Proteolysis of human bone collagen by cathepsin K: characterization of the cleavage sites generating by cross-linked N-telopeptide neoepitope

Bone formation is not impaired by hibernation (disuse) in black bears Ursus americanus

Disuse by bed rest, limb immobilization or space flight causes rapid bone loss by arresting bone formation and accelerating bone resorption. This net bone loss increases the risk of fracture upon remobilization. Bone loss also occurs in hibernating ground squirrels, golden hamsters, and little brown bats by arresting bone formation and accelerating bone resorption. There is some histological evidence to suggest that black bears Ursus americanus do not lose bone mass during hibernation (i.e. disuse). There is also evidence suggesting that muscle mass and strength are preserved in black bears during hibernation. The question of whether bears can prevent bone loss during hibernation has not been conclusively answered. The goal of the current study was to further assess bone metabolism in hibernating black bears. Using the same serum markers of bone remodeling used to evaluate human patients with osteoporosis, we assayed serum from five black bears, collected every 10 days over a 196-day period, for bone resorption and formation markers. Here we show that bone resorption remains elevated over the entire hibernation period compared to the pre-hibernation period, but osteoblastic bone formation is not impaired by hibernation and is rapidly accelerated during remobilization following hibernation.

Different acute responses of serum type I collagen telopeptides, CTX, NTX and ICTP, after repeated ingestion of calcium

BACKGROUND

N- and C-terminal fragments of type I collagen such as NTX, CTX and ICTP are released into circulation during bone resorption and can be quantified in serum. Their respective sensitivity as indices of osteoclastic activity was compared after a short-term inhibition of resorption induced by repeated drinking of calcium-fortified water.

METHODS

Serum NTX, CTX and ICTP were measured by specific immunoassays in one group of 15 subjects sampled at 08.00, 11.00, 14.00 and 17.00 (referred to as T0, T3h, T6h and T9h) and having ingested in two experimental periods 660 ml of either low-calcium mineral water or the same low-calcium mineral water fortified with calcium (300 mg/l) at three times (08.00, 11.00 and 14.00).

RESULTS

Oral intake of calcium-fortified water resulted in progressive decrease in serum CTX (by 38.7% at T3h, 61.0% at T6h and 60.4% at T9h) and NTX (by 19.0% at T3h, 24.1% at T6h and 25.2% at T9h) while serum ICTP concentrations were not significantly affected. Since ingestion of low-calcium water induced a modest but significant decrease in both CTX (-19.4%) and NTX (-10.6%) we compared the two sets of assays with repeated-measures two-factor analysis of variance with interaction. Ingestion of calcium-fortified water vs. low-calcium water resulted in a significant decrease in both serum CTX (time, P<0.0001; treatment, P<0.0001; time-by treatment, P<0.0001) and NTX (time, P<0.0001; treatment, P=0.0001; time-by treatment, P=0.0066).

CONCLUSIONS

CTX is more sensitive than NTX while ICTP is not sensitive to calcium-induced acute changes in osteoclastic activity. The present results stress the importance of choosing appropriate biochemical bone markers to demonstrate the effects of calcium on bone resorption.

Matrix metalloproteinases (MMP) and cathepsin K contribute differently to osteoclastic activities

The best established proteolytic event of osteoclasts is bone matrix solubilization by the cysteine proteinase cathepsin K. Here, however, we draw the attention on osteoclastic activities depending on matrix metalloproteinases (MMPs). We discuss the observations supporting that MMPs contribute significantly to bone matrix solubilization in specific areas of the skeleton and in some developmental and pathological situations. Our discussion takes into account (1) the characteristics of the bone remodeling persisting in the absence of cathepsin K, (2) the ultrastructure of the resorption zone in response to inactivation of MMPs and of cathepsin K in different bone types, (3) bone resorption levels in MMP knockout mice compared to wild-type mice, (4) the identification of MMPs in osteoclasts and surrounding cells, and (5) the effect of different bone pathologies on the serum concentrations of specific collagen fragments believed to discriminate between cathepsin K and MMP cleavage. Next, we provide evidence that MMPs are very critical for osteoclast migration, thereby controlling also the cell-matrix interactions required for cell attachment/detachment. The evidence supporting this role is based on a model of osteoclast recruitment in primitive long bones, an assay of osteoclast invasion through collagen gel, and the effect of proteinase inhibitors/knockouts in these models. Furthermore, we mention observations indicating a role of MMPs in initiation of bone resorption. Finally, we emphasize the many distinct ways MMPs may alter focally the extracellular environment thereby regulating the osteoclast behavior. Although the understanding of MMPs in osteoclast biology is rapidly expanding, it is suspected that important roles remain to be discovered.

Bone metabolic markers in bisphosphonate therapy for skeletal metastases in patients with breast cancer

The use of bisphosphonates for skeletal metastasis of breast cancer is now well established. Although clinical judgement for treating skeletal metastasis is based on symptoms and imaging studies, accurate or quantitative means are few. Various bone metabolic markers have been developed and these were evaluated in patients with metastasis to bone. Bone metabolic markers, especially resorption markers, have been shown to be a good tool for the monitoring the response to therapy for skeletal metastasis. This is also true for bisphosphonate treatment for skeletal metastasis. Bone metabolic markers are produced by different mechanisms. There are some different classes of resorption markers; tartrate-resistant acid phosphatase (TRAP) is secreted by osteoclast, N- and C-terminal cross-linking telopeptide of type I collagen (NTx and CTx) are the degradation the products of type I collagen, mainly produced by cathepsin K, and pyridinoline cross-linked carboxyl-terminal telopeptides of type I collagen (I CTP) is also a degradation product of type I collagen, by matrix metalloproteases. Even though bone resorption markers are a good tool to monitor response to bisphosphonate therapy, there remains the question of which class of bone resorption markers is best suited to the task.

S3 to S3' subsite specificity of recombinant human cathepsin K and development of selective internally quenched fluorescent substrates

We have systematically examined the S3 to S3' subsite substrate specificity requirements of cathepsin K using internally quenched fluorescent peptides derived from the lead sequence Abz-KLRFSKQ-EDDnp [where Abz is o -aminobenzoic acid and EDDnp is N -(2,4-dinitrophenyl)ethylenediamine]. We assayed six series of peptides, in which each position except Gln was substituted with various natural amino acids. The results indicated that the S3-S1 subsite requirements are more restricted than those of S1'-S3'. Cathepsin K preferentially accommodates hydrophobic amino acids with aliphatic side chains (Leu, Ile and Val) in the S2 site. Modifications at P1 residues also have a large influence on cathepsin K activity. Positively charged residues (Arg and Lys) represent the best accepted amino acids in this position, although a particular preference for Gly was found as well. Subsite S3 accepted preferentially basic amino acids such as Lys and Arg. A broad range of amino acids was accommodated in the remaining subsites. We further explored the acceptance of a Pro residue in the P2 position by cathepsin K in order to develop specific substrates for the enzyme. Two series of peptides with the general sequences Abz-KXPGSKQ-EDDnp and Abz-KPXGSKQ-EDDnp (where X denotes the position of the amino acid that is altered) were synthesized. The substrates Abz-KPRGSKQ-EDDnp and Abz-KKPGSKQ-EDDnp were cleaved by cathepsin K at the Arg-Gly and Gly-Ser bonds respectively, and have been shown to be specific for cathepsin K when compared with other lysosomal cysteine proteases such as cathepsins L and B and with the aspartyl protease cathepsin D.

Measurement of serum beta-crosslaps is influenced by proteolytic conditions

Serum beta-crosslaps are an established laboratory test to investigate bone metabolism. However, lytic conditions may affect the measurement of serum beta-crosslaps. Therefore, we investigated the effect of cathepsin C and human leukocyte lysate on serum beta-crosslaps in relation to temperature and time. We divided eight serum samples with elevated beta-crosslaps levels into three aliquots and stored them at 4, 21 and 37 degrees C. Another five serum samples were divided into three aliquots and adjusted to contain different cathepsin C concentrations (50, 250, 500 IU/l). These aliquots were divided again and stored at 4, 21 and 37 degrees C. Finally, three aliquots from three additional serum samples were treated with human leukocyte lysate (100, 300, 500 microl), divided again and stored at 4, 21 and 37 degrees C. Measurements of serum beta-crosslaps were then carried out before and immediately after manipulation, and after 2 and 5 days of storage. When stored at 21 degrees C, serum beta-crosslaps diminished significantly (25% after 5 days), but no significant change was detectable when they were stored at 4 degrees C. Cathepsin C induced up to a 14% increase in beta-crosslaps while human leukocyte lysate caused up to a 17% decrease. This study demonstrates that the influence of proteolytic conditions on the serum concentration of beta-crosslaps is not uniform. Leukocyte lysate decreased serum beta-crosslaps while the addition of cathepsin C increased their concentration. Therefore, serum should be separated from the whole blood immediately after coagulation and stored until analysis in a deep freezer.

Bone metabolic markers as gauges of metastasis to bone: a review

Currently, imaging techniques are the leading methods used to diagnose of metastasis to bone. However, these techniques are expensive, expose patients to toxic and radioactive compounds, and monitor response to treatment poorly; these drawbacks have prompted the search for alternative screening methods. Therefore, bone metabolic markers have been evaluated as possible methods to diagnose and monitor the development and progression of metastatic bone disease. Although bone metabolic markers are often grouped as either resorption or formation markers, studies have revealed that each marker has its own biologic meaning and clinical relevance. Recent milestones in the use of bone metabolic markers as screening methods for metastatic bone disease and as evaluation methods for treatment response are shown in the following lists. 1. Bone metabolic marker measurements provide insight into mechanisms of metastasis to bone. 2. Although promising data have been reported, bone metabolic markers are not yet considered to be reliable screening methods for metastasis to bone. 3. Bone metabolic markers are reliable indicators of response to both conventional and bisphosphonate therapies. 4. Preliminary results indicate bone metabolic markers might be an independent prognostic factor in patients whose tumors metastasize to bone. 5. New or refined assays for bone metabolic markers are expected to improve the sensitivity and specificity of bone metabolic marker use in diagnosing and monitoring metastasis to bone.

A potent small molecule, nonpeptide inhibitor of cathepsin K (SB 331750) prevents bone matrix resorption in the ovariectomized rat

Inhibition of the cyteine proteinase, cathepsin K (E.C. 3.4.22.38) has been postulated as a means to control osteoclast-mediated bone resorption. The preferred animal models for evaluation of antiresorptive activity are in the rat. However, the development of compounds that inhibit rat cathepsin K has proven difficult because the human and rat enzymes differ in key residues in the active site. In this study, a potent, nonpeptide inhibitor of rat cathepsin K (K(i) = 4.7 nmol/L), 5-(2-morpholin-4-yl-ethoxy)-benzofuran-2-carboxylic acid ((S)-3-methyl-1-(3-oxo-1-[2-(3-pyridin-2-yl-phenyl)-ethenoyl]-azepan-4-ylcarbanoyl)-butyl)-amide (SB 331750), is described, which is efficacious in rat models of bone resorption. SB 331750 potently inhibited human cathepsin K activity in vitro (K(i) = 0.0048 nmol/L) and was selective for human cathepsin K vs. cathepsins B (K(i) = 100 nmol/L), L (0.48 nmol/L), or S (K(i) = 14.3 nmol/L). In an in situ enzyme assay, SB 331750 inhibited osteoclast-associated cathepsin activity in tissue sections containing human osteoclasts (IC(50) approximately 60 nmol/L) and this translated into potent inhibition of human osteoclast-mediated bone resorption in vitro (IC(50) approximately 30 nmol/L). In vitro, SB 331750 partially, but dose-dependently, prevented the parathyroid hormone-induced hypercalcemia in an acute rat model of bone resorption. To evaluate the ability of SB 331750 to inhibit bone matrix degradation in vivo, it was administered for 4 weeks at 3, 10, or 30 mg/kg, intraperitoneally (i.p.), u.i.d. in the ovariectomized (ovx) rat. Both 10 and 30 mg/kg doses of compound prevented the ovx-induced elevation in urinary deoxypyridinoline and prevented the ovx-induced increase in percent eroded perimeter. Histological evaluation of the bones from compound-treated animals indicated that SB 331750 retarded bone matrix degradation in vivo at all three doses. The inhibition of bone resorption at the 10 and 30 mg/kg doses resulted in prevention of the ovx-induced reduction in percent trabecular area, trabecular number, and increase in trabecular spacing. These effects on bone resorption were also reflected in inhibition of the ovx-induced loss in trabecular bone volume as assessed using microcomputerized tomography (microCT; approximately 60% at 30 mg/kg). Together, these data indicate that the cathepsin K inhibitor, SB 331750, prevented bone resorption in vivo and this inhibition resulted in prevention of ovariectomy-induced loss in trabecular structure.

Recovery of bone mass and normalization of bone turnover in long-term survivors of allogeneic bone marrow transplantation

Osteoporotic fractures are potential long-term complications of bone marrow transplantation (BMT). We previously reported that bone mineral density (BMD) of patients undergoing allogeneic BMT decreased by 6% to 9% during the first 6 months after BMT and that bone turnover rate was still increased 1 year after BMT. BMT patients do not need lifelong immunosuppressive treatment, which should offer favorable circumstances for the recovery of BMD. Thus, 27 (14 women, 13 men) of 29 long-term survivors of our previous study were invited to a follow-up study at a median of 75 months after BMT. From 12 months after BMT the BMD of the lumbar spine had increased by 2.4% (P = 0.002). The respective changes in femoral sites were +4.1% in the femoral neck (P = 0.087), 4.0% in the trochanter (P = 0.095), +4.7% in Ward's triangle (P = 0.072) and +1.4% in the total hip (P = 0.23). The markers of bone formation, serum osteocalcin and type I procollagen aminoterminal propeptide (PINP) had returned to control levels, but out of the markers of bone resorption the mean level of serum type I carboxyterminal telopeptide (ICTP) was 41% higher (P = 0.0001) and that of urinary type I collagen N-terminal telopeptide/creatinine (NTx) 41% lower (P = 0.0002) in patients than in controls. The mean serum 25-hydroxyvitamin D [25(OH)D] was 33% lower in patients (P = 0.0002), most of whom had hypovitaminosis D [serum 25(OH)D < or = 37 nmol/l]. Except for two, males had serum testosterone level lower than before BMT and four men had hypogonadism. In conclusion, in long-term survivors of allogeneic BMT BMD recovers and bone turnover state normalizes as compared to the situation 1 year after BMT. More attention should be paid to the vitamin D status of all recipients and to possible hypogonadism of male patients.

Potent and selective inhibition of human cathepsin K leads to inhibition of bone resorption in vivo in a nonhuman primate

Cathepsin K is a cysteine protease that plays an essential role in osteoclast-mediated degradation of the organic matrix of bone. Knockout of the enzyme in mice, as well as lack of functional enzyme in the human condition pycnodysostosis, results in osteopetrosis. These results suggests that inhibition of the human enzyme may provide protection from bone loss in states of elevated bone turnover, such as postmenopausal osteoporosis. To test this theory, we have produced a small molecule inhibitor of human cathepsin K, SB-357114, that potently and selectively inhibits this enzyme (Ki = 0.16 nM). This compound potently inhibited cathepsin activity in situ, in human osteoclasts (inhibitor concentration [IC]50 = 70 nM) as well as bone resorption mediated by human osteoclasts in vitro (IC50 = 29 nM). Using SB-357114, we evaluated the effect of inhibition of cathepsin K on bone resorption in vivo using a nonhuman primate model of postmenopausal bone loss in which the active form of cathepsin K is identical to the human orthologue. A gonadotropin-releasing hormone agonist (GnRHa) was used to render cynomolgus monkeys estrogen deficient, which led to an increase in bone turnover. Treatment with SB-357114 (12 mg/kg subcutaneously) resulted in a significant reduction in serum markers of bone resorption relative to untreated controls. The effect was observed 1.5 h after the first dose and was maintained for 24 h. After 5 days of dosing, the reductions in N-terminal telopeptides (NTx) and C-terminal telopeptides (CTx) of type I collagen were 61% and 67%, respectively. A decrease in serum osteocalcin of 22% was also observed. These data show that inhibition of cathepsin K results in a significant reduction of bone resorption in vivo and provide further evidence that this may be a viable approach to the treatment of postmenopausal osteoporosis.

Role of biochemical markers in the management of osteoporosis

Several serum and urine biochemical markers of bone resorption and formation have been developed. Biochemical bone markers have been used as intermediate end-points in all major studies of anti-osteoporotic therapies. Bone resorption markers, in particular, may add an independent, predictive value to the assessment of bone loss and fracture risk. There are also potential advantages in monitoring anti-osteoporotic treatment in the short-term in addition to bone densitometry, to rapidly identify non-responders to therapy, or non-compliance. Despite these recent advances, until now bone markers have simply been very useful research tools, with their clinical utility being limited by intra-individual and diurnal variability. However, the probability of the true bone mineral density response to hormone replacement therapy for the individual patient may be predicted using algorithms based on a spectrum of cut-off bone marker levels with varying false positive and negative rates. Thus, the transition of biochemical bone markers into everyday clinical practice may be rapidly approaching.

The slow-binding inhibition of cathepsin K by its propeptide

A peptide corresponding to the full-length proregion (amino acids 16-114) of human cathepsin K was expressed and purified from Escherichia coli. This recombinant propeptide was investigated for its ability to inhibit the activity of three cysteine proteinases: cathepsins K, L, and B. Kinetic studies showed the propeptide to be a potent slow-binding inhibitor of its parent enzyme with a K(i) = 2. 61 nM at pH 6. This inhibition was pH-dependent, with a decrease in pH from 6 to 4 leading to a concomitant increase in K(i) to 147 nM. The propeptide also inhibited cathepsin L with a K(i) = 26.1 nM at pH 6, but showed little inhibition of cathepsin B at concentrations up to 400 nM.