Calcitonin, prolactin, and growth hormone gene expression as model systems for the characterization of neuroendocrine regulation

Regulation of calcitonin gene transcription by vitamin D metabolites in vivo in the rat

Calcitonin is secreted by the C cells of the thyroid in response to a raised serum calcium, and acts on bone to lower serum calcium. The C cells have specific receptors for the dihydroxymetabolite of vitamin D3, 1,25(OH)2D3. Moreover, calcitonin stimulates the synthesis of 1,25(OH)2D3 in the kidney. Parathyroid hormone (PTH), the third calciotrophic hormone, is also trophic to the renal synthesis of 1,25(OH)2D3, and in turn 1,25(OH)2D3 inhibits PTH gene transcription and synthesis. We report here the marked inhibition of calcitonin gene transcription by the injection of physiologically relevant doses of 1,25(OH)2D3 to normal rats that did not raise serum calcium. Calcitonin mRNA levels after 100 pmol 1,25(OH)2D3 decreased to 6% of basal at 6 h and 4% at 48 h, and a dose response showed a marked effect even after 12.5 pmol 1,25(OH)2D3, with no appreciably greater effect with larger doses (up to 200 pmol). Control genes, actin, thyroglobulin (thyroid follicular cells), somatostatin (thyroid C-cells) were not affected by 1,25(OH)2D3. Gel blots showed that 1,25(OH)2D3 decreased calcitonin mRNA levels without any change in its size. In vitro nuclear transcription showed that 1,25(OH)2D3-treated (100 pmol) rats' calcitonin transcription was 10% of control, while thyroglobulin and actin were 100%. We propose that calcium is the major regulator of PTH and calcitonin secretion, while 1,25(OH)2D3 is an important regulator of PTH and calcitonin gene transcription. We believe this to be the first demonstration of an effect of 1,25(OH)2D3 on the C cells thereby establishing a new target organ and site of action of vitamin D. Calcitonin is trophic to 1,25(OH)2D3 synthesis, which in turn inhibits calcitonin synthesis, which are the components of a new endocrinological feedback loop.

Calcium, cyclic AMP and protein kinase C--partners in mitogenesis

Evidence is steadily mounting that the proto-oncogenes, whose products organize and start the programs that drive normal eukaryotic cells through their chromosome replication/mitosis cycles, are transiently stimulated by sequential signals from a multi-purpose, receptor-operated mechanism (consisting of internal surges of Ca2+ and bursts of protein kinase C activity resulting from phosphatidylinositol 4,5-bisphosphate breakdown and the opening of membrane Ca2+ channels induced by receptor-associated tyrosine-protein kinase activity) and bursts of cyclic AMP-dependent kinase activity. The bypassing or subversion of the receptor-operated Ca2+/phospholipid breakdown/protein kinase C signalling mechanism is probably the basis of the freeing of cell proliferation from external controls that characterizes all neoplastic transformations.

Calcitonin gene-related peptide and its binding sites in the human central nervous system and pituitary

Binding sites for synthetic human 125I-labeled calcitonin gene-related peptide (125I-CGRP) have been demonstrated in membranes of the human nervous system. Binding was high in the cerebellar cortex (1.35 +/- 0.27 fmol/mg of tissue; mean +/- SEM), spinal cord (1.06 +/- 0.27 to 1.27 +/- 0.23 fmol/mg), and nucleus dentatus (1.02 +/- 0.15 fmol/mg), intermediate in the inferior colliculus (0.80 +/- 0.14 fmol/mg) and substantia nigra (0.75 +/- 0.14 fmol/mg), low in the neocortex, globus pallidus, nucleus caudatus, hippocampus, amygdala, superior colliculus, thalamus, and hypothalamus (0.15-0.32 fmol/mg), and negligible in spinal and sympathetic ganglia and pituitary (less than 0.04 fmol/mg). Autoradiography showed distinct 125I-CGRP binding over the molecular and Purkinje cell layers of the cerebellar cortex and over the substantia gelatinosa posterior of the spinal cord. The highest levels of CGRP-like components were recognized in the dorsal part of the spinal cord and the pituitary gland. In the ventral part of the spinal cord as well as in the pituitary and thyroid glands, CGRP values were higher when measured by radioreceptorassay as compared to RIA, indicating that at least two CGRP-like components are present. The predominant CGRP-like peak on HPLC had the retention time of synthetic human CGRP. Immunohistochemistry revealed the presence of a dense plexus of CGRP immunoreactive nerve fibers in the dorsal horn of the spinal cord.