Kaisa K. Ivaska

Biochemical Markers of Bone Metabolism and Prediction of Fracture in Elderly Women

We studied the ability of various markers of bone turnover to predict fracture in 1040 randomly recruited 75‐year‐old women. A total of 178 of the women sustained at least one fracture during follow‐up (mean, 4.6 years). In elderly women, TRACP5b and urinary fragments of osteocalcin are promising new markers for prediction of fracture, in particular, vertebral fracture.

Serum TRACP 5b is a useful marker for monitoring alendronate treatment: comparison with other markers of bone turnover

We studied clinical performance of serum TRACP 5b and other bone turnover markers, including S‐CTX, U‐DPD, S‐PINP, S‐BALP, and S‐OC, for monitoring alendronate treatment. TRACP 5b had higher clinical sensitivity, area under the ROC curve, and signal‐to‐noise ratio than the other markers.

Associations Between Homocysteine, Bone Turnover, BMD, Mortality, and Fracture Risk in Elderly Women†

Homocysteine has been suggested to be a risk factor for fracture, but the causal relationship is not clear. In 996 women from the OPRA study, high homocysteine level was associated with high bone marker levels and low BMD at baseline. During a mean 7‐year follow‐up, high homocysteine level was associated with mortality, but no clear association to fracture risk existed.

Bone Turnover Markers and Prediction of Fracture: A Prospective Follow-Up Study of 1040 Elderly Women for a Mean of 9 Years

Osteoporosis is characterized by compromised bone mass and strength, predisposing to an increased risk of fracture. Increased bone metabolism has been suggested to be a risk factor for fracture. The aim of this study was to evaluate whether baseline bone turnover markers are associated with long‐term incidence of fracture in a population‐based sample of 1040 women who were 75 years old (Malmö OPRA study). Seven bone markers (S‐TRACP5b, S‐CTX‐I, S‐OC[1–49], S‐TotalOC, S‐cOC, S‐boneALP, and urinary osteocalcin) were measured at baseline and 1‐year follow‐up visit. During the mean follow‐up of 9.0 years (range 7.4–10.9), 363 women sustained at least one fracture of any type, including 116 hip fractures and 103 clinical vertebral fractures. High S‐TRACP5b and S‐CTX‐I levels were associated with increased risk of any fracture with hazard ratios [HRs (95% confidence interval)] of 1.16 (1.04–1.29) and 1.13 (1.01–1.27) per SD increase, respectively. They also were associated with increased risk of clinical vertebral fracture with HRs of 1.22 (1.01–1.48) and 1.32 (1.05–1.67), respectively. Markers were not associated with risk for hip fracture. Results were similar when we used resorption markers, including urinary osteocalcin, measured at the 1‐year visit or an average of the two measurements. The HRs were highest for any fracture in the beginning of the follow‐up period, 2.5 years from baseline. For vertebral fractures, the association was more pronounced and lasted for a longer period of time, at least for 5 years. In conclusion, elevated levels of S‐TRACP5b, S‐CTX‐I, and urinary osteocalcin are associated with increased fracture risk for up to a decade in elderly women.

Serial assessment of serum bone metabolism markers identifies women with the highest rate of bone loss and osteoporosis risk.

CONTEXT One of the important challenges in the management of osteoporosis is to identify women who are at high risk of developing osteoporosis and fragility fractures. OBJECTIVE Our objective was to evaluate whether assessment of bone metabolism at multiple occasions can identify women with the highest risk for bone loss. DESIGN The Malmö Osteoporosis Prospective Risk Assessment study is an ongoing longitudinal study. Participants have been evaluated at baseline and after 1, 3, and 5 yr. SETTING We conducted a population-based study. PARTICIPANTS Participants included 1044 women, all 75 yr old at baseline. MAIN OUTCOME MEASURES Seven bone turnover markers were assessed at baseline and at 1, 3, and 5 yr (n = 573). The 5-yr change in areal bone mineral density (aBMD) was also determined. RESULTS Baseline markers correlated weakly to change in total body aBMD. The associations were more pronounced when the average of the baseline and 1-yr measurements was used (standardized regression coefficients -0.12 to -0.23, P < 0.01). Adding the 3-yr and 5-yr measurement further strengthened the correlation (regression coefficients up to -0.30, P < 0.001). Women with constantly high turnover lost significantly more bone at total body assessment (-2.6%) than women with intermediate (-1.6%) or low turnover (-0.2%, P for trend < 0.001). They also had a greater decrease in hip BMD (-8.3, -6.0, and -5.1%, respectively, P = 0.010). Results were similar also in the subgroup of women with osteopenia. CONCLUSIONS Our results suggest that serial assessment of bone turnover improves the identification of women with the highest rate of bone loss and osteoporosis risk.

Variation in the Osteocalcin Gene: Association Study of BMD, Fracture and Changes in Body Fat Mass in Elderly Women.

Infantile ARO is a genetic disorder characterized by osteoclast dysfunction that leads to osteopetrosis. We describe a novel mutation affecting the OSTM1 locus responsible for ARO. In addition to common clinical features of osteopetrosis, the patient developed a unique neuronal pathology that provided evidence for an essential role of OSTM1 in normal neuronal cell development.

Serum Osteocalcin Is Not Associated with Glucose but Is Inversely Associated with Leptin across Generations of Nondiabetic Women

CONTEXT The skeleton is recognized as an important player in energy metabolism through its interactions with other tissues. Whether the association of osteocalcin with glucose metabolism is age dependent has not been fully addressed. OBJECTIVE The objective of the study was to examine the age-specific association between different forms of osteocalcin and glucose and adipokines. DESIGN This was a family-based study across three generations. SETTING The study was conducted at a university laboratory. PARTICIPANTS Sixty-four daughter-premenopausal mother-maternal grandmother trios participated in the study. METHODS Fasting plasma glucose and insulin concentrations, serum total (tOC), carboxylated (cOC), and uncarboxylated (ucOC = tOC - cOC) osteocalcin, leptin, and adiponectin levels, and fat masses were assessed. Generalized estimating equations (GEE) model was used to assess the associations of bone biomarkers with glucose metabolism variables and adipokines. RESULTS No significant difference in insulin was found between generations, whereas glucose and leptin increased with age. Levels of tOC, cOC, and ucOC were highest in girls and lowest in mothers (P < 0.01). Grandmothers had higher leptin and adiponectin than mothers and girls. Despite the differences in insulin and glucose between the low and high homeostasis model assessment insulin resistance index (HOMA-IR) groups within generations, no significant differences in tOC, cOC, and ucOC were found. Compared with their low HOMA-IR counterparts, the high HOMA-IR group had significantly higher leptin and lower adiponectin in mothers and grandmothers. The tOC, cOC, and ucOC levels did not correlate with HOMA-IR, leptin, or adiponectin when the three generations were evaluated together, but when separated by generation, leptin was inversely correlated with tOC (P = 0.003) and cOC (P = 0.047) in mothers and with ucOC in grandmothers (P = 0.042). CONCLUSIONS Osteocalcin, glucose, and adipokines change with age but in a noncommensurate manner. We infer that the association between osteocalcin and glucose metabolism is minor and age specific in nondiabetic women. Leptin, however, strongly correlated with insulin resistance independently of fat masses, suggesting that obesity, as a metabolic disorder risk factor, affects glucose metabolism, partly through the role of leptin.

Suppressed Bone Turnover in Obesity: A Link to Energy Metabolism? A Case-Control Study

CONTEXT Observations in rodents suggest that osteocalcin (OC) participates in glucose metabolism. Based on human studies, it remains unclear whether circulating OC is simply a bone turnover marker (BTM) or also a mediator in interactions between the skeleton and glucose homeostasis. OBJECTIVE The objective of the study was to determine the responses of BTMs, including OC, to oral glucose tolerance test (OGTT) in a case-control setting. DESIGN AND PATIENTS Thirty-four normoglycemic young adults [mean age 19 y (SD 2.3)] with severe childhood-onset obesity and their gender- and age-matched nonobese controls underwent a standard 2-hour OGTT. MAIN OUTCOME MEASURES Glucose, insulin, and six BTMs including total and carboxylated OC (cOC) were determined at baseline and at 30, 60, 90, and 120 minutes during OGTT. RESULTS The obese and control subjects were similar in height; the mean body mass indices were 40.4 and 21.9 kg/m(2), respectively. The homeostasis model assessment index was 2.7 times greater in the obese subjects. All BTMs, except bone-specific alkaline phophatase, were lower in the obese subjects compared with the controls: the differences at baseline were 40%, 35%, 17%, 31%, and 32% for N-terminal propeptides of type I collagen, cross-linked telopeptides of type I collagen, tartrate-resistant acid phosphatase, total OC, and carboxylated OC (P < .05 for all) after adjusting for whole-body bone area. All BTMs decreased during OGTT. The relative values for the OGTT responses for total, but not for cOC (measured as area under the curve) differed between the two groups (P = .029 and P = .139, respectively): the decrease in total OC during the OGTT was less pronounced in the obese subjects. Responses in other BTMs were similar between the groups. No associations were observed between glucose metabolism and OCs during OGTT with linear regression. CONCLUSIONS Bone turnover markers were substantially lower in obese subjects compared with controls. Total OC and cOC showed less pronounced decrease during the OGTT in obese subjects compared with controls, whereas other BTMs responded similarly in the two groups. The role of OC, if anything, in glucose homeostasis is indirect and may be mediated via other factors than glucose or insulin.

Identification of novel proteolytic forms of osteocalcin in human urine

In this study, we report the isolation and characterization of osteocalcin in human urine using mass spectrometry and N-terminal sequencing. Multiple proteolytic forms of osteocalcin were found, which consisted of 16-27 residues from the middle region of the molecule. Several fragments had residue Gly7 at the N-terminus and the most predominant was fragment 7-31. Additional fragments starting from residue Asp14 were detected in the samples of children and young adults. Immunochemical detection of urine osteocalcin fragments had a statistically significant negative correlation to bone mineral density in evaluation of urine samples from 75-year-old women. Thus, the measurement of osteocalcin fragments in urine may have potential applications in diagnostics related to disorders of bone metabolism.

Overexpression of cathepsin K accelerates the resorption cycle and osteoblast differentiation in vitro

Bone resorption is a multistep process including osteoclast attachment, cytoskeletal reorganization, formation of four distinct plasma membrane domains, and matrix demineralization and degradation followed by cell detachment. The present study describes the intracellular mechanisms by which overexpression of cathepsin K in osteoclasts results in enhanced bone resorption. Osteoclasts and bone marrow-derived osteoclast and osteoblast precursors were isolated from mice homozygous (UTU17(+/+)) and negative for the transgene locus. Cells cultured on bovine cortical bone slices were analyzed by fluorescence and confocal laser scanning microscopy, and bone resorption was studied by measurements of biochemical resorption markers, morphometry, and FESEM. Excessive cathepsin K protein and enzyme activity were microscopically observed in various intracellular vesicles and in the resorption lacunae of cathepsin K-overexpressing osteoclasts. The number of cathepsin K-containing vesicles in UTU17(+/+) osteoclasts was highly increased, and co-localization with markers for the biosynthetic and transcytotic pathways was observed throughout the cytoplasm. As a functional consequence of cathepsin K overexpression, biochemical resorption markers were increased in culture media of UTU17(+/+) osteoclasts. Detailed morphometrical analysis of the erosion in bone slices indicated that the increased biosynthesis of cathepsin K was sufficient to accelerate the osteoclastic bone resorption cycle. Cathepsin K overexpression also enhanced osteogenesis and induced the formation of exceptionally small, actively resorbing osteoclasts from their bone marrow precursors in vitro. The present study describes for the first time how enhancement in one phase of the osteoclastic resorption cycle also stimulates its other phases and further demonstrate that tight control and temporal coupling of mesenchymal and hematopoietic bone cells in this multistep process.