Serum markers of collagen synthesis and degradation in acute respiratory failure patients.

BACKGROUND: Procollagen-derived propeptides reflect the rate of collagen synthesis and type I cross-linked collagen telopeptides (ICTP) collagen I degradation. We studied the collagen metabolism to find out if changes seen in acute respiratory distress syndrome patients are observed in patients with acute respiratory failure (ARF), and whether multiple organ dysfunction (MOD) has impact on it. METHODS: ARF patients with prolonged hospitalisation at least 21 days were included to the study. Blood samples for serum procollagen aminoterminal propeptide I (PINP) and III (PIIINP), and ICTP measurements were collected at study admission (day 0) and on days 2, 7, and 21. RESULTS: The study population comprised 68 patients. Forty-three patients (63%) developed MOD during the first week. PIIINP levels increased in all patients over time. The increase was slightly more pronounced in patients with MOD. During the first week, the synthesis of PIIINP increased more than PINP, and PINP degradation exceeded its production. By day 21, the balance of collagen metabolites returned to baseline. CONCLUSION: The collagen metabolism was altered in ARF patients. The first week was dominated by degradation of type I collagen and production of type III collagen, but by day 21, the collagen composition returned to more stable form.

Influence of high-altitude grazing on bone metabolism of growing sheep.

The objective of this study was to identify the effect of high alpine grazing, associated with varying pasture grass qualities and more pronounced exercise on typically steep slopes, on bone metabolism by improving bone density and enhancing bone turnover in growing sheep. Twenty-four 5-month-old sheep were randomly assigned to two groups. One group was kept at high altitude (HA; 2000-2200 m a.s.l.) for 3 months, and the other group (C; control) remained in the lowlands (400 m a.s.l.). Both groups were kept in grazing pastures with access to good-quality swards. Before the start of the experiment, blood samples were taken, the sheep were weighed, and the left metatarsus of each animal was analysed by quantitative computer tomography. After 1 month, blood samples were taken and body weight was measured, followed by biweekly sampling. Finally, the animals were slaughtered, and the bones were collected for analysis of various bone parameters. Body weight development did not differ between the groups. Concentrations of 25-OH-Vitamin D, carboxy-terminal telopeptide of type I collagen and activities of bone-specific alkaline phosphatase were always higher in the HA group than in the C group, except on the last two sampling dates. Bone mineral content and density increased in both groups during the experiment, but more intensively in the HA group. In addition, the cortical thickness of the HA group increased. The present study demonstrates an increase in bone turnover and mineral content of the bones of the growing sheep grazing in high alpine pastures. The factors associated with HA grazing, therefore, clearly seem to improve bone composition.

Effect of oral clodronate on structural damage and bone turnover in rheumatoid arthritis.

OBJECTIVES: To study the effect of oral clodronate on structural damage and bone metabolism in rheumatoid arthritis (RA). METHODS: In this 2-year proof-of-concept study, sixty patients with at least moderately active RA were randomised to receive anti-rheumatic therapy alone or together with oral clodronate 1600 mg daily. Radiographs of hands and feet and serum samples for bone biomarkers were studied at baseline and at 24 months. RESULTS: At 24 months, progression of radiographic joint damage was similar in the 2 groups. Clodronate suppressed carboxyterminal cross-linked peptide of type I collagen (p=0.03) and aminoterminal propeptide of type I procollagen (p=0.01). Eight patients (27%) withdrew from clodronate therapy due to adverse drug reactions. CONCLUSIONS: Oral clodronate did not retard the focal bone damage in RA despite its beneficial effect on overall bone metabolism, as judged by the decrease in the reference bone biomarkers.

Toll-like receptor 9 ligands enhance mesenchymal stem cell invasion and expression of matrix metalloprotease-13.

Human mesenchymal stem cells (hMSCs) are multipotent cells that are found in the bone marrow. Inflammation and tissue damage mobilize MSCs and induce their migration towards the damaged site through mechanisms that are not well defined. Toll-like receptor-9 (TLR9) is a cellular receptor for microbial and vertebrate DNA. Stimulation of TLR9 induces inflammatory and invasive responses in TLR9-expressing cells. We studied here the expression of TLR9 in human MSCs and the effects of synthetic TLR9-agonists on their invasion. Constitutive expression of TLR9 was detected in human MSCs but the expression was suppressed when MSCs were induced to differentiate into osteoblasts. Using standard invasion assays and a novel organotypic culture model based on human myoma tissue, we discovered that stimulation with the TLR9 agonistic, CpG oligonucleotides increased the invasion capacity of undifferentiated MSCs. Simultaneously, an increase in MMP-13 synthesis and activity was detected in the CpG-activated MSCs. Addition of anti-MMP-13 antibody significantly diminished the CpG-induced hMSC invasion. We conclude that treatment with TLR9-ligands increases MSC invasiveness, and this process is at least partially MMP-13-mediated.

Genitourinary prolapse and joint hypermobility are associated with altered type I and III collagen metabolism.

PURPOSE: The aim of this study was to determine whether benign joint hypermobility (BJH) is associated with urogenital prolapse and altered collagen metabolism. METHODS: 43 postmenopausal women with previous vaginal hysterectomy operated due to genitourinary prolapse were recruited. Each patient was also evaluated for joint hypermobility. The collagen metabolism was studied measuring serum concentrations of type I and III procollagen aminoterminal propeptides and trivalently cross-linked carboxyterminal telopeptide of type I collagen. RESULTS: Clinical joint hypermobility was found in 35% patients. Women with joint hypermobility had higher concentration of aminoterminal propeptide for type I procollagen and the values were statistically significant (P < 0.0178). Recurrent prolapse was found in 47% of the patients with BJH as compared to non-hypermobile group (25%). In this subgroup the results were statistically significant (P < 0.0085) for type III collagen. Also, the mean serum concentration for type III procollagen was significantly increased above the reference limit. CONCLUSIONS: Women with joint hypermobility have more recurrent genital prolapse as compared to women with normal joint mobility. Plain hypermobility was associated with higher concentrations for type I procollagen. Patients with recurrent prolapse and joint hypermobility have significantly high concentrations for type III procollagen.

Clearance of NH2-terminal propeptides of types I and III procollagen is a physiological function of the scavenger receptor in liver endothelial cells.

This study was undertaken to determine the fate of circulating NH2-terminal propeptide of type I procollagen (PINP) in rats. Radiolabeled PINP showed a biphasic serum decay curve after intravenous injection. 79% of the material disappeared from the blood during the initial alpha-phase (t1/2 alpha = 0.6 min), while the remaining 21% was eliminated with a t1/2 beta of 3.3 min. The major site of uptake was the liver, 78, 1, and 21% of its radioactivity being recovered in isolated liver endothelial cells (LEC), Kupffer cells, and parenchymal cells, respectively. In LEC, fluorescently labeled PINP accumulated in small (0.1 microns) peripheral and larger (> 0.1 microns) perinuclear vesicles within 10 min at 37 degrees C after a binding pulse at 4 degrees C. These grew in size with increasing chasing time, reaching a maximum diameter of 1 microns or more after 30 min, and taking the shape of rings that were stained only along their periphery. At chase intervals exceeding 30 min, the size of the vesicles decreased, and after 60 min the stain appeared in smaller, densely stained perinuclearly located vesicles. Degradation of 125I-PINP to free smaller fragments and 125I- was significant after 30 min. Only formaldehyde-treated albumin, acetylated LDL, polyinosinic acid and NH2-terminal propeptide of type III procollagen (PIIINP) competed with PINP for uptake. These findings indicate that clearance of PINP and PIIINP, which are normal waste products generated in large quantities, is a physiological function of the scavenger receptor in LEC.

Sensitivity and specificity of autoantibodies binding to citrullinated carboxyterminal telopeptides of types I and II collagens in an early arthritis series.

OBJECTIVE: To assess the specificity and sensitivity of autoantibodies binding to citrullinated carboxyterminal telopeptides of types I and II collagens in an early arthritis series. METHODS: A cohort of 146 patients from the Kuopio 2000 Arthritis Survey having RA, AS, PsA, ReA, uSpA or undifferentiated arthritis were studied. Autoantibodies binding citrullinated types I and II carboxytelopeptides were measured in two different inhibition ELISA assays. Sera from 135 adult persons were used as controls. RESULTS: In RA, the sensitivities were 0.83 with long type I telopeptide and 0.78 with long type II telopeptide and the respective specificities were 0.94 and 0.93, while the corresponding values in other inflammatory joint diseases were much lower. The likelihood ratio in RA increased with longer peptides from 4.20 to 14.06 for type I telopeptide and from 2.74 to 11.67 for type II telopeptide. CONCLUSION: The antibody assay using long telopeptide from type I collagen was the most specific and sensitive method in every diagnostic category, although in the arthritides other than RA, binding was much less abundant and possibly citrulline-independent.

PINP: a serum biomarker of bone formation in the rat.

Serum PINP has emerged as a reliable marker of bone turnover in humans and is routinely used to monitor bone formation. However, the effects of PTH (1-34) on bone turnover have not been evaluated following short-term treatment. We present data demonstrating that PINP is an early serum biomarker in the rat for assessing bone anabolic activity in response to treatment with PTH (1-38). Rat serum PINP levels were found to increase following as few as 6 days of treatment with PTH (1-38) and these increases paralleled expression of genes associated with bone formation, as well as, later increases in BMD. Additionally, PINP levels were unaffected by treatment with an antiresorptive bisphosphonate. PINP may be used to detect PTH-induced early bone formation in the rat and may be more generally applicable for preclinical testing of potential bone anabolic drugs.

The effects of first gestation and lactation on bone metabolism in dairy goats and milk sheep.

The goal of the present study was to compare mobilization rate of calcium (Ca) from bone in pregnant and lactating goats and sheep. Blood samples were collected from goats and sheep monthly during pregnancy and at 1, 2, and 4 weeks postpartum (pp) and monthly during lactation until 6 months after parturition. Total bone mineral content (BMC) and total bone mineral density (BMD) were quantified using peripheral quantitative computed tomography at the same intervals as the blood was taken. Bone resorption was assessed by immunoassays quantitating two epitopes of the carboxyterminal telopeptide of type I collagen (ICTP, CTX). Bone formation was estimated by quantifying serum osteocalcin (OC) and bone-specific alkaline phosphatase (bAP). In addition, Ca and 1,25-dihydroxy vitamin D (1,25-VITD) concentrations were determined in serum. Mean ICTP and CTX concentrations of both animal species increased the first week after parturition. By the second week pp, the concentrations of both markers had decreased toward early gestation levels. In contrast, mean OC concentrations continually decreased until the 1st week pp. By the 2nd week pp, the mean concentrations of OC started to increase again. Mean bAP activities decreased during gestation and reached a nadir in the first week pp in goats and 4 weeks pp in sheep. Afterwards, mean bAP activities increased again in goats and sheep. 1,25-VITD concentrations peaked the first week pp and returned to early gestation values thereafter. Total BMC and BMD decreased from the 4th month of pregnancy until the 1st week pp in both species. Afterwards, BMC increased throughout the first month pp in goats and the first 3 months pp in sheep. BMD levels of sheep and goats returned to prepartum levels during lactation. The resorptive phase of bone remodeling is accelerated at parturition and in early lactation and is uncoupled from the process of bone formation. This allows the animal to achieve Ca homeostasis at the expense of bone. Increased bone remodeling during lactation may represent physiological mechanisms to help replace the maternal skeleton lost as the animal adapted to enormously increased Ca losses to the fetus and milk in late gestation and early lactation.

Amino-terminal propeptide of type III procollagen: a new prognosis indicator in human ovarian cancer.

To investigate the clinical usefulness of the amino-terminal propeptide of type III procollagen (PIIINP) as an indicator of ovarian cancer behavior, 30 patients with advanced epithelial malignancy were monitored with serial serum PIIINP and CA-125 determinations before and during treatment. Initially, PIIINP and CA-125 concentrations were each separately increased in 87% of the cases and, simultaneously, in 77% of the cases. In monitoring treatment responses, PIIINP and CA-125 were identical in 17 patients (57%), both being good predictors of the clinical behavior of the disease in 16 cases and poor predictors in one case. In 13 patients (43%) they were complementary to each other. In three cases PIIINP alone and in one case CA-125 alone were clinically useful prognosis indicators. During the period of complete clinical response to cytotoxic chemotherapy of 16 patients, the CA-125 concentrations decreased to normal before the clinical disappearance of the tumor in eight cases. PIIINP did so in only two cases, thus correlating more precisely with the presence of malignancy. In second-look laparotomies, PIIINP concentrations correlated with the presence of occult cancer better than those of CA-125. In predicting recurrent malignancy in patients with transient complete response, PIIINP and CA-125 were clinically equal. According to the present data, PIIINP concentrations often give information not obtainable by CA-125, thus being useful in monitoring the clinical behavior of ovarian cancer.

Progressive ovarian carcinoma induces synthesis of type I and type III procollagens in the tumor tissue and peritoneal cavity.

Increased serum concentrations of aminoterminal propeptide of type III procollagen occur in advanced ovarian cancer. To study their origin, we compared the expressions of type I and type III procollagens in ovarian tumor tissue and the peritoneal cavity with immunoassays for the propeptide domains of these procollagens. Samples of tumor cyst fluid, peritoneal ascitic fluid, tumor vein blood, and peripheral blood were obtained at operation from 50 women with malignant ovarian neoplasms and 61 women with benign neoplasms. The ascitic fluid concentrations of both type I and type III procollagen antigens were significantly higher in the malignant tumors than in the benign ones, but this difference was evident only for type I procollagen in the tumor cysts. The aminoterminal propeptide of type III procollagen concentration in the peripheral blood was higher in the patients with malignant tumors, whereas the concentrations were similar in the tumor veins. The enhanced type I procollagen synthesis in the malignant tumors did not affect the corresponding antigen in the blood. The findings suggest that progressive ovarian carcinoma invariably induces a fibroproliferative response, characterized by active expression of type I and type III procollagens. The increased circulating aminoterminal propeptide of type III procollagen is derived from the peritoneal cavity rather than from the tumor tissue via the ovarian vein.

Aggressive breast cancer leads to discrepant serum levels of the type I procollagen propeptides PINP and PICP.

The propeptides PICP and PINP are derived from the synthesis of type I collagen, a major matrix protein of bone and soft tissues. The aim of this cross-sectional study was to investigate their value as indicators of the aggressivity of breast cancer. Serum PINP, PICP, and total alkaline phosphatase were determined from 89 breast cancer patients. Forty had major bone and/or soft tissue metastases with an aggressive disease course: the progressive disease (PD) group. Forty-nine had either none or minor bone and/or soft tissue metastases with a stable clinical course: the stable disease group (SD). The mean value of PINP in the PD group was 7.2 times higher than that in the SD group (276 +/- 79 microg/l versus 38 +/- 3 microg/l, respectively; P = 0.005), whereas PICP mean value was only 1.7 times higher in the PD group (174 +/- 20 microg/l versus 100 +/- 5 microg/l; P = 0.001). The ratio of PICP to PINP was 1.02 +/- 0.07 in the PD group and 3.07 +/- 0.18 in the SD group (P < 0.001). The correlation between PICP and PINP was linear in the SD group and nonlinear in the PD group. The results indicate that high serum PICP and PINP concentrations and a low PICP:PINP ratio are associated with a highly aggressive nature of breast cancer. Determination of PINP, in particular, may be valuable when evaluating the clinical status of a breast cancer patient.

Intracellular enzymes of collagen biosynthesis in rat kidney in streptozotocin diabetes.

The activities of the four enzymes catalyzing intracellular post-translational modifications of the collagen polypeptide chains were assayed in the kidneys of rats with streptozotocin diabetes. When the changes in the four enzyme activities were expressed per milligram of protein in the 15,000 X g supernatant of the kidney homogenates, there were no changes in any of the enzyme activities at four weeks and only slight increases in the prolyl and lysyl hydroxylase activities at 12 weeks after the induction of diabetes. When the changes were expressed as total enzyme activities per two kidneys, again no changes were found in any enzyme activity at four weeks, but at 12 weeks significant increases were found in all four enzyme activities, namely prolyl hydroxylase, lysyl hydroxylase, collagen galactosyltransferase, and collagen glucosyltransferase. The data would be consistent with an increased collagen synthesis in diabetic kidneys, but they do not support the hypothesis that there might be specific changes in some of these enzyme activities or in the level of certain posttranslational modifications of the collagen polypeptide chains in this disease.

Type I and III collagen metabolites as predictors of clinical outcome in epithelial ovarian cancer.

We evaluated the significance of biochemical tumor markers, ie, aminoterminal propeptide of type III procoliagen, trivalently cross-linked COOH-terminal telopeptide of type I collagen (ICTP), aminoterminal propeptide of type I procollagen, and CA 125 in the prediction of ovarian cancer outcome and compared them with several classical indicators of prognosis. The concentrations of biochemical markers were determined from the preoperative serum specimens of 55 patients with epithelial ovarian cancer. In the univariate analysis, all biochemical markers except PINP and all conventional prognostic indicators except histological subtype correlated significantly with survival. In the multivariate Cox analysis of biochemical markers, serum ICTP remained the only significant prognostic indicator of overall survival. Among all variables, clinical stage and ICTP were the only independent and significant determinants of prognosis. Because the content of trivalently cross-linked, mature type I collagen (the breakdown of which is detectable in the ICTP test) in malignant ovarian cancer tissue has been reported to be lower and that of bivalently cross-linked and non-cross-linked collagen has been reported to be higher than in benign tumors, the source of excess ICTP in the circulation of ovarian cancer patients is most likely the degradative damage of soft tissues surrounding the progressively growing malignant lesions. The serum ICTP concentration can thus be regarded as an indicator of the invasion of ovarian cancer. Such information is not available by conventional methods. Therefore, the ICTP test will improve the accuracy of predicting clinical outcome in this disease.

Elevated serum levels of type I collagen degradation marker ICTP and tissue inhibitor of metalloproteinase (TIMP) 1 are associated with poor prognosis in lung cancer.

PURPOSE: The aim of this study was to evaluate the correlation between type I collagen degradation marker ICTP, MMP (matrix metalloproteinase)-9, and tissue inhibitor of metalloproteinase (TIMP)-1 and to compare their value as prognostic factors in lung cancer. EXPERIMENTAL DESIGN: From the sera of 141 lung cancer patients, we assessed markers of type I collagen synthesis (PINP and PICP) and degradation (ICTP) by radioimmunoassays, and we assessed MMP-9 and its tissue inhibitor TIMP-1 by ELISA. There were 62 squamous cell carcinomas, 42 adenocarcinomas, 14 small cell carcinomas, and 23 cases with other histology. Seventeen of these patients had advanced disease. Sixty-seven patients had been operated on, 33 had received radiation therapy, 7 had received chemotherapy, and the rest had received other treatment combinations. RESULTS: We examined the relationship between these markers and found a correlation between ICTP and MMP-9 (r = 0.201; P = 0.01) or TIMP-1 (r = 0.415; P = 0.00). Elevated serum concentrations of ICTP (>5 microg/liter) and/or TIMP-1 (>300 ng/ml) correlated with poor prognosis. In univariate regression analysis, ICTP had prognostic value (odds ratio, 1.6462; P < 0.03): the patients with elevated serum concentrations of ICTP (>5 microg/liter) had a 64% higher risk of dying from lung cancer than did patients with opposite values. CONCLUSIONS: Our results indicated that ICTP and TIMP-1 are good prognostic markers in lung cancer. The association between serum MMP-9 and ICTP suggests that MMP-9 could play a role in the degradation of the extracellular matrix producing ICTP in this pathological situation.

Procollagen synthesis and extracellular matrix deposition in MG-63 osteosarcoma cells.

We compared the procollagen synthetic properties of MG-63 osteosarcoma cells with those of cultured human skin fibroblasts. In both cells, the expressions of type I and III procollagens are largely dependent on the constant presence of ascorbate and coordinately decreased by the neutral polymer dextran T-40. The amino-terminal propeptides of pro-alpha 1 and pro-alpha 2 chains of type I procollagen are phosphorylated and those of the pro-alpha 1 and pN-alpha 1 chains of type III procollagen both phosphorylated and sulfated, there being no difference in net charge in the propeptides between these cell types. The major differences between MG-63 and normal fibroblasts are the exceptionally high relative synthesis of type III procollagen by MG-63 cells, up to about 40% of the total of types I and III (6% in cultured skin fibroblasts), and the inability of ascorbate-supplemented MG-63 cells to deposit collagens into an insoluble pericellular matrix. A longer dextran treatment shifts up to one-fourth of the proline-labeled extracellular macromolecules into the matrix fraction within 4 days (in control 4%). Despite processing of the procollagens to the respective collagens in the matrix, neither control matrices nor those induced by dextran induced increased production of alkaline phosphatase. In cultures up to 4 days postconfluence the proportion of type III collagen produced tended to increase over that in early confluent cultures. With respect to collagen production, the MG-63 cell line is not a representative of the osteoblast lineage but rather resembles a proliferative wound fibroblast.

Diurnal variation in serum markers of type I collagen synthesis and degradation in healthy premenopausal women.

There are several indications that the functions of human osteoblasts and osteoclasts have circadian rhythms with peak activities occurring at night. It is not known, however, whether the principal function of these cells, namely synthesis and degradation of the organic matrix of bone, of which about 90% is type I collagen, also has a circadian rhythm. This was therefore investigated for both the formation of type I collagen and the degradation of type I collagen in bone using two newly developed serum markers: the serum concentration of the carboxy-terminal propeptide of type I procollagen (PICP) as a marker of formation and the serum concentration of the carboxy-terminal pyridinoline cross-linked telopeptide of type I collagen (ICTP) as a marker of degradation. PICP and ICTP were measured by RIA in samples taken every 3 h over a 24 h period in 12 healthy premenopausal women (age 32 +/- 5 years, mean +/- SD). Both PICP (p = 0.003) and ICTP (p = 0.00003) showed a significant circadian rhythm, with about 20% higher values at night than in the afternoon. We conclude that serum markers of both the formation of new type I collagen and the degradation of old type I collagen in bone exhibit a clear circadian rhythm, with increased activity of both osteoblasts and osteoclasts at night. The etiology of this circadian rhythm is still unknown.

Effects of a combined estrogen-gestagen regimen on serum levels of the carboxy-terminal propeptide of human type I procollagen in osteoporosis.

Estrogen stimulates osteoblastic collagen production in vitro, but whether the same stimulation takes place in vivo is still unknown. To test the stimulatory effects of a combined estrogen-gestagen regimen in vivo we monitored serum levels of the carboxy-terminal propeptide of human type I procollagen (S-PICP) in a group of 12 osteoporotic women over a 150 week treatment period. Spinal bone mineral content (BMC) increased to a maximum of 5% over pretreatment values around week 90. Serum alkaline phosphatase (S-AP) and serum bone gla protein (S-BGP) both fell from initial values of 220 U/liter and 39 ng/ml, respectively, to 146 U/liter (p less than 0.01) and 27.2 ng/ml (NS) around week 60 and remained reduced over the remaining treatment period. S-PICP also fell from 117 to 68 micrograms/liter at week 60 and 70 micrograms/ml at week 150 (P less than 0.01). This is equal to a reduction to 32 +/- 10% pretreatment levels. The reduction in S-PICP was not significantly different from that of the other two markers of bone formation (S-AP and S-BGP). Thus, provided the metabolic clearance of PICP remains unaltered after hormone replacement therapy, no major stimulation of osteoblastic collagen type I synthesis was demonstrable during estrogen-gestagen treatment in this population of osteoporotic women. The changes in bone markers seen in this study are therefore consistent with an estrogen-mediated reduction in the frequency of remodeling activation. Because of the reduction in bone turnover and methodologic limitations of bone marker assays, however, smaller increases in the amount of bone formed per activation could remain undetectable.

Serum markers of type I collagen formation and degradation in metabolic bone disease: correlation with bone histomorphometry.

Type I collagen makes up more than 90% of bone matrix. Therefore, analysis of antigens related to collagen formation and degradation in bone should provide good and specific estimates of both bone resorption and bone formation rates. In this study we measured serum levels of the pyridinoline cross-linked telopeptide domain of type I collagen (ICTP) as a marker of bone resorption and serum carboxy-terminal propeptide of type I procollagen (PICP) as a marker of bone formation. Serum levels of the two antigens were correlated to histomorphometric indices of bone resorption and bone formation calculated from iliac crest bone biopsies in a group of 18 individuals with high- and low-turnover bone disease (myxedema, primary hyperparathyroidism, and thyrotoxicosis). After logarithmic transformation the regression of S-ICTP on volume-referent resorption rate (BRs/R/BV) was significant (r = 0.61, p < 0.01, SEM/Y = 56%). S-ICTP also showed a significant regression on the volume-referent cancellous bone balance (r = -0.45, p < 0.05, SEM/Y = 412%). S-PICP was significantly correlated to the mineral appositional rate (r = 0.53, p < 0.05) and volume-referent bone formation rate (r = 0.61, p < 0.01, SEM/Y = 48%). The correlation to bone turnover as expressed in the activation frequency was also highly significant (r = 0.61, p < 0.01, SEM/Y = 51%). No significant correlation with wall thickness or bone balance was demonstrable per remodeling cycle. Thus, assays employing antigens that reflect collagen formation and degradation are useful instruments for the evaluation of rates of bone remodeling in metabolic bone disease.

The type I collagen fragments ICTP and CTX reveal distinct enzymatic pathways of bone collagen degradation.

Bone resorption may generate collagen fragments such as ICTP and CTX, which can be quantified in serum and/or urine by using specific immunoassays, and which are used as clinical markers. However, the relative abundance of ICTP and CTX varies according to the type of bone pathology, suggesting that these two fragments are generated through distinct collagenolytic pathways. In this study, we analyzed the release of ICTP and CTX from bone collagen by the proteinases reported to play a role in the solubilization of bone matrix. Cathepsin K released large amounts of CTX, but did not allow a detectable release of ICTP. Conversely, the matrix metalloproteinases (MMPs) MMP-2, -9, -13, or -14 released ICTP, but did not allow a detectable release of CTX. Next we analyzed the release of ICTP and CTX from bone explants cultured in the presence of well-established inhibitors of these proteinases and of matrix solubilization. An inhibitor of cysteine proteinases including cathepsin K, inhibited the release of CTX, but not the release of ICTP. MMP inhibitors inhibited the release of ICTP, but also that of CTX, in agreement with the putative role of MMPs in the initiation of bone resorption in addition to matrix solubilization. Similarly the treatment of mice bearing bone metastasis with an MMP inhibitor led to a significant reduction of serum ICTP and CTX, and osteolytic lesions. We conclude that the generation of ICTP and CTX depends on different collagenolytic pathways. This finding may explain why these two markers may discriminate between different bone pathologies.