Angela M. Inzerillo

Forty years of calcitonin—where are we now? A tribute to the work of Iain Macintyre, FRS

Calcitonin was discovered as a hypocalcemic principal that was initially thought to originate from the parathyroid gland. This view was corrected subsequently, and an origin from the thyroid C cells was documented. The purification and sequencing of various calcitonins soon followed. Calcitonin is a 32-amino-acid-long peptide with an N-terminal disulfide bridge and a C-terminal prolineamide residue. The peptide was shown to potently inhibit bone resorption; however, a direct osteoclastic action of the peptide was confirmed only in the early 1980s. Several osteoclast calcitonin receptors have subsequently been cloned and sequenced. Specific regions of the receptor necessary for ligand binding and intracellular signaling through cyclic AMP and calcium have been identified through systematic deletion mutagenesis and chimeric receptor studies. Calcitonins potent antiresorptive effect has led to its use in treating Pagets disease of bone, osteoporosis, and hypercalcemia. This review retraces key aspects of the synthesis and structure of calcitonin, its cellular and molecular actions, and its therapeutic uses as they have emerged over the 40 years since its discovery. The review also examines the implications of these findings for future clinical applications as a tribute to early workers to whom credit must be given for creation of an important and expanding field. Notable are the new approaches currently being used to enhance calcitonin action, including novel allosteric activators of the calcitonin receptor, modulation of the release of endogenous calcitonin by calcimimetic agents, as well as the development of oral calcitonins.

Osteoporosis and Diabetes Mellitus

Osteoporosis is defined as microarchitectural deterioration of bone with increased susceptibility to fragility fractures [1]. Between fifteen and twenty million persons in the United States currently have osteoporosis [2]. The presence of a generalized osteoporosis related to diabetes mellitus (DM) is less acknowledged and controversial, although localized bone lesions of the foot are well-recognized complications of DM. Due to the different pathogenetic mechanisms in Type 1 DM and Type 2 DM, the association with osteoporosis is even is less clear. Factors associated with diabetes-osteoporosis, which may account for the pathogenesis of diabetic bone loss have been studied. These include vascular and possibly neuropathic mechanisms, poor glycemic control, abnormalities of calcium and vitamin D metabolism, and hypercalciuria with secondary increase in parathyroid hormone (PTH) secretion. Vitamin D levels have been found to be normal or low. Histomorphometry has revealed decreased bone turnover. Other factors include a negative association with sex hormone binding globulin (SHBG), higher free testosterone, and free estrogen levels. This article will review the relevant literature relating to diabetes and osteoporosis including cellular and animal models. The role of insulin, insulin-like growth factor-1 (IGF-1), glycosylated end products, poor glycemic control, acidosis, ketosis, vitamin D metabolism, BMD, and bone markers will be investigated. Duration of diabetes as well as microvascular complications will also be addressed. The data on cellular mechanisms and experimental models is extensive but despite this, the relevance to the clinical situation is unclear. The presence of Type 1 DM imparts several deleterious consequences for skeletal health, including diminished peak height velocity during the pubertal growth spurt, decreased adult bone density, an increased risk for osteoporosis and fracture, poor bone healing and diabetes-induced skeletal embryopathy, processes which rely on de novo bone formation. Clinical studies in Type 1 DM appears to be concentrated on bone density and bone growth in adolescents, the importance of insulin control, nutrition, celiac disease, and bone markers. The fracture data in this cohort is either absent, or poorly documented. In Type 2 DM, the influence of body weight, fracture incidence, BMD and bone turnover appear to be the prime factors and will be addressed. General recommendations for screening and treatment will be briefly discussed, but once again, the evidence in terms of effective therapy specifically tailored to the diabetic patient is again lacking.

Disorders Associated With Acute Rapid and Severe Bone Loss

We describe a constellation of bone diseases characterized by the common feature of acute, rapid, and severe bone loss accompanied by dramatic fracture rates. These disorders are poorly recognized, resulting mainly from systemic diseases, frailty, immobilization, and immunosuppressive drugs, such as glucocorticoids and the calcineurin inhibitors. The opportunity to prevent or treat fractures is commonly missed because they are often not detected. Ideally, patients need to be identified early and preventative therapy initiated promptly to avoid the rapid bone loss and fractures. The most effective therapy at present seems to be the bisphosphonates, particularly when bone resorption is predominant. However, more severe forms of bone loss that result from an osteoblastic defect and reduced bone formation may benefit potentially more from newer anabolic agents, such as recombinant human parathyroid hormone (rhPTH).

Calcitonin: the other thyroid hormone.

Calcitonin was originally discovered as a hypocalcemic factor synthesized by thyroid parafollicular C cells. Early experiments demonstrated that calcitonin inhibited bone resorption and decreased calcium efflux from isolated cat tibiae and subsequent histologic and culture studies confirmed the osteoclast as its major site of action. Its potent antiresorptive effect and analgesic action have led to its clinical use in treatment of Pagets bone disease, osteoporosis, and hypercalcemia of malignancy. This review surveys the cellular and molecular basis of these physiologic and clinical actions.

Calcitonin: physiological actions and clinical applications.

Calcitonin (CT) was first reported as a hypocalcemic principle, initially thought to originate from the parathyroid gland, a view subsequently corrected to an origin from parafollicular C-cells. Human CT is a 32 amino acid peptide with an N-terminal disulphide bridge and a C-terminal prolineamide residue, shown to potently inhibit bone resorption. More recent studies have demonstrated that this may take place through a direct osteoclastic action. A number of osteoclast CT receptors have subsequently been characterized and particular receptor regions necessary for ligand binding and intracellular signaling identified. Its potent anti-resorptive effect has led to its use in treating Pagets bone disease, osteoporosis, hypercalcaemia and osteogenesis imperfecta. This review summarises some key aspects of its synthesis, structure and its actions at the cellular and molecular levels, and leads on to its therapeutic uses that have emerged since its discovery as well as possibilities for future clinical applications.

Molecular and Clinical Pharmacology of Calcitonin

Publisher Summary Calcitonin is a 32 amino-acid-long peptide with an N-terminal disulfide bridge and a C-terminal prolineamide residue. The calcitonin gene complex comprises two known genes, the α and β genes. In man, both genes are located on chromosome 11 between the catalase and parathyroid hormone genes. The α calcitonin gene has six exons, where the first three exons are shared between calcitonin and the alternative splice product, calcitonin gene-related peptide (CGRP). The organization of the β gene is similar to that of the α gene, and gives rise to β-CGRP, but not to a second calcitonin at least in man. The cloning and sequencing of the calcitonin promoter has revealed a negative response element to vitamin D, a cAMP-response element (CRE), and a transcriptionally active octomer sequence (CRE-O). Biologically active calcitonin comprises 32 amino acids and it has an N-terminal disulfide bridge between residues one and seven in contrast to CGRP, in which the latter is positioned between residues two and seven. Modifications of the primary structure that cause a reduction in biological activity include deletion of the C-terminal proline amide, shortening of the C-terminal end, cleavage of the disulfide bond or oxidation of the methionine at position 8. Calcitonin inhibits basal and stimulated resorption of intact bone in organ culture. In bone sections, calcitonin acts directly on the osteoclast to cause a rapid loss of the ruffled border, and when applied for a longer term, there is a reduction in the number of osteoclasts in bone. When applied in vitro to isolated osteoclasts, femtomolar concentrations of calcitonin result in an acute cessation of cytoplasmic motility followed by gradual pseudopodial retraction. Calcitonins utility as a pharmaceutical agent is primarily due to its inhibitory effect on osteoclastic bone resorption.

The Molecular Pharmacology of Osteoporosis

Skeletal remodeling requires a balance between the resorption and formation of bone. Osteoclasts resorb bone, while osteoblasts are bone forming. These two processes are closely coupled but independently regulated by various hormones and cytokines. Osteoporosis is a disease characterized by excess osteoclastic activity compared to osteoblastic activity. Because of excess osteoclastic activity bones become fragile and may fracture, typically in the hip, spine, and wrist. Pharmacological therapy for osteoporosis should be aimed at preventing bone loss, whether the cause is involutional, postmenopausal, or secondary.

Transplantation Bone Disease Induced by Non-Steroid Immunosuppressants

Publisher Summary This chapter focuses on the effect of drugs other than glucocorticoids on bone. The recognition and identification of the role of T lymphocytes and B lymphocyte in mediating the immune reaction involved in virtually every disease, including organ transplantation, allowed development of the class of drugs termed “immune modulators.” These drugs either enhance or suppress the immune reaction depending upon what type of modulation is required to affect the disease outcome. These drugs include the calcineurin inhibitors (CIs) cyclosporine and tacrolimus and the non-CIs rapamycin, mycophenolate mofetil, methotrexate, and azathioprine. These immune-modulating drugs have had an enormous impact on prolonging the lifespan of patients but it adversely affects the bone. The non-steroidal immunosuppressants belonging to the calcineurin inhibiting family have been shown, experimentally and clinically, to produce severe and rapid high-turnover bone loss. Other immunosuppressants, such as sirolimus, azathioprine, and mycophenolate mofetil have not yet been clearly demonstrated experimentally to have adverse effects on bone. A development of immunosuppressant drugs that can prevent organ rejection and other adverse side effects, including bone loss would be a major advancement in the field of organ transplantation.

The Cellular and Molecular Aspects of Immunosuppressant Osteoporosis

The advent of immunosuppressant agents that prevent organ rejection and prolong life has revolutionized the field of organ transplantation. However, these drugs possess numerous side effects, including bone loss that results in osteoporosis and fracture. This chapter will focus on the mechanism of action of the major immunosuppressants used clinically. These include glucocorticoids, the calcineurin inhibitors (cyclosporin A [CsA] and tacrolimus [FK506]), rapamycin, and several others, such as azathiaprine and mycophenolate mofetil (MMF). We will describe in vitro and in vivo studies that have elucidated their action on bone.