Muscle-bone and fat-bone interactions in regulating bone mass: do PTH and PTHrP play any role?

Correlation of body composition metrics with bone mineral density and computed tomography-based trabecular attenuation

PURPOSE

To investigate the relationship of body composition metrics with bone mineral density (BMD) and trabecular attenuation in a cohort of healthy individuals.

METHODS

We retrospectively analyzed data of consecutively evaluated individuals who underwent dual-energy X-ray absorptiometry (DXA) and abdominopelvic computed tomography (CT) on the same day during routine medical check-ups between January 2021 and December 2021. Trabecular attenuation was measured at L1 level, while body composition metrics, including skeletal muscle index (SMI), skeletal muscle attenuation (SMA), visceral fat index (VFI), and subcutaneous fat index (SFI), were measured at L3 level. The association of body composition metrics with BMD and trabecular attenuation was analyzed using partial correlation analysis.

RESULTS

A total of 634 patients (median age, 56 years; range 50-62 years; 392 men) were included. In men, the SMI and SMA were positively correlated with BMD and trabecular attenuation, both before (r, 0.157-0.344; p < 0.05) and after (r, 0.103-0.246; p < 0.05) adjusting for age and body mass index. The VFI showed negative correlations with trabecular attenuation in both men (r, -0.170; p = 0.001) and women (r, -0.394; p < 0.001), which remained significant after adjusting for age and body mass index (r, -0.181 to -0.122; p < 0.05).

CONCLUSION

Low skeletal muscle mass and attenuation were significantly correlated with low BMD and trabecular attenuation in men. Visceral adiposity was associated with reduced BMD and trabecular attenuation in both men and women, demonstrating a stronger correlation with trabecular attenuation.

Research on biochemical indexes of bone metabolism in bipolar disorder: A cross-sectional study with newly diagnosed, drug-naïve patients

BACKGROUND

In recent years, the metabolic abnormalities associated with bipolar disorder (BD) have attracted people's attention. However, clinical studies on bone metabolism in individuals with BD are unavailable. This study was designed to assess biochemical indexes of bone metabolism and related influencing factors.

METHODS

We measured bone turnover markers (BTMs), including procollagen Ⅰ N-terminal propeptide (PⅠNP), osteocalcin (OC) and C-terminal cross-linking telopeptide of type I collagen (CTX-I), and index of calcium and phosphorus metabolism in 100 drug-naïve individuals with BD (DSM-5) and 91 healthy volunteers. Besides, sociodemographic and clinical assessment were collected. Between-group comparisons and within subgroup analysis were performed.

RESULTS

The PⅠNP (t = 3.715, p < 0.001), OC (t = 2.117, p = 0.036), parathyroid hormone (PTH, t = 3.877, p < 0.001), vitamin D (t = 2.065, p = 0.041), insulin (t = 4.208, p < 0.001) and insulin resistance (t = 2.888, p = 0.004) levels in the drug-naive BD group was significantly higher than those in the healthy control (HC) group. The level of calcium (t = -2.124, p = 0.035) in the drug-naive BD group was significantly lower than that of the HC group. But OC and vitamin D loses statistical significance after Bonferroni correction. However, there was no significant difference in the CTX-I level between the two groups. There are gender differences in the level of BMTs in individuals with BD, but this phenomenon was not found in the HC subgroup. It is shown that diagnosed BD, gender, age and BMI may affect the PINP levels through multiple linear regression analysis.

CONCLUSION

The biochemical indexes of bone metabolism in drug-naive individuals with BD were more active than that of the healthy controls in a sample from the Chinese Han nationality. The finding provides new evidence for our understanding of bone metabolism in individuals with BD.

Upstream Regulators of Fibroblast Growth Factor 23

Fibroblast growth factor 23 (FGF23) has been described as an important regulator of mineral homeostasis, but has lately also been linked to iron deficiency, inflammation, and erythropoiesis. FGF23 is essential for the maintenance of phosphate homeostasis in the body and activating mutations in the gene itself or inactivating mutations in its upstream regulators can result in severe chronic hypophosphatemia, where an unbalanced mineral homeostasis often leads to rickets in children and osteomalacia in adults. FGF23 can be regulated by changes in transcriptional activity or by changes at the post-translational level. The balance between O-glycosylation and phosphorylation is an important determinant of how much active intact or inactive cleaved FGF23 will be released in the circulation. In the past years, it has become evident that iron deficiency and inflammation regulate FGF23 in a way that is not associated with its classical role in mineral metabolism. These conditions will not only result in an upregulation of FGF23 transcription, but also in increased cleavage, leaving the levels of active intact FGF23 unchanged. The exact mechanisms behind and function of this process are still unclear. However, a deeper understanding of FGF23 regulation in both the classical and non-classical way is important to develop better treatment options for diseases associated with disturbed FGF23 biology. In this review, we describe how the currently known upstream regulators of FGF23 change FGF23 transcription and affect its post-translational modifications at the molecular level.

Identification of osteogenic progenitor cell-targeted peptides that augment bone formation

Activation and migration of endogenous mesenchymal stromal cells (MSCs) are critical for bone regeneration. Here, we report a combinational peptide screening strategy for rapid discovery of ligands that not only bind strongly to osteogenic progenitor cells (OPCs) but also stimulate osteogenic cell Akt signaling in those OPCs. Two lead compounds are discovered, YLL3 and YLL8, both of which increase osteoprogenitor osteogenic differentiation in vitro. When given to normal or osteopenic mice, the compounds increase mineral apposition rate, bone formation, bone mass, and bone strength, as well as expedite fracture repair through stimulated endogenous osteogenesis. When covalently conjugated to alendronate, YLLs acquire an additional function resulting in a “tri-functional” compound that: (i) binds to OPCs, (ii) targets bone, and (iii) induces “pro-survival” signal. These bone-targeted, osteogenic peptides are well suited for current tissue-specific therapeutic paradigms to augment the endogenous osteogenic cells for bone regeneration and the treatment of bone loss.

Activation of osteogenic cells is essential for bone regeneration. Here, the authors screen a peptide library and identify 2 compounds that promote osteogenic progenitor cell differentiation in vitro, and show that they increase bone formation and fracture repair in mice.

Physical Activity-Dependent Regulation of Parathyroid Hormone and Calcium-Phosphorous Metabolism

Exercise perturbs homeostasis, alters the levels of circulating mediators and hormones, and increases the demand by skeletal muscles and other vital organs for energy substrates. Exercise also affects bone and mineral metabolism, particularly calcium and phosphate, both of which are essential for muscle contraction, neuromuscular signaling, biosynthesis of adenosine triphosphate (ATP), and other energy substrates. Parathyroid hormone (PTH) is involved in the regulation of calcium and phosphate homeostasis. Understanding the effects of exercise on PTH secretion is fundamental for appreciating how the body adapts to exercise. Altered PTH metabolism underlies hyperparathyroidism and hypoparathyroidism, the complications of which affect the organs involved in calcium and phosphorous metabolism (bone and kidney) and other body systems as well. Exercise affects PTH expression and secretion by altering the circulating levels of calcium and phosphate. In turn, PTH responds directly to exercise and exercise-induced myokines. Here, we review the main concepts of the regulation of PTH expression and secretion under physiological conditions, in acute and chronic exercise, and in relation to PTH-related disorders.

Which factors are associated with sarcopenia and frailty in elderly persons residing in the community?

Abstract Objective: to broaden knowledge about the factors associated with sarcopenia and frailty in elderly persons residing in the community. Method: an integrative systematic review based on the PRISMA recommendations was carried out, using articles published from 2012 to March 2017 in the PubMED, SciELO, Virtual Health Library, CINAHL and Springer electronic databases with the following descriptors: frail elderly, sarcopenia and etiology and their synonyms. The articles identified by the initial search strategy were independently assessed by two researchers, according to the eligibility criteria, and the articles selected were evaluated for methodological quality. Results: the results of this survey show that frailty may be associated with sarcopenia, low serum vitamin D levels, anemia, subclinical hyperthyroidism in men, while the greatest evolution in women was for osteoporosis. An association between sarcopenia and advanced age was also observed, with worsening quality of life, physical-functional capacity, nutritional status and comorbidities, as well as an increased risk of death in sarcopenic elderly persons. Conclusion: this systematic review showed that low serum levels of vitamin D are associated with frailty and factors that predispose this condition. It is therefore important to monitor the serum levels of this vitamin in the elderly population, and it is suggested that new studies are carried out related to supplements of this vitamin in frail elderly persons.

Novel bone metabolism-associated hormones: the importance of the pre-analytical phase for understanding their physiological roles

The endocrine function of bone is now a recognized feature of this tissue. Bone-derived hormones that modulate whole-body homeostasis, are being discovered as for the effects on bone of novel and classic hormones produced by other tissues become known. Often, however, the data regarding these last generation bone-derived or bone-targeting hormones do not give about a clear picture of their physiological roles or concentration ranges. A certain degree of uncertainty could stem from differences in the pre-analytical management of biological samples. The pre-analytical phase comprises a series of decisions and actions (i.e., choice of sample matrix, methods of collection, transportation, treatment and storage) preceding analysis. Errors arising in this phase will inevitably be carried over to the analytical phase where they can reduce the measurement accuracy, ultimately, leading discrepant results. While the pre-analytical phase is all important, in routine laboratory medicine, it is often not given due consideration in research and clinical trials. This is particularly true for novel molecules, such as the hormones regulating the endocrine function of bone. In this review we discuss the importance of the pre-analytical variables affecting the measurement of last generation bone-associated hormones and describe their, often debated and rarely clear physiological roles.

Implications of exercise-induced adipo-myokines in bone metabolism

Physical inactivity has been recognized, by the World Health Organization as the fourth cause of death (5.5 % worldwide). On the contrary, physical activity (PA) has been associated with improved quality of life and decreased risk of several diseases (i.e., stroke, hypertension, myocardial infarction, obesity, malignancies). Bone turnover is profoundly affected from PA both directly (load degree is the key determinant for BMD) and indirectly through the activation of several endocrine axes. Several molecules, secreted by muscle (myokines) and adipose tissues (adipokines) in response to exercise, are involved in the fine regulation of bone metabolism in response to the energy availability. Furthermore, bone regulates energy metabolism by communicating its energetic needs thanks to osteocalcin which acts on pancreatic β-cells and adipocytes. The beneficial effects of exercise on bone metabolism depends on the intermittent exposure to myokines (i.e., irisin, IL-6, LIF, IGF-I) which, instead, act as inflammatory/pro-resorptive mediators when chronically elevated; on the other hand, the reduction in the circulating levels of adipokines (i.e., leptin, visfatin, adiponectin, resistin) sustains these effects as well as improves the whole-body metabolic status. The aim of this review is to highlight the newest findings about the exercise-dependent regulation of these molecules and their role in the fine regulation of bone metabolism.

Mitogen-activated protein kinase phosphatase-1: function and regulation in bone and related tissues

In this review, we have highlighted work that has clearly demonstrated that mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1), a negative regulator of MAPKs, is an important signaling mediator in bone, muscle, and fat tissue homeostasis and differentiation. Further, we examined recent studies with particular focus on MKP-1 overexpression or deletion and its impact on tissues connected to bone. We also summarized regulation of MKP-1 by known skeletal regulators like parathyroid hormone (PTH)/PTH-related peptide (PTHrP) and bone morphogenic proteins. MKP-1's integration into the pathophysiological state of osteoporosis, osteoarthritis, rheumatoid arthritis, obesity, and muscular dystrophy are examined to emphasize possible involvement of MKP-1 both at the molecular level and in disease complications such as sarcopenia- or diabetes-related osteoporosis. We predict that understanding the mechanism of MKP-1-mediated signaling in bone-muscle-fat crosstalk will be a key in coordinating their activities and developing therapeutics to improve clinical outcomes for diseases associated with advanced age.

Exercise does not enhance aged bone's impaired response to artificial loading in C57Bl/6 mice

Bones adapt their structure to their loading environment and so ensure that they become, and are maintained, sufficiently strong to withstand the loads to which they are habituated. The effectiveness of this process declines with age and bones become fragile fracturing with less force. This effect in humans also occurs in mice which experience age-related bone loss and reduced adaptation to loading. Exercise engenders many systemic and local muscular physiological responses as well as engendering local bone strain. To investigate whether these physiological responses influence bones' adaptive responses to mechanical strain we examined whether a period of treadmill exercise influenced the adaptive response to an associated period of artificial loading in young adult (17-week) and old (19-month) mice. After treadmill acclimatization, mice were exercised for 30 min three times per week for two weeks. Three hours after each exercise period, right tibiae were subjected to 40 cycles of non-invasive axial loading engendering peak strain of 2250 με. In both young and aged mice exercise increased cross-sectional muscle area and serum sclerostin concentration. In young mice it also increased serum IGF1. Exercise did not affect bone's adaptation to loading in any measured parameter in young or aged bone. These data demonstrate that a level of exercise sufficient to cause systemic changes in serum, and adaptive changes in local musculature, has no effect on bone's response to loading 3h later. This study provides no support for the beneficial effects of exercise on bone in the elderly being mediated by systemic or local muscle-derived effects rather than local adaptation to altered mechanical strain.

Inverse relationship between a genetic risk score of 31 BMI loci and weight change before and after reaching middle age

Background/Objective:

Genome-wide-association studies have identified numerous body mass index (BMI)-associated variants, but it is unclear how these relate to weight gain in adults at different ages.

Methods:

We examined the association of a genetic risk score (GRS), consisting of 31 BMI-associated variants, with an annual weight change (AWC) and a substantial weight gain (SWG) of 10% by comparing self-reported weight at 20 years (y) with baseline weight (mean: 58 y; s.d.: 8 y) in 21407 participants from the Malmö Diet and Cancer Study (MDCS), and comparing baseline weight to weight at follow-up (mean: 73 y; s.d.: 6 y) among 2673 participants. Association between GRS and AWG and SWG was replicated in 4327 GLACIER (Gene x Lifestyle interactions And Complex traits Involved in Elevated disease Risk) participants (mean: 45 y; s.d.: 7 y) with 10 y follow-up. Cohort-specific results were pooled by fixed-effect meta-analyses.

Results:

In MDCS, the GRS was associated with increased AWC (β: 0.003; s.e: 0.01; P: 7 × 10⁻⁸) and increased odds for SWG (odds ratio (OR) 1.01 (95% confidence interval (CI): 1.00, 1.02); P: 0.013) per risk-allele from age 20y, but unexpectedly with decreased AWC (β: −0.006; s.e: 0.002; P: 0.009) and decreased odds for SWG OR 0.96 (95% CI: 0.93, 0.98); P: 0.001) between baseline and follow-up. Effect estimates from age 20 y to baseline differed significantly from those from baseline to follow-up (P: 0.0002 for AWC and P: 0.0001 for SWG). Similar to MDCS, the GRS was associated with decreased odds for SWG OR 0.98 (95% CI: 0.96, 1.00); P: 0.029) from baseline to follow-up in GLACIER. In meta-analyses (n=7000), the GRS was associated with decreased AWC (β: −0.005; s.e.m. 0.002; P: 0.002) and decreased odds for SWG OR 0.97 (95% CI: 0.96, 0.99); P: 0.001) per risk-allele.

Conclusions:

Our results provide convincing evidence for a paradoxical inversed relationship between a high number of BMI-associated risk-alleles and less weight gain during and after middle-age, in contrast to the expected increased weight gain seen in younger age.

Preservation of high-fat diet-induced femoral trabecular bone loss through genetic target of TNF-α

Obesity and osteoporosis are two common chronic diseases, however, the basis for the correlation between them remains largely unknown. The pro-inflammation cytokine tumor necrosis factor-alpha (TNF-α) plays important roles in lipid and bone metabolisms, which may be a good candidate in the correlation between obesity and osteoporosis. We investigated the pathological roles of TNF-α in high-fat diet (HFD)-induced bone loss. Wild-type (WT) mice and TNF-α knockout (TNF-α(-/-)) mice were fed with the standard diet or the HFD for 12 weeks. Bone marrow stromal cells (BMSCs) from both genotypes were induced to differentiate into osteoblasts and treated with palmitic acid (PA). Bone mass and microstructure of femurs were evaluated by micro-CT. Lipid and bone metabolisms were investigated by histological and plasma analyses, and real-time PCR. On the HFD, both TNF-α(-/-) and WT mice presented notable visceral obesity, dyslipidemia. Adipogenesis and osteoclastogenesis were enhanced, while osteoblastogenesis was reduced in both genotypes. However, the changes were significantly different between TNF-α(-/-) and WT mice after the HFD. The gain of body and fat-pad weight was less and adipocyte area was smaller by 22 % in TNF-α(-/-) mice. Osteoclast numbers and plasma CTX level were lower by 40 % and by 23 % in TNF-α(-/-) mice. There were more ALP positive cells in the PA-treated TNF-α(-/-) BMSCs. mRNA expression of PPAR-γ was lower while that of Runx2 was higher in the bone from TNF-α(-/-) HFD group and in the PA-treated TNF-α(-/-) BMSCs, compared to WT on the same treatment. Furthermore, femoral trabecular bone mass and trabecular bone number were significantly decreased in WT mice on the HFD, whereas they were increased by 1.56-fold and 1.53-fold, respectively, in TNF-α(-/-) mice on the same diet (P < 0.05). Our results demonstrated that TNF-α gene knockout retained HFD-induced femoral trabecular bone loss mainly by suppressing adipogenesis and osteoclastogenesis, and enhancing osteoblastogenesis, which suggests that TNF-α plays a critical role in the development of HFD-related bone metabolic disorders and it may be a new potential therapeutic target for obesity-related bone loss.

Hypovitaminosis D in adolescents living in the land of sun is correlated with incorrect life style: a survey study in Campania region

PURPOSE

The aim of this study is to investigate in a population of adolescents living in Regione Campania, undergoing sun exposure at least 9 months per year the prevalence of severe deficiency (<20 ng/ml) or insufficiency (21-29 ng/ml) of 25-Hydroxyvitamin D (25(OH)D) levels and its relationship with individual body weight, use of smoking, and exercise performance.

METHODS

From October 2012 to October 2013, 373 healthy subjects (153 girls, 223 boys 11-20 years) without chronic diseases were consecutively enrolled in a campaign to prevent metabolic, cardiovascular, and oncological diseases. 25(OH)D assay, BMI, and lifestyle habitudes (smoking and exercise indoor or outdoor) were assessed.

RESULTS

In this population, median 25(OH) level was 25.0 ng/ml (95 %CI 23.8-25.2) without any difference between girls (25.0 (95 %CI 23.1-25.7)) and boys (24.3 (95 %CI 23.8-25.2)). Severe deficiency was found in 6 girls (0.02 %), while insufficiency was found in 296 patients (110 girls, 79.3 %). Normal levels were found in 71 patients (37 girls, 19 %). Vitamin D levels were significantly correlated with BMI (r = -0.429, p < 0.0001), smoking (r = -0.241; p < 0.0001), and exercise performance (r = 0.791; p < 0.0001). At the multistep regression analysis, Vitamin D levels were best predicted by exercise performance (t = 19.6, p < 0.0001), less smoking addiction (t = -4.97, p < 0.001), and lower BMI (t = -4.69, p < 0.0001).

CONCLUSIONS

The current study demonstrates that Vitamin D levels are commonly unsatisfactory in adolescents. Lower levels were found in overweight or obese adolescent, smokers and with low or absence of physical activity outdoors.

A four-season molecule: osteocalcin. Updates in its physiological roles

Osteocalcin (OC) is the main non-collagenous hydroxyapatite-binding protein synthesized by osteoblasts, odontoblasts, and hypertrophic chondrocytes. It has a regulatory role in mineralization and it is considered a marker of bone cell metabolism. Recent findings evidenced new extra-skeletal roles for OC, depicting it as a real hormone. OC shares many functional features with the common hormones, such as tissue-specific expression, circadian rhythm, and synthesis as a pre-pro-molecule. However, it has some peculiar features making it a unique molecule: OC exists in different forms based on the degree of carboxylation. Indeed, OC has three glutamic acid residues, in position 17, 21, and 24, which are subject to γ-carboxylation, through the action of a vitamin K-dependent γ-glutamyl carboxytransferase. The degree of carboxylation, and thus the negative charge density, determines the affinity for the calcium ions deposited in the extracellular matrix of the bone. The modulation of the carboxylation could, thus, represent the mechanism by which the body controls the circulating levels, and hence the hormonal function, of OC. There are evidences linking OC, and the bone metabolism, with a series of endocrine (glucose metabolism, energy metabolism, fertility) physiological (muscle activity) and pathological functions (ectopic calcification). Aim of this review is to give a full overview of the physiological roles of OC by collecting the newest experimental findings on this intriguing molecule.