis of rat liver revealed an approximately 1.8-kilobase TBG mRNA. Its amount increased markedly following thyroidectomy and decreased with thyroxine treatment in a dose-dependent manner.

Prolactin secretion in patients with idiopathic diabetes insipidus

It has been demonstrated that hyperprolactinemia is sometimes present even in patients with idiopathic diabetes insipidus (DI). In this study, we examined the responses of serum prolactin (PRL) to hypertonic saline infusion and TRH injection in 11 patients with idiopathic DI diagnosed by clinical examinations. Serum sodium in these patients (147.5 +/- 3.2 mEq/L) was significantly higher at baseline than in normal subjects (139.7 +/- 2.4 mEq/L). The plasma arginine vasopressin (AVP) level was significantly lower in DI (0.42 +/- 0.24 pg/ml) at baseline than in normal subjects (2.53 +/- 1.03 pg/ml). However, the serum PRL level in both groups did not differ significantly except in one patient with idiopathic DI (35.6 ng/ml). There was no significant correlation between the basal serum sodium and basal serum PRL in either group. After an infusion of hypertonic saline, the serum sodium level gradually increased to 155.6 +/- 3.4 mEq/L in DI and to 146.5 +/- 4.3 mEq/L in the normal subjects. However, this increase did not affect PRL secretion in either group. PRL response to TRH was essentially normal in all patients with idiopathic DI. These results indicate that the secretion of PRL is not generally affected by chronic mild hypernatremic hypovolemia in the patients with idiopathic DI.

Aggravation of subclinical diabetes insipidus during pregnancy

BACKGROUND

Transient polyuria and polydipsia during pregnancy are rare, and their cause is not entirely clear. Possible explanations include the exacerbation of preexisting abnormalities in the secretion or action of vasopressin and abnormally large increases in plasma vasopressinase activity.

METHODS

We studied two women in whom overt polyuria and polydipsia developed during the third trimester of pregnancy and disappeared after delivery. The secretion and action of vasopressin were studied both when the women had polyuria and polydipsia and later, when their water intake and urine volume were normal.

RESULTS

One patient had partial nephrogenic diabetes insipidus. She had little increase in urine osmolality in response to water deprivation, hypertonic-saline infusion, and vasopressin injection and no response to desmopressin acetate (1-deamino-8-D-arginine vasopressin) during the immediate postpartum period. Her basal and stimulated plasma vasopressin concentrations were high (16.5 to 203.4 pmol per liter) before and during hypertonic-saline infusion 30 months post partum. The other patient had partial neurogenic diabetes insipidus. She had subnormal basal plasma vasopressin concentrations, a subnormal increase in the plasma vasopressin level and a subnormal decrease in urine flow in response to the administration of vasopressin, and a normal response to desmopressin. After pregnancy, when her urine volume was normal, she had no increase in plasma vasopressin in response to hypertonic-saline infusion, but she had a normal rise in the plasma vasopressin level and a normal renal response to vasopressin administration.

CONCLUSIONS

Pregnancy may unmask subclinical forms of both nephrogenic and neurogenic diabetes insipidus. This exacerbation may result from both increased vasopressinase activity and diminished renal responsiveness to vasopressin.

A single base substitution in the coding region for neurophysin II associated with familial central diabetes insipidus

To elucidate the molecular mechanism of familial central diabetes insipidus (FDI), we sequenced the arginine vasopressin-neurophysin II (AVP-NPII) gene in 2 patients belonging to a pedigree that is consistent with an autosomal dominant mode of inheritance. 10 patients with idiopathic central diabetes insipidus (IDI) and 5 normals were also studied. The AVP-NPII gene, locating on chromosome 20, consists of three exons that encode putative signal peptide, AVP, NPII, and glycoprotein. Using polymerase chain reaction, fragments including the promoter region and all coding regions were amplified from genomic DNA and subjected to direct sequencing. Sequences of 10 patients with IDI were identical with those of normals, while in 2 patients with FDI, a single base substitution was detected in one of two alleles of the AVP-NPII gene, indicating they were heterozygotes for this mutation. It was a G----A transition at nucleotide position 1859 in the second exon, resulting in a substitution of Gly for Ser at amino acid position 57 in the NPII moiety. It was speculated that the mutated AVP-NPII precursor or the mutated NPII molecule, through their conformational changes, might be responsible for AVP deficiency.

Possible coupling of prostaglandin E2 receptor with pertussis toxin-sensitive guanine nucleotide-binding protein in osteoblast-like cells

In cloned osteoblast-like cells, MC3T3-E1, prostaglandin E2 (PGE2) stimulated the formation of inositol phosphates in a dose-dependent manner in the range between 10 nM and 10 microM. Pertussis toxin inhibited the effect of PGE2 dose-dependently in the range between 1 ng/ml and 1 micrograms/ml. In the cell membranes, pertussis toxin catalyzed ADP-ribosylation of a protein with an Mr of about 40,000. Pretreatment of membranes with 10 microM PGE2 in the presence of 2.5 mM MgCl2 and 100 microM GTP markedly attenuated this pertussis toxin-catalyzed ADP-ribosylation of the protein in a time-dependent manner. G12 was detected in these cells by immunoblotting with purified anti-G12 alpha antibodies. The results indicate the possible coupling of PGE2 signalling with pertussis toxin-sensitive GTP-binding protein, which is probably G12, in osteoblast-like cells.