Binding of Herpes Simplex Virus Glycoprotein B (gB) to Paired Immunoglobulin-Like Type 2 Receptor α Depends on Specific Sialylated O-Linked Glycans on gB

Paired immunoglobulin-like type 2 receptor α (PILRα) is an inhibitory receptor expressed on both hematopoietic and nonhematopoietic cells. Its binding to a cellular ligand, CD99, depends on the presence of sialylated O-linked glycans on CD99. Glycoprotein B (gB) of herpes simplex virus type 1 (HSV-1) binds to PILRα, and this association is involved in HSV-1 infection. Here, we found that the presence of sialylated O-glycans on gB is required for gB to associate with PILRα. Furthermore, we identified two threonine residues on gB that are essential for the addition of the principal O-glycans acquired by gB and that are also essential for the binding of PILRα to gB.Four envelope glycoproteins, gB, gD, gH, and gL, are required for herpes simplex virus type 1 (HSV-1) to enter into host cells. Paired immunoglobulin (Ig)-like type 2 receptor α (PILRα) binds to gB and functions as an entry receptor during HSV-1 infection in concert with an interaction between gD and gD receptors (10). An X-ray structure of gB suggested that it is a class III fusogenic glycoprotein with internal fusion loops (6), and evidence that these loops can associate with lipid membranes was presented recently (5). The interaction between PILRα and gB might help the fusion loops of gB to associate with cellular membranes. However, it has remained unclear how PILRα associates with gB. PILRα also binds to CD99, which is expressed mainly on T-cell subsets (12). Specific O-glycan structures on CD99 are required for recognition of CD99 by PILRα (15). Here, we addressed whether O-glycans on gB are involved in the association between PILRα and gB. One approach was to use benzyl-α-GalNAc, which specifically blocks the extension of O-glycans through its ability to compete with GalNAc-O-Ser/Thr, a substrate for β1-3-galactosyl-transferases, which generate core 1 structures of O-glycans (8). 293T cells transfected with gB (HSV-1 strain KOS) were treated with benzyl-α-GalNAc (Sigma) at 37°C for 48 h and were then stained with PILRα-Ig (15) or anti-gB monoclonal antibody ([MAb] clone 1105; Rumbaugh-Goodwin Institute) (Fig. ​(Fig.1A).1A). Recognition of gB by PILRα was abrogated almost completely by the treatment of gB transfectants with benzyl-α-GalNAc, whereas cell surface expression of gB was not affected. Because benzyl-α-GalNAc functions competitively, the weak binding of PILRα-Ig to benzyl-α-GalNAc-treated gB transfectants might have been due to an insufficient effect of benzyl-α-GalNAc on O-glycans. Benzyl-α-GalNAc did not affect the viability of cells (data not shown). Similarly, Western blot analysis showed that recognition of gB by PILRα-Ig was reduced by treatment with benzyl-α-GalNAc in a dose-dependent manner (Fig. ​(Fig.1B).1B). The molecular weight of gB expressed on cells treated with benzyl-α-GalNAc was slightly lower than that of gB on untreated cells. Thus, the presence of O-glycans on gB is critical for the interaction between PILRα and gB, as it is for the interaction between PILRα and CD99 (15).Open in a separate windowFIG. 1.Requirement of sialylated O-glycans on gB for the interaction with PILRα. (A) 293T cells transfected with gB were treated with benzyl-α-GalNAc (10 mM) and were then stained with PILRα-Ig or anti-gB MAb (solid line). As a control, the transfectants were stained with control Ig or control MAb (dotted line). Histograms show fluorescence intensity measured in arbitrary units on a log scale (x axis) and relative cell number on a linear scale (y axis). (B) Total cell lysates of mock (M)- or gB-transfected 293T cells treated with benzyl-α-GalNAc at the indicated concentrations were separated by SDS-PAGE, followed by blotting with anti-gB antiserum (R74; see reference 2) or PILRα-Ig. (C) gB (left)- or gD (right)-transfected 293T cells were incubated in the presence or absence of sialidase (0.01 U/ml) for 3 h and were stained with PILRα-Ig (solid line), nectin-Ig (solid line), or control Ig (dotted line). Expression of gB or gD was analyzed by using anti-gB MAb (solid line), anti-gD MAb (solid line), or control MAb (dotted line). Histograms show fluorescence intensity measured in arbitrary units on a log scale (x axis) and relative cell number on a linear scale (y axis).HSV gB is sialylated, and sialic acids on virions play an essential role in HSV-1 infection (14). Interestingly, sialic acids on O-glycans are required for recognition of CD99 by PILRα (13, 15). Therefore, we analyzed the involvement of sialic acids on gB in the interaction with PILRα. gB-transfected 293T cells treated with neuraminidase (Vibrio cholera; Roche) at 37°C for 3 h were not recognized by PILRα-Ig, whereas nontreated cells were recognized by PILRα-Ig (Fig. ​(Fig.1C).1C). Neuraminidase treatment did not affect the binding of nectin-Ig to gD transfectants or the cell surface expression of gB and gD.Four to 10% of the amino acids of PILRα are identical to Siglec (sialic acid-binding Ig-like lectin) family proteins, which recognize sialic acids on glycans (15). An arginine residue that is essential for sialic acid recognition by Siglecs is conserved in PILRα. Indeed, PILRα-Ig with this arginine residue mutated did not recognize gB or CD99 (data not shown). These results suggest that sialic acids on gB are involved in the recognition of gB by PILRα, as they are in the recognition of CD99 by PILRα. Along with the result that O glycosylation on gB is important for association with PILRα, sialylated O-glycans on gB are involved in the interaction with PILRα.We analyzed the glycosylation sites on gB that are responsible for recognition by PILRα. Although the NetOGlyc 3.1 algorithm (www.cbs.dtu.dk/services/NetOGlyc/) is useful in predicting potential O glycosylation sites, prediction of O glycosylation sites is still imprecise. The NetOGlyc 3.1 algorithm predicted seven threonine or serine residues (threonines at 37, 44, 53, 64, 67, and 480 and serine at 487) to be potential O glycosylation sites. Of note, five threonines were located near the N terminus. In order to analyze whether this N-terminal region is involved in recognition by PILRα, we constructed a gB chimeric molecule (gB30-115) possessing a BM-40 signal sequence (amino acid residues 1 to 40), a Flag-tag, an N-terminal gB fragment from its signal peptide cleavage site (amino acid residues 30 to 115) containing the five possible O glycosylation sites, and a transmembrane region of mouse PILRα (amino acid residues 196 to 256; GenBank accession number, {"type":"entrez-nucleotide","attrs":{"text":"NM_153510","term_id":"256355021","term_text":"NM_153510"}}NM_153510) to serve as an anchor to cellular membranes. This short N-terminal fragment of gB expressed on the cell surface was stained with both anti-Flag MAb and a PILRα-Ig fusion protein similar to wild-type (WT) gB (Fig. ​(Fig.2).2). In order to identify the amino acid residues of gB that are involved in association with PILRα, we generated a series of mutations of the N-terminal gB fragment. The gB fragment, in which all possible O glycosylation sites were mutated to alanine (gB30-115m), was not recognized by PILRα-Ig, whereas cell surface expression was not affected by these mutations. A revertant that has a threonine at amino acid residue 53 (A53T gB30-115m) was recognized by PILRα-Ig. In contrast, a WT N-terminal gB fragment in which only threonine 53 (T53) was mutated to alanine (T53A gB30-115) was not recognized by PILRα-Ig. Furthermore, the binding of PILRα-Ig to the A53T gB30-115m revertant was abrogated by sialidase or benzyl-α-GalNAc treatment (data not shown). Therefore, T53 is the only threonine within residues 30 to 115 of gB whose O glycosylation is required for the association of gB with PILRα.Open in a separate windowFIG. 2.Mutational analyses of O glycosylation sites in the N terminus domain of gB. Flag-tagged N terminus fragments of gB (amino acid residues 30 to 115) containing five potential O glycosylation sites or point mutations of these possible O glycosylation sites were transfected into 293T cells. The transfectants were stained with control Ig (dotted line) or PILRα-Ig (solid line). Expression of the N terminus domain of gB was analyzed by staining with anti-Flag MAb (solid line) or control MAb (dotted line). Histograms show fluorescence intensity measured in arbitrary units on a log scale (x axis) and relative cell number on a linear scale (y axis).We generated full-length gB in which T53 was mutated to alanine (T53A gB). The single point mutation at T53 partially affected the recognition of full-length gB by PILRα-Ig, whereas cell surface expression of gB was not affected (Fig. ​(Fig.3A).3A). This finding suggests that T53 is a dominant O glycosylation site on gB, which is involved in interactions with PILRα, although additional potential O glycosylation sites other than those near the N terminus might also be involved. Interestingly, gB with a mutation at threonine 480 (T480) in addition to T53 (T53AT480A) was not recognized by PILRα, whereas, similar to T53A gB, gB with an additional mutation at serine 487 (T53AS487A) was recognized by PILRα-Ig. gB with a mutation at T480 alone (T480A) was recognized by PILRα, as was WT gB. None of these mutations affected the cell surface expression of gB. Similar results were obtained using several other cell lines, such as COS cells (data not shown). These data suggest that two O glycosylation sites of gB, T53 and T480, are involved in the association with PILRα.Open in a separate windowFIG. 3.Mutational analyses of O glycosylation sites of full-length gB. (A) 293T cells were transfected with various mutated gBs, and the transfectants were stained with control Ig (dotted line) or PILRα-Ig (solid line). Expression of gB was analyzed by staining with anti-gB MAb (solid line) or control MAb (dotted line). The histograms show fluorescence intensity measured in arbitrary units on a log scale (x axis) and relative cell number on a linear scale (y axis). (B) Membrane proteins prepared from COS-7 cells transfected with WT gB and mutated gBs were boiled or left unheated in SDS sample buffer in reducing or nonreducing conditions, respectively. Samples were separated by SDS-PAGE, followed by blotting with anti-gB MAb.Both T53 and T480 are located in a proline-rich region, which may be important for protein folding (16). It has been reported that functional gB forms oligomers (1, 3, 6). Therefore, we analyzed whether the point mutations of gB affected oligomer formation. The oligomeric structure of gB is resistant in sodium dodecyl sulfate (SDS) sample buffer but is denatured by boiling (9). As shown in Fig. ​Fig.3B,3B, there was no difference in SDS resistance between WT gB and the mutated gBs. This suggests that point mutations of the O glycosylation sites at T53 and T480 of gB did not greatly affect the physical characteristics of gB. Moreover, there was no difference in the molecular weight between WT and mutated gB or in cell surface expression. Because the molecular weight of gB is relatively high and gB has several N glycosylation sites, mutations of one or two O glycosylation sites alone did not affect the total molecular weight of gB. However, the molecular weight of gB expressed in benzyl-α-GalNAc-treated cells was slightly lower than that of gB expressed in nontreated cells (Fig. ​(Fig.1B).1B). Because benzyl-α-GalNAc treatment inhibits synthesis of all the O-glycans on gB, other O glycosylation sites on gB might exist. However, it is noteworthy that only O glycosylation sites at T53 and T480 are involved in association with PILRα.Although mutations at T53 and T480 of gB completely abrogated recognition by PILRα, there is no direct evidence to suggest that these two residues are O glycosylated. In order to analyze O glycosylation on gB, we employed a novel method to label O-linked glycans, using Click-iT O-GalNAz metabolic glycoprotein-labeling reagent (azido-GalNAc) (Invitrogen). Because O-linked glycans generally possess peptide-proximal GalNAc residues (7), we cultured cells transfected with WT gB or mutated gB for 3 days in the presence of azido-GalNAc, which is metabolically incorporated into O-linked glycoproteins (4). gBs were immunoprecipitated with anti-gB MAb, and the azido-GalNAc incorporated into gB was treated with phosphine-Flag, which specifically reacts with the azido-GalNAc (11), followed by detection with anti-Flag MAb by Western blotting. WT gB, T53A-mutated gB, and T480A-mutated gB were blotted with anti-Flag MAb, whereas T53AT480A gB was only weakly blotted with anti-Flag MAb (Fig. ​(Fig.4).4). In contrast, there was no significant difference in the total amount of gB expressed. This result suggests that T53 and T480 of gB are O glycosylated. However, weak detection of O-glycans on the T53AT480A gB suggest the presence of O glycosylation sites other than T53 and T480 on gB.Open in a separate windowFIG. 4.Analysis of O-glycans on WT and mutated gB. O-glycans on gB expressed on 293T cells were metabolically labeled with Ac₄GalNAz and were then immunoprecipitated with anti-gB MAb. The labeled O-glycans in immunoprecipitates were modified with phosphine-Flag and were then analyzed by Western blotting. The labeled O-glycans and total amount of gB were detected by anti-Flag or anti-gB MAb, respectively.PILRα specifically associates with HSV-1 gB (10), but not with other HSV-1 glycoproteins, although some other envelope proteins are known to be O glycosylated. Recently, it was shown that insertion mutations in gB could reduce the binding of gB to PILRα, suggesting that the conformation of gB is also involved in the interaction (2). Therefore, PILRα does not associate with glycans alone and seems to recognize both protein structure and O-glycans (13, 15), which may be a reason that PILRα specifically associates with gB. It is interesting that both T53 and T480 are involved in the interaction with PILRα, because these two residues are widely separated on the polypeptide chain. Because PILRα bound to gB by Western blotting, PILRα might recognize linear epitopes in gB; therefore, PILRα might bind to the two sites in gB independently. Alternatively, elements of higher-order structure retained in the unreduced samples examined by SDS-polyacrylamide gel electrophoresis (PAGE) (Fig. ​(Fig.1B)1B) could have been necessary for the binding of PILRα-Ig to the blots. Thus, the binding of PILRα might depend upon the close proximity of the O-glycans attached to T53 and T480 in the trimeric conformation of gB. Determination of the structure of gB associated with PILRα will facilitate understanding the mechanism of membrane fusion during HSV-1 infection.     

Evaluation of the importance of lactate for the activation of ethanolic fermentation in lettuce roots in anoxia

According to the Davies–Roberts hypothesis, plants primarily respond to oxygen limitation by a burst of lactate production and the resulting pH drop in the cytoplasm activates ethanolic fermentation. To evaluate this system in lettuce ( Lactuca sativa L.), seedlings were subjected to anoxia and in vitro activities of alcohol dehydrogenase (ADH, EC 1.1.1.1), pyruvate decarboxylase (PDC, EC 4.1.1.1) and lactate dehydrogenase (LDH, EC 1.1.1.27) and concentrations of ethanol, acetaldehyde and lactate were determined in roots of the seedlings. The in vitro activities of ADH and PDC in the roots increase in anoxia, whereas no significant increase was measured in LDH activity. At 6 h, the ADH and PDC activities in the roots kept in anoxia were 2.8- and 2.9-fold greater than those in air, respectively. Ethanol and acetaldehyde in the roots accumulated rapidly in anoxia and increased 8- and 4-fold compared with those in air by 6 h, respectively. However, lactate concentration did not increase and an initial burst of lactate production was not found. Thus, ethanol and acetaldehyde production occurred without an increase in lactate synthesis. Treatments with antimycin A and salicylhydroxamic acid, which are respiratory inhibitors, to the lettuce seedlings in the presence of oxygen increased the concentrations of ethanol and acetaldehyde but not of lactate. These results suggest that ethanolic fermentation may be activated without preceding activation of lactate fermentation and may be not regulated by oxygen concentration directly.     

Role of caveolin-1 and cytoskeletal proteins, actin and vimentin, in adipogenesis of bovine intramuscular preadipocyte cells

We investigated the involvement of caveolin-1 and the cytoskeletal proteins, actin and vimentin, in the adipogenesis of bovine intramuscular preadipocyte (BIP) cells. Immunoblot analysis demonstrated that levels of caveolin-1 and actin gradually increased during adipose conversion in BIP cells, whereas a slight decrease was observed for vimentin. We found that part of the vimentin was clearly distributed to caveolin-1-enriched membrane fractions in BIP cells, but actin was not. During adipogenesis of BIP cells, treatment with the tubulin depolymerizer, nocodazole, significantly increased intracellular triglyceride accumulation compared to non-treated cells. Immunocytochemical analysis showed that actin microfilaments were significantly disrupted in nocodazole-treated cells. Also, a decrease in the localization of vimentin in caveolin-1-enriched fractions and a failure of vimentin to co-immunoisolate with caveolin-1 were observed in nocodazole-treated cells. These results suggest that a rearrangement of cytoskeletal proteins has a role in the intracellular accumulation of lipid droplets during adipogenesis of BIP cells.     

Characterization of ppGalNAc-T18, a member of the vertebrate-specific Y subfamily of UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases

The first step of mucin-type O-glycosylation is catalyzed by members of the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (ppGalNAc-T; EC 2.4.1.41) family. Each member of this family has unique substrate specificity and expression profiles. In this report, we describe a new subfamily of ppGalNAc-Ts, designated the Y subfamily. The Y subfamily consists of four members, ppGalNAc-T8, -T9, -T17 and -T18, in which the conserved YDX(5)WGGENXE sequence in the Gal/GalNAc-T motif of ppGalNAc-Ts is mutated to LDX(5)YGGENXE. Phylogenetic analysis revealed that the Y subfamily members only exist in vertebrates. All four Y subfamily members lack in vitro GalNAc-transferase activity toward classical substrates possibly because of the UDP-GalNAc-binding pocket mutants. However, ppGalNAc-T18, the newly identified defining member, was localized in the endoplasmic reticulum rather than the Golgi apparatus in lung carcinoma cells. The knockdown of ppGalNAc-T18 altered cell morphology, proliferation potential and changed cell O-glycosylation. ppGalNAc-T18 can also modulate the in vitro GalNAc-transferase activity of ppGalNAc-T2 and -T10, suggesting that it may be a chaperone-like protein. These findings suggest that the new Y subfamily of ppGalNAc-Ts plays an important role in protein glycosylation; characterizing their functions will provide new insight into the role of ppGalNAc-Ts.     

Higher susceptibility to osmolality of the medaka (Oryzias latipes) mutants in orthologue genes of mammalian skin transglutaminases

Transglutaminase (TG) is an essential enzyme to catalyze cross-linking reactions of epidermal proteins. Recently, we biochemically characterized human skin TG orthologues for medaka (Oryzias latipes), a model fish. By genome editing, gene-modified fishes for the two orthologues were obtained, both of which lack the ordinal enzymes. These fish appeared to exhibit higher susceptibility to osmolality at the period of larvae.     

REFOLDdb: a new and sustainable gateway to experimental protocols for protein refolding

Background

More than 7000 papers related to “protein refolding” have been published to date, with approximately 300 reports each year during the last decade. Whilst some of these papers provide experimental protocols for protein refolding, a survey in the structural life science communities showed a necessity for a comprehensive database for refolding techniques. We therefore have developed a new resource – “REFOLDdb” that collects refolding techniques into a single, searchable repository to help researchers develop refolding protocols for proteins of interest.

Results

We based our resource on the existing REFOLD database, which has not been updated since 2009. We redesigned the data format to be more concise, allowing consistent representations among data entries compared with the original REFOLD database. The remodeled data architecture enhances the search efficiency and improves the sustainability of the database. After an exhaustive literature search we added experimental refolding protocols from reports published 2009 to early 2017. In addition to this new data, we fully converted and integrated existing REFOLD data into our new resource. REFOLDdb contains 1877 entries as of March 17^th, 2017, and is freely available at http://p4d-info.nig.ac.jp/refolddb/.

Conclusion

REFOLDdb is a unique database for the life sciences research community, providing annotated information for designing new refolding protocols and customizing existing methodologies. We envisage that this resource will find wide utility across broad disciplines that rely on the production of pure, active, recombinant proteins. Furthermore, the database also provides a useful overview of the recent trends and statistics in refolding technology development.

     

Structure and host recognition of serotype 13 glycopeptidolipid from Mycobacterium intracellulare

The Mycobacterium avium-M. intracellulare complex (MAIC) is divided into 28 serotypes by a species-specific glycopeptidolipid (GPL). Previously, we clarified the structures of serotype 7 GPL and two methyltransferase genes (orfA and orfB) in serotype 12 GPL. This study elucidated the chemical structure, biosynthesis gene, and host innate immune response of serotype 13 GPL. The oligosaccharide (OSE) structure of serotype 13 GPL was determined to be 4-2'-hydroxypropanoyl-amido-4,6-dideoxy-β-hexose-(1 → 3)-4-O-methyl-α-L-rhamnose-(1 → 3)-α-L-rhamnose-(1 → 3)-α-L-rhamnose-(1 → 2)-α-L-6-deoxy-talose by using chromatography, mass spectrometry, and nuclear magnetic resonance (NMR) analyses. The structure of the serotype 13 GPL was different from those of serotype 7 and 12 GPLs only in O-methylations. We found a relationship between the structure and biosynthesis gene cluster. M. intracellulare serotypes 12 and 13 have a 1.95-kb orfA-orfB gene responsible for 3-O-methylation at the terminal hexose, orfB, and 4-O-methylation at the rhamnose next to the terminal hexose, orfA. The serotype 13 orfB had a nonfunctional one-base missense mutation that modifies serotype 12 GPL to serotype 13 GPL. Moreover, the native serotype 13 GPL was multiacetylated and recognized via Toll-like receptor 2. The findings presented here imply that serotypes 7, 12, and 13 are phylogenetically related and confirm that acetylation of the GPL is necessary for host recognition. This study will promote better understanding of the structure-function relationships of GPLs and may open a new avenue for the prevention of MAIC infections.     

Cleavage of syndecan-1 by membrane type matrix metalloproteinase-1 stimulates cell migration

The transmembrane heparan sulfate proteoglycan syndecan-1 was identified from a human placenta cDNA library by the expression cloning method as a gene product that interacts with membrane type matrix metalloproteinase-1 (MT1-MMP). Co-expression of MT1-MMP with syndecan-1 in HEK293T cells promoted syndecan-1 shedding, and concentration of cell-associated syndecan-1 was reduced. Treatment of cells with MMP inhibitor BB-94 or tissue inhibitor of MMP (TIMP)-2 but not TIMP-1 interfered with the syndecan-1 shedding promoted by MT1-MMP expression. In contrast, syndecan-1 shedding induced by 12-O-tetradecanoylphorbol-13-acetate treatment was inhibited by BB-94 but not by either TIMP-1 or TIMP-2. Shedding of syndecan-1 was also induced by MT3-MMP but not by other MT-MMPs. Recombinant syndecan-1 core protein was shown to be cleaved by recombinant MT1-MMP or MT3-MMP preferentially at the Gly245-Leu246 peptide bond. HT1080 fibrosarcoma cells stably transfected with the syndecan-1 cDNA (HT1080/SDC), which express endogenous MT1-MMP, spontaneously shed syndecan-1. Migration of HT1080/SDC cells on collagen-coated dishes was significantly slower than that of control HT1080 cells. Treatment of HT1080/SDC cells with BB-94 or TIMP-2 induced accumulation of syndecan-1 on the cell surface, concomitant with further retardation of cell migration. Substitution of Gly245 of syndecan-1 with Leu significantly reduced shedding from HT1080/SDC cells and cell migration. These results suggest that the shedding of syndecan-1 promoted by MT1-MMP through the preferential cleavage of Gly245-Leu246 peptide bond stimulates cell migration.     

In vivo administration of 1,25-dihydroxyvitamin D3 suppresses the expression of RANKL mRNA in bone of thyroparathyroidectomized rats constantly infused with PTH

It is known that pharmacological or toxic doses of vitamin D induce bone resorption both in vivo and in vitro, whereas physiological doses of the vitamin have a protective effect on bone in vivo. To investigate the discrepancies of the dose-dependent effect of vitamin D on bone resorption, we examined the in vivo effect of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] on the expression of the receptor activator of nuclear factor-kappaB (NF-kappaB) ligand (RANKL) and osteoprotegerin (OPG) mRNAs in bone of thyroparathyroidectomized (TPTX) rats infused with or without parathyroid hormone (PTH). Continuous infusion of 50 ng/h of PTH greatly increased the expression of RANKL mRNA in bone of TPTX rats. Expression of OPG mRNA was not altered by PTH infusion. When graded doses of 1,25(OH)(2)D(3) was daily administered orally for 14 days to normocalcemic TPTX rats constantly infused with PTH, 0.01 and 0.1 microg/kg of 1,25(OH)(2)D(3) inhibited the PTH-induced RANKL mRNA expression, but 0.5 microg/kg of the vitamin did not inhibit it. Regulator of G protein signaling-2 (RGS-2) gene expression was suppressed by 1,25(OH)(2)D(3) dose-dependently, but PTH/PTHrP receptor mRNA expression was not altered. Bone morphometric analyses revealed that 1,25(OH)(2)D(3) suppressed PTH-induced osteoclast number in vivo. These results suggest that pharmacological or toxic doses of 1,25(OH)(2)D(3) stimulate bone resorption by inducing RANKL, but a certain range of physiological doses of the vitamin inhibit PTH-induced bone resorption, the latter mechanism appeared to be mediated, at least in part, by the suppression of the PTH/PTHrP receptor-mediated signaling.