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L D Quarles

Publications and source records attributed to L D Quarles.

At least 37 records · Page 2Linked to original sources
PubMed1998

Intrinsic mineralization defect in Hyp mouse osteoblasts.

X-linked hypophosphatemia (XLH) is caused by inactivating mutations of PEX, an endopeptidase of uncertain function. This defect is shared by Hyp mice, the murine homologue of the human disease, in which a 3' Pex deletion has been documented. In the present study, we report that immortalized osteoblasts derived from the simian virus 40 (SV40) transgenic Hyp mouse (TMOb-Hyp) have an impaired capacity to mineralize extracellular matrix in vitro. Compared with immortalized osteoblasts from the SV40 transgenic normal mouse (TMOb-Nl), osteoblast cultures from the SV40 Hyp mouse exhibit diminished 45Ca accumulation into extracellular matrix (37 +/- 6 vs. 1,484 +/- 68 counts . min-1 . microgram protein-1) and reduced formation of mineralization nodules. Moreover, in coculture experiments, we found evidence that osteoblasts from the SV40 Hyp mouse produce a diffusible factor that blocks mineralization of extracellular matrix in normal osteoblasts. Our findings indicate that abnormal PEX in osteoblasts is associated with the accumulation of a factor(s) that inhibits mineralization of extracellular matrix in vitro.

Alkaline Phosphatase↗
PubMed1998

Predictors of short-term changes in serum intact parathyroid hormone levels in hemodialysis patients: role of phosphorus, calcium, and gender.

Several factors have been identified as important in the pathogenesis of secondary hyperparathyroidism in end-stage renal disease, including serum calcium, phosphorus, and calcitriol. To examine the independent effects of key factors, we prospectively studied 52 new hemodialysis patients with mild secondary hyperparathyroidism (PTH, 110-670 pg/mL) treated with a standardized regimen of calcium supplements, phosphorus binders, and no vitamin D derivatives. We used simple and multivariable linear regression analysis to examine the relationship between changes in PTH (deltaPTH) levels observed over a 4-week period and various biochemical and demographic variables. By simple linear regression we found that changes in serum phosphorus (r2 = 0.31; beta = 41.6; P = 0.0001), initial phosphorus concentration (r2 = 0.15; beta = 33.4; P = 0.005), initial PTH level (r2 = 0.29; beta = 0.58; P = 0.0001), changes in serum calcium (r2 = 0.12; beta = -74.0; P = 0.01), and gender (r2 = 0.07; beta = 76.1; P = 0.05) were significantly associated with deltaPTH. However, upon multivariable regression analysis, only the changes in phosphorus (partial r2 = 0.31; beta = 37.0; P = 0.0001), initial PTH level (partial r2 = 0.23; beta = 0.50; P = 0.0001), and gender (partial r2 = 0.05; beta = 63.1; P = 0.02) remained significantly associated with deltaPTH. Neither the serum concentration of 1,25-dihydroxyvitamin D3, bicarbonate, aluminum, or albumin nor changes in the serum bicarbonate concentration, the presence of diabetes, KT/V, or age were significantly associated with the deltaPTH. Our findings are consistent with independent effects of phosphorus and gender on parathyroid gland function in patients with dialysis-dependent renal failure through mechanisms that remain to be defined.

Adult↗
PubMed1997

Identification of putative transmembrane receptor sequences homologous to the calcium-sensing G-protein-coupled receptor.

The sensing of extracellular calcium is a general paradigm for regulating diverse cellular functions in many tissues. A calcium-sensing receptor (Casr) belonging to the metabotropic glutamate family of G-protein-coupled receptors (GPCR) that transduces the effects of extracellular calcium in the parathyroid gland as well as other tissues has been identified. The diversity of GPCR families and the recent finding of calcium sensing in cells lacking the known Casr suggest the existence of additional receptors related to Casr. By polymerase chain reaction (PCR) amplification and screening of genomic libraries, we have identified multiple Casr-related sequences (Casr-rs) in the mouse. Using primers designed to regions of the first and third intracellular loops of Casr, we initially PCR amplified a 497-bp Casr-related sequence (Casr-rs1) with high homology to Casr. The deduced protein sequence of Casr-rs1 is 63% similar and 40% identical to Casr over the available transmembrane region. We screened a mouse genomic library with a Casr-rs1 probe and identified two additional Casr-related sequences (Casr-rs2 and Casr-rs3). In the predicted transmembrane domain, Casr-rs2 and Casr-rs3 are 95% identical to Casr-rs1. We mapped Casr-rs1 to mouse Chromosome (Chr) 7 by interspecific backcross analysis, whereas the known Casr localizes to mouse Chr 16. By fluorescence in situ hybridization, Casr-rs2 also localized to mouse Chr 7 and Casr-rs3 mapped to mouse Chr 4. We were able to distinquish Casr-rs1 from Casr-rs2 by PCR using specific primers, suggesting that they are distinct genes clustered on Chr 7. By RT-PCR, we identified additional Casr-rs transcripts in mouse kidney, brain, testis, embryo, and MC3T3-E1 osteoblasts, but not in lung or liver. The homologous sequence in mouse kidney, embryo, and MC3T3-E1 osteoblasts, designated Casr-rs4, has a deduced amino acid sequence that is 100% similar and 97% identical to that of Casr-rs1. The sequence amplified from mouse brain, Casr-rs5, has a deduced protein sequence that is 96% similar and 92% identical to that of Casr-rs1. Our findings establish the existence of a novel multimembered family of Casr-related sequences in the mouse which may encode receptors that transduce responses to diverse extracellular cations.

Amino Acid Sequence↗
PubMed1997

Quantitative analysis of the calcium-sensing receptor messenger RNA in parathyroid adenomas.

BACKGROUND: In primary hyperparathyroidism, hypercalcemia fails to suppress adequately secretion of parathyroid hormone by the parathyroid gland, which may result from failure of the cell-surface calcium receptor (CaR) to sense calcium correctly. Quantification of mRNA concentrations should provide important information on the role of expression of Call in primary hyperparathyroidism. METHODS: We have developed a quantitative reverse transcriptase-polymerase chain reaction assay with a competitive template (CaR-M). Amplified cDNAs for CaR and CaR-M are quantified, and the concentration of CaR mRNA is determined from the ratio of CaR-M/CaR versus known CaR-M concentrations. RESULTS: In parathyroid adenomas (n = 12) the CaR mRNA was 19.2 +/- 2.4 (mean +/- SE) fg/ng total RNA (range, 7.4 to 32.8 fg/ng). Extracellular ionized calcium levels ranged from 1.38 to 1.74 mmol/L (normal 1.19 to 1.31 mmol/L) and parathyroid hormone from 69 to 345 pg/ml (normal, 14 to 65 pg/ml). In spite of the wide variability in CaR expression in the various adenomas, there was no correlation between mRNA and either extracellular ionized calcium (r2 = 0.013) parathyroid hormone levels (r2 = 0.001). Normal human parathyroid glands gave values of 8.0 and 16.6 fg/ng, whereas normal bovine parathyroid glands had a mean of 20 +/- 0.6 fg/ng (n = 4). CONCLUSIONS: There is no apparent relationship between CaR mRNA levels in adenomas and preoperative Ca and PTH levels. Our findings suggest that defective Ca sensing in adenomas may involve post-translational modification or signal transduction distal to the receptor. Our highly sensitive assay for CaR mRNA should prove useful in examining further the role of CaR in Ca sensing in parathyroid tissue.

Adenoma↗
PubMed1997

Specific coupling of a cation-sensing receptor to G protein alpha-subunits in MDCK cells.

Extracellular cations such as Ca2+ stimulate a G protein-coupled, cation-sensing receptor (CaR). We used microphysiometry to determine whether an extracellular cation-sensing mechanism exists in Madin-Darby canine kidney (MDCK) cells. The CaR agonists Ca2+ and Gd3+ caused cellular activation in a concentration-dependent manner. mRNA for the CaR was identified by reverse transcription and polymerase chain reaction (PCR) using nested CaR-specific primers, identification of an appropriately located restriction site, and sequencing of the subcloned fragment obtained by PCR. G protein activation was evaluated using the GTP photoaffinity label [alpha-32P]GTP azidoanalide (AA-GTP). After stimulation with Gd3+ and cross-linking, plasma membranes were solubilized and immunoprecipitated with antisera specific for Gq/11 alpha and Gi alpha family members. Gd3+ increased incorporation of AA-GTP into Gq/11 alpha precipitates by 146 +/- 48% and into G alpha i-2 and G alpha i-3 to a lesser extent but not into G alpha i-1. Direct effects of Gd3+ on the G proteins were ruled out using partially purified mammalian G proteins expressed in Escherichia coli or Sf9 cells. We conclude that MDCK cells possess a cell-surface CaR that activates Gq/11 alpha, G alpha i-2, and G alpha i-3 but not G alpha i-1.

Affinity Labels↗
PubMed1997

A distinct cation-sensing mechanism in MC3T3-E1 osteoblasts functionally related to the calcium receptor.

The presence of a cation-sensing mechanism in osteoblasts is suggested by the ability of specific cations to stimulate osteoblastic proliferation in culture and to induce de novo bone formation in some experimental models. Our study examines whether extracellular cations stimulate osteoblasts through the recently identified G protein-coupled calcium receptor (CaR). We found that CaR agonists, calcium (Ca2+), gadolinium (Gd3+), aluminum (Al3+), and neomycin, stimulated DNA synthesis in murine-derived MC3T3-E1 preosteoblasts, whereas magnesium (Mg2+), nickel (Ni2+), cadmium (Cd2+), and zinc (Zn2+) had no effect. With the exception of Mg2+, the cation specificities and apparent affinities were similar to that reported for CaR. CaR agonists also stimulated DNA synthesis in C3HT10(1/2) fibroblasts, but not in mesangial PVG, CHO, hepatic HTC, COS-7 cells, or malignant transformed ROS17/2.8 and UMR-106 osteoblasts. In addition, similar to other growth factors, CaR agonists activated transcription of a serum response element luciferase reporter construct (SRE-Luc) stably transfected into MC3T3-E1 osteoblasts, but had no effect on SRE-Luc transfected into CHO and COS-7 cells. We were unable to detect CaR expression by Northern analysis using a mouse CaR-specific probe or to amplify CaR mRNA by reverse transcribed polymerase chain reaction in MC3T3-E1 osteoblasts. These findings suggest that an extra-cellular cation-sensing mechanism is present in murine-derived osteoblasts that is functionally similar to but molecularly distinct from CaR.

Amino Acid Sequence↗
PubMed1997

Cloning and sequencing of human PEX from a bone cDNA library: evidence for its developmental stage-specific regulation in osteoblasts.

Inactivating mutations of the neutral endopeptidase, PEX, have been identified as the cause of X-linked hypophosphatemia (XLH). Though the function of PEX is unknown, current information suggests that impaired renal phosphate conservation in XLH is due to the failure of PEX to either degrade an undefined phosphaturic factor or activate a novel phosphate-conserving hormone. The physiologically relevant target tissue for the XLH mutation has not been identified. An apparent intrinsic defect of osteoblast function in XLH implicates bone as a possible site of PEX expression. In the current investigation, we employed a polymerase chain reaction (PCR) strategy to amplify a PEX cDNA from a human bone cell cDNA library. We found that the human PEX cDNA encodes a 749 amino acid protein belonging to the type II integral membrane zinc-dependent endopeptidase family. The predicted PEX amino acid sequence shares 96.0% identify to the recently cloned mouse Pex cDNA and has 27-38% identity to other members of the metalloendopeptidase family. Using reverse transcriptase (RT)-PCR with PEX-specific primers, we detected PEX transcripts in both human osteosarcoma-derived MG-63 osteoblasts and in differentiated mouse MC3T3-E1 clonal osteoblasts but not in immature MC3T3-E1 preosteoblasts. The association of impaired mineralization of bone in XLH and the apparent developmental stage-specific expression of PEX in osteoblasts suggest that bone is a physiologically relevant site of PEX expression and that PEX may play an active role in osteoblast-mediated mineralization.

3T3 Cells↗
PubMed1997

Developmental regulation of osteocalcin expression in MC3T3-E1 osteoblasts: minimal role of the proximal E-box cis-acting promoter elements.

Osteoblasts undergo a temporal sequence of development characterized by transcriptional upregulation of osteoblast-specific genes. Basic helix-loop-helix (bHLH) transcription factors may control this developmental process through binding to E-box cis-acting elements in developmentally regulated genes. To investigate the role of bHLH proteins in MC3T3-E1 osteoblasts, which undergo a developmental sequence in vitro, we analyzed the transcriptional control of osteocalcin gene expression by stable transfection of an osteocalcin promoter-luciferase chimeric gene (p637OC-luc) and assessed the role of E-box cis-acting elements in osteocalcin promoter by DNA binding assays. We compared our findings in MC3T3-E1 osteoblasts with transient DNA transfections and DNA binding experiments in Ros 17/2.8 osteoblasts. We found that the activity of 637-OC luciferase promoter was low in undifferentiated 5-day-old cultures but increased in parallel with endogenous osteocalcin message expression in mature MC3T3-E1 osteoblasts, consistent with developmental stage-specific transcriptional upregulation of the osteocalcin gene. We identified two putative E-box elements in the proximal osteocalcin promoter, E-box 1 (CACATG) at -102 and E-box 2 (CAGCTG) at position -149. In gel mobility shift assays, factors present in nuclear extracts derived from differentiated osteoblast bound to oligonucleotide probes containing the E-box 1 and E-box 2 elements. Binding to the E-box 2 probe was not specific for the core CAGCTG element, whereas the CACATG site in E-box 1 oligonucleotide was required for specific binding of these nuclear factors. Stable transfection of p637OC-luc containing a mutant E1 site (p637OC-luc E1m), however, did not alter the developmental upregulation of osteocalcin promoter activity in MC3T3-E1 osteoblasts. Moreover, the E-box 1 mutation had no effect on either basal or vitamin D-stimulated activity of the osteocalcin promoter in Ros 17/2.8 osteoblasts in transient transfection experiments. These data suggest that osteoblasts contain underfined factors that bind to the E-box 1 CACATG site in the proximal osteocalcin promoter; however, this E-box element does not play a significant role in the developmental stage-specific regulation of the osteocalcin gene in MC3T3-E1 osteoblasts.

3T3 Cells↗
PubMed1996

Differential regulation of receptor-stimulated cyclic adenosine monophosphate production by polyvalent cations in MC3T3-E1 osteoblasts.

Extracellular cations have paradoxical trophic and toxic effects on osteoblast function. In an effort to explain these divergent actions, we investigated in MC3T3-E1 osteoblasts if polyvalent cations differentially modulate the agonist-stimulated cyclic adenosine monophosphate (cAMP) pathway, an important regulator of osteoblastic function. We found that a panel of cations, including gadolinium, aluminum, calcium, and neomycin, inhibited prostaglandin E1 (PGE)-stimulated cAMP accumulation but paradoxically potentiated parathyroid hormone (PTH)-stimulated cAMP production. In contrast, these cations had no effect on forskolin- or cholera toxin-induced increases in cAMP, suggesting actions proximal to adenylate cyclase and possible modulation of receptor interactions with G proteins. Phorbol 12-myristate 13-acetated (PMA) mimicked the effects of cations on PGE1- and PTH-stimulated cAMP accumulation in MC3T3-E1 cells, respectively, diminishing and augmenting the responses. Moreover, down-regulation of protein kinase C (PKC) by overnight treatment with PMA prevented gadolinium (Gd3+) from attenuating PGE1- and enhancing PTH-stimulated cAMP production, indicating involvement of PKC-dependent pathways. Cations, however, activated signal transduction pathways not coupled to phosphatidylinositol-specific phospholipase C (PI-PLC), since there was no corresponding increase in inositol phosphate formation or intracellular calcium concentrations. In addition, pertussis toxin treatment failed to prevent Gd(3+)-mediated suppression of PGE1-stimulated cAMP, suggesting actions independent of Gm. Thus, polyvalent cations may either stimulate or inhibit hormone-mediated cAMP accumulation in osteoblasts. These differential actions provide a potential explanation for the paradoxical trophic and toxic effects of cations on osteoblast function that occur in vivo under different hormonal conditions.

Adenylate Cyclase Toxin↗
PubMed1996

Calcitriol administration in end-stage renal disease: intravenous or oral?

1,25-Dihydroxyvitamin D deficiency plays an important role in the pathogenesis of secondary hyperparathyroidism, and adequate replacement of this hormone is considered essential to normalize parathyroid gland function and restore bone homeostasis in patients with advanced renal failure. Although initial uncontrolled clinical trials suggested the superiority of intravenous calcitriol treatment, more recent controlled investigations show that different routes (oral versus intravenous), frequency (daily versus intermittent), and dosing (physiological versus pharmacological) of calcitriol administration are clinically equivalent. Overall, the response to calcitriol treatment depends more on the severity of secondary hyperparathyroidism and the presence of confounding variables, such as hyperphosphatemia and acquired abnormalities of parathyroid cell function, than the method of calcitriol administration.

Administration, Oral↗
PubMed1996

Non-suppressible parathyroid hormone secretion is related to gland size in uremic secondary hyperparathyroidism.

To determine the relative importance of parathyroid gland enlargement and alterations in calcium sensing (set-point changes) in the pathogenesis of uremic secondary hyperparathyroidism (2 degrees HPT), we investigated the relationship between estimates of parathyroid gland size and calcium-mediated parathyroid hormone (PTH) suppression in 19 normocalcemic 2 degrees HPT patients on chronic maintenance hemodialysis. We compared our results to calcium-mediated PTH suppression in 12 normal volunteers, 12 patients with familial benign hypocalciuric hypercalcemia (FBHH), a disorder of abnormal calcium sensing, and 9 subjects with primary hyperparathyroidism (1 degree HPT), which is characterized by both calcium set-point abnormalities and parathyroid gland enlargement. We found that the 2 degrees HPT group displayed a distinctive pattern of calcium-mediated PTH suppression characterized by a failure to normally suppress PTH at supraphysiologic ionized calcium concentrations, similar to 1 degree HPT, but without the rightward shift of the calcium-PTH suppression curve that characterizes calcium sensing abnormalities in FBHH and 1 degree HPT. In the patients with 2 degrees HPT, hypercalcemic suppression resulted in an ending PTH (as a percent of baseline) that was significantly higher (39.8 +/- 4.47%), and a slope of the calcium-PTH suppression curve that was significantly less negative (-4.8 +/- 0.53), compared to respective values of 19.4 +/- 1.81% (P = 0.0009) and -9.0 +/- 1.02 (P = 0.001) in normals and 19.1 +/- 2.49% (P = 0.001) and -9.6 +/- 1.11 (P = 0.0006) in FBHH. Values of ending PTH and slope in 2 degrees HPT patients, however, were similar to those found in 1 degree HPT (49.8 +/- 6.35%, P = 0.21 and -4.5 +/- 0.74, P = 0.72). The ionized calcium concentration required to attain half maximal PTH suppression (EC50) in 2 degrees HPT (1.20 +/- 0.02 mmol/liter) was not significantly different from normals (1.25 +/- 0.01 mmol/liter, P = 0.12) but was significantly less than in 1 degree HPT (1.52 +/- 0.02 mmol/liter, P < 0.0001) and in FBHH (1.44 +/- 0.02 mmol/liter, P < 0.0001). More importantly, we found a significant linear correlation between the natural logarithm of gland size and ending PTH suppression (r = 0.71, P < 0.001) and slope of the calcium-PTH curve (r = 0.67, P = 0.002) in 2 degrees HPT. Thus, calcium non-suppressible PTH secretion in 2 degrees HPT does not represent a simple set-point error, but rather correlates with the degree of parathyroid gland enlargement.

Calcium↗
PubMed1995

Differentiation of MC3T3-E1 osteoblasts is associated with temporal changes in the expression of IGF-I and IGFBPs.

We examined the relationship between osteoblast maturation and temporal changes in the secretion of IGF-I and the IGF-binding proteins (IGFBPs) in the MC3T3-E1 model of osteoblast development. IGF-I was present at low levels in conditioned media in proliferating preosteoblasts (3.7 +/- 1.7 ng/micrograms DNA and 3.9 +/- 0.6 at culture (days 3 and 9) and increased progressively in postmitotic differentiating osteoblasts, reaching a maximal concentration of 13.1 +/- 1.5 ng/micrograms DNA by day 25 of culture. We also observed an increase in IGF-I mRNA expression. Using Western ligand blot and immunoblot techniques, we found that IGFBP-2, -4, and -5 also displayed temporal differences in expression during MC3T3-E1 development. We observed a sustained increase in IGFBP-2, -4, and -5 mRNA expression between days 10-14, coincident with the onset of differentiation. IGFBP-2 and IGFBP-4 protein concentrations increased in parallel with IGFBP mRNA expression, but IGFBP-5 levels peaked between days 8-14 of culture, and declined thereafter in spite of persistent IGFBP-5 mRNA levels. These findings suggest complex transcriptional and post-transcriptional regulation of IGFBP metabolism during osteoblast development. Thus, IGF-I and IGFBP production are regulated during osteoblast development. In turn, time-dependent changes in IGF-I and modulation of IGF-I bioavailability by IGFBPs may regulate the osteoblastic developmental sequence.

Animals↗
PubMed1995

Forskolin inhibits protein kinase C-induced mitogen activated protein kinase activity in MC3T3-E1 osteoblasts.

We recently demonstrated that stimulation of DNA synthesis in MC3T3-E1 osteoblasts involves cross-talk between protein kinase C (PKC)-dependent pathways and activation of possible nonreceptor tyrosine kinases. In the current investigation we examined whether the Raf-1/MAP kinase kinase (MKK)/mitogen-activated protein kinase (MAPK) cascade integrates cross-talk between G protein-coupled second messengers and protein tyrosine phosphorylation in osteoblasts. We investigated the effects on DNA synthesis, protein tyrosine phosphorylation, and Raf-1, MKK, and MAPK activities of PKC activation by phorbol 12-myristate 13-acetate (PMA) and of cAMP elevation by forskolin (FSK) in MC3T3-E1 osteoblasts. We found that PMA-stimulated DNA synthesis was associated with increments in tyrosine phosphorylation of p44mapk (ERK1) and p42mapk (ERK2) and activation of Raf-1, MKK, and MAPK in these cells. FSK treatment of osteoblasts, which raised intracellular cAMP levels and inhibited DNA synthesis, blocked PKC-stimulated tyrosine phosphorylation of p44mapk (ERK1) and p42mapk (ERK2) as well as inhibited PKC-stimulated MAPK and Raf-1 activities. Despite this, PMA activated the intermediate MKK step of the Raf-1/MKK/MAPK cascade in the presence of FSK. The differential inhibition of PMA-stimulated Raf-1 and MKK activities by FSK suggests that PKC activates both Raf-1-dependent and -independent pathways in MC3T3-E1 osteoblasts. Moreover, the noncoordinate effects of FSK on PMA-stimulated MKK and MAPK activities indicates the presence of a additional distal cAMP-dependent inhibitory mechanisms.

Calcium-Calmodulin-Dependent Protein Kinases↗
PubMed1995

Characterization of insulin-like growth factor-binding protein 5-degrading proteases produced throughout murine osteoblast differentiation.

Insulin-like growth factor (IGF)-binding protein-5 (IGFBP-5) is uniquely regulated throughout MC3T3-E1 osteoblast differentiation: IGFBP-5 is first detectable in conditioned medium (CM) of replicating preosteoblasts (day 5); IGFBP-5 levels peak between culture days 8-12, then decline to almost undetectable levels in mature osteoblast cultures (> day 18) despite the persistence of IGFBP-5 messenger RNA. These observations suggest that IGFBP-5 concentrations may be regulated by posttranslational mechanisms. To determine whether proteolysis contributes to the disappearance of IGFBP-5 in CM of mature osteoblasts, serial samples of MC3T3-E1 cell CM obtained during a 30-day culture period were analyzed for IGFBP-5-degrading protease activity. Using [125I]recombinant human IGFBP-5 substrate zymography, we demonstrated that proteases with M(r) of 52-72 and 97 kilodaltons (kDa) were present in CM, and protease activity increased in concentration as cultures matured. The 52- to 72-kDa proteases were cation dependent and were inhibited by tissue inhibitor of metalloproteinase 1, a specific inhibitor of matrix metalloproteinases (MMPs), identifying them as MMPs. Furthermore, antisera to human MMP-1 and -2 immunoprecipitated IGFBP-5-degrading proteases with M(r) of 52 and 69/72 kDa, respectively, suggesting that homologous murine MMPs degrade IGFBP-5. Finally, MC3T3-E1 cell CM contained immunoreactive MMP-1 and -2, and MMP-2, in particular, increased significantly throughout differentiation. In contrast, the 97-kDa protease was neither inhibited by tissue inhibitor of metalloproteinase 1 nor immunoprecipitated by antisera to MMPs, suggesting that the 97-kDa protease is not a MMP. Together, these data suggest that MMPs along with an unidentified 97-kDa protease degrade IGFBP-5 in MC3T3-E1 cell cultures. Because truncated forms of IGFBP-5 have been shown to enhance the action of IGF in bone cells, IGFBP-5 proteases may be instrumental in IGF-mediated bone morphogenesis.

Animals↗
PubMed1995

Oral versus intravenous calcitriol: is the route of administration really important?

Deficiency of 1,25-dihydroxyvitamin D plays an important role in the pathogenesis of secondary hyperparathyroidism. Adequate replacement of this hormone is required to normalize parathyroid gland function and restore bone homeostasis in patients with advanced renal failure. Controversy exists regarding the best method of administering 1,25-dihydroxyvitamin D. Although initial, uncontrolled clinical trials suggested the superiority of intravenous calcitriol treatment, more recent controlled investigations have shown that different routes (oral versus intravenous), frequency (daily versus intermittent) and dosing (physiologic versus pharmacologic) of calcitriol administration are equivalent. Overall, the response to calcitriol treatment depends more on the severity of secondary hyperparathyroidism and the presence of confounding variables, such as hyperphosphatemia and acquired abnormalities of parathyroid cell function, than on the method of calcitriol administration.

Administration, Oral↗
PubMed1994

Molecular to pharmacologic control of osteoblast proliferation and differentiation.

Control of osteoblast growth and development can be characterized from receptor mediated events to nuclear messengers controlling gene transcription. From this analysis it is possible to formulate a model to explain the reciprocal relationship between growth and differentiation as well as differential cytokine modulation of osteoblast function. Central to this model are putative tissue specific transcriptional switches (possibly of the bHLH gene superfamily) that may repress proliferation and permit the regulation of mature osteoblast phenotypic characteristics. This model proposes that in post-mitotic differentiated osteoblasts, tissue specific transcription factors determine the capacity to express osteoblastic characteristic, whereas receptor activated signalling cascades, namely, cAMP/protein kinase A, receptor serine/threonine kinase, and vitamin D receptor-dependent pathways, regulate mature osteoblast-specific gene expression. Activated differentiation switches also may feedback to transcriptionally repress proliferation. Conversely, in preosteoblasts, in which differentiation switches are turned off, distinct signalling cascades involving tyrosine kinases, PKC, and calcium/calmodulin regulate proliferation. Proliferating preosteoblasts also exhibit negative modulation of maturation either through inactivation of putative tissue-specific transcription factors and/or through AP-1 dependent phenotype suppression of genes expressed in mature osteoblast. Thus, the final outcome of transcriptional regulation of osteoblast function results from complex interactions between signalling pathways and permissive differentiating transcription factors. Though many aspects of this model remain speculative and require confirmation, it serves as a useful conceptual framework to further investigate the differential control of osteoblast proliferation and differentiation that may lead to improved pharmacologic ways to manipulate bone formation in vivo.

Cell Differentiation↗