Regulation of cell growth mediated by the calcitonin receptor.

Calcitonin (CT) is a 32 amino acid peptide hormone of thyroidal origin, whose main recognised physiological role is the inhibition of osteoclast--mediated bone resorption. There is also evidence that CT might modulate bone formation. However, both CT and its receptors (CTR) have also been identified in a large number of other cell types and tissue sites, suggesting roles for the CT/CTR system distinct from those involving calcium homeostasis. Evidence has accumulated consistent with the involvement of CT in cell growth and differentiation and in tissue development and remodelling. The close proximity of cells expressing CT, or CT receptors (CTR), during development, and during pregnancy and lactation, is consistent with important roles for CT in morphogenesis. It thus appears that, in tissues such as the uterus, breast and pituitary, CT acts in a paracrine manner to Influence cell proliferation and function, as distinct from its endocrine actions to regulate calcium stores in the skeleton. In vitro studies have shown that CT can be either mitogenic or can inhibit cell proliferation, depending on the cell type and the conditions of the experiment. More recently, evidence has also been obtained for a role for CT in cell survival, in cells as diverse as osteoblast--like and osteocyte--like cells, osteoclasts and neurons.