Osteogenic PTHs and vascular ossification—Is there a danger for osteoporotics?

Vascular ossification: Pathology, mechanisms, and clinical implications

In recent years, the mechanisms and clinical significance of vascular calcification have been increasingly investigated. For over a century, however, pathologists have recognized that vascular calcification is a form of heterotopic ossification. In this review, we aim to describe the pathology and molecular processes of vascular ossification, to characterize its clinical significance and treatment options, and to elucidate areas that require further investigation. The molecular mechanisms of vascular ossification involve the activation of regulators including bone morphogenic proteins and chondrogenic transcription factors and the loss of mineralization inhibitors like fetuin-A and pyrophosphate. Although few studies have examined the gross pathology of vascular ossification, the presence of these molecular regulators and evidence of microfractures and cartilage have been demonstrated on heart valves and atherosclerotic plaques. These changes are often triggered by common inflammatory and metabolic disorders like diabetes, hyperlipidemia, and chronic kidney disease. The increasing prevalence of these diseases warrants further research into the clinical significance of vascular ossification and future treatment options.

Calcium-sensing receptor, calcimimetics, and cardiovascular calcifications in chronic kidney disease

Renal function impairment goes along with a disturbed calcium, phosphate, and vitamin D metabolism, resulting in secondary hyperparathyroidism (sHPT). These mineral metabolism disturbances are associated with soft tissue calcifications, particularly arteries, cardiac valves, and myocardium, ultimately associated with increased risk of mortality in patients with chronic kidney disease (CKD). sHPT may lead to cardiovascular calcifications by other mechanisms including an impaired effect of parathyroid hormone (PTH), and a decreased calcium-sensing receptor (CaR) expression on cardiovascular structures. PTH may play a direct role on vascular calcifications through activation of a receptor, the type-1 PTH/PTHrP receptor, normally attributed to PTH-related peptide (PTHrP). The CaR in vascular cells may also play a role on vascular mineralization as suggested by its extremely reduced expression in atherosclerotic calcified human arteries. Calcimimetic compounds increasing the CaR sensitivity to extracellular calcium efficiently reduce serum PTH, calcium, and phosphate in dialysis patients with sHPT. They upregulate the CaR in vascular cells and attenuate vascular mineralization in uremic states. In this article, the pathophysiological mechanisms associated with cardiovascular calcifications in case of sHPT, the impact of medical and surgical correction of sHPT, the biology of the CaR in vascular structures and its function in CKD state, and finally the role played by the CaR and its modulation by the calcimimetics on uremic-related cardiovascular calcifications are reviewed.

The influence of parathyroid hormone on the adult hematopoietic stem cell niche

Adult hematopoietic stem cells (HSCs) reside in the bone marrow in stable microenvironments known as the stem cell niche. One key component of the stem cell niche is cells of the osteoblastic lineage. Factors that are known to affect osteoblast activity, such as parathyroid hormone (PTH), have also been shown to affect the HSCs. Treatment of mice with PTH has led to beneficial effects on the HSC pool, which have led to clinical trials of PTH treatment to enhance HSC-based therapies.

Serum parathyroid hormone is associated with carotid intima-media thickness in postmenopausal women

OBJECTIVES

To test whether parathyroid hormone (PTH) might be related to the development of atherosclerosis in postmenopausal women, we measured serum levels of PTH, the carotid intima-media thickness (IMT), and other clinical and biochemical parameters.

METHODS

One hundred and seven postmenopausal women were recruited for this study. The serum level of PTH was measured by immunoradiometric assay, and carotid IMT was measured with high resolution B-mode ultrasonography.

RESULTS

On the basis of bivariate correlation analyses or unpaired t-tests, the body mass index (BMI), waist circumference, estimated glomerular filtration rate, and 25-hydroxyvitamin D were not associated with carotid IMT. In contrast, age (r = 0.414, p < 0.001), serum level of PTH (r = 0.304, p = 0.001), hypertension (p < 0.001), and hypercholesterolaemia (p = 0.004) were related to carotid IMT. On the basis of multiple regression analysis, the serum level of PTH (beta = 0.198, p = 0.029), as well as age (beta = 0.309, p = 0.001) and hypertension (beta = 0.262, p = 0.006), were independent predictors of carotid IMT.

CONCLUSIONS

Our results have demonstrated that serum PTH is an independent determinant of carotid IMT in postmenopausal women. This result suggests that serum PTH, even in the reference range, might be associated with the development of atherosclerosis or cardiovascular diseases in postmenopausal women. Further study is necessary in males and premenopausal women to fully elucidate the clinical significance of this finding.

Parathyroid hormone and the cardiovascular system

The classical actions of parathyroid hormone (PTH) are well recognized, but its effects on other target tissues, such as the cardiovascular system, are less appreciated. Several studies have evaluated the effects of PTH in patients with primary hyperparathyroidism in order to understand potential cardiovascular effects in terms of hypertension, cardiovascular mortality, left ventricular function, and endothelial function. We review these studies and evaluate the cellular mechanisms that may affect these outcomes.

Parathyroid hormone (PTH) and hematopoiesis: New support for some old observations

Forty-seven years ago, the parathyroid hormone (PTH) in one injection of Lilly's old bovine parathyroid extract, PTE, was found to greatly increase the 30-day survival of heavily X-irradiated rats when given from 18 h before to as long as 3 h after irradiation but no later. This was the first indication that PTH might stimulate hematopoiesis. Recent studies have confirmed the relation between PTH and hematopoiesis by showing that hPTH-(1-34)OH increases the size of the hematopoietic stem cell pool in mice. The peptide operates through a cyclic AMP-mediated burst of Jagged 1 production in osteoblastic cells lining the stem cells' niches on trabecular bone surfaces. The osteoblastic cells' Jagged 1 increases the hematopoietic stem cell pool by activating Notch receptors on attached stem cells. PTH-triggered cyclic AMP signals also directly stimulate the proliferation of the hematopoietic stem cells. However, the single PTH injection in the early experiments using PTE probably increased the survival of irradiated rats mainly by preventing the damaged hematopoietic progenitors from irreversibly initiating self-destructive apoptogenesis during the first 5 h after irradiation. It has also been shown that several daily injections of hPTH-(1-34)OH enable lethally irradiated mice to survive by stimulating the growth of transplanted normal bone marrow cells. If the osteogenic PTHs currently entering or on the verge of entering the market for treating osteoporosis can also drive hematopoiesis in humans as well as rodents, they could be potent tools for reducing the damage inflicted on bone marrow by cytotoxic cancer chemotherapeutic drugs and ionizing radiation.