Back to the future: revisiting parathyroid hormone and calcitonin control of bone remodeling.

This review reflects on the past, present, and future of translational research on calcitropic hormones and bone metabolism. Calcitonin (CT) and parathormone (PTH) are complementary hormones involved in the acquisition and maintenance of bone mass and regulation of calcium metabolism. Early research demonstrated that these hormones could have an important role in the treatment of osteoporosis. Calcitonin was approved for this indication by the FDA more than two decades ago, and PTH gained regulatory approval for the treatment of osteoporosis nearly ten years ago. Unfortunately, basic research underlying the mechanism of action of these agents has lagged behind drug approval, and the role of these hormones in bone remodeling is still not firmly established. Moreover, research in bone biology shifted from these hormones to smaller molecules and paracrine regulators of skeletal remodeling. Although important, this development was somewhat unfortunate because without a clearer understanding of how calcitropic hormones work, we cannot be sure that they are being used optimally in the management of osteoporosis. In this review, we look at what is known about CT and PTH and the cells that they target, namely osteoblasts, osteoclasts, and osteocytes. We then identify gaps in knowledge and the research needed to fill them. The conduct of mechanistic studies may point to important factors, such as diurnal variation and dose responsiveness that would lead to improved treatment regimens. By reopening lines of basic and clinical investigation and applying those findings at the bedside, we hope to restart the cycle of translational research in this area.