The concentration of calcium in blood and extracellular fluids is controlled largely by parathyroid hormone (PTH). PTH is the major regulator of the concentration of serum calcium. In turn, extracellular calcium acts to suppress PTH secretion, a mechanism that is disordered in hyperparathyroidism. The mechanism by which extracellular calcium, an extracellular signal, is transmitted to the inside of the cell to result in the suppression of PTH secretion is not known. There are several systems that have been studied including the second messengers: cytosolic calcium, cAMP and diacylglycerol (DG). None of these have fully and satisfactorily explained how extracellular calcium controls PTH secretion. One of these second messengers DG results in unique actions in parathyroid cells. In most cells, diacylglycerol activates the enzyme protein kinase C (PKC). This action is largely mimicked in most tissues by the compound phorbol myristate acetate. In the presence of high extracellular calcium, parathyroids cells are unusual in that they exhibit enhanced cellular DG, but suppressed PKC activity; moreover the action of phorbol myristate acetate is not mimicked by states that enhance cellular DG in the parathyroid. Recently a unique second messenger pathway has been described resulting in the formation of the lipid sphingosine, an inhibitor of PKC. Sphingosine inhibits PTH secretion. Sphingosine formation results from the activation of the enzyme sphingomyelinase which results in ceramide production which is then converted to sphingosine. Sphingomyelinase is activated in the parathyroid by DG, high calcium, vitamin D but not PMA.
The aims of this proposal are to further describe the control of this pathway, especially by extracellular calcium, vitamin D and conditions that effect cellular DG metabolism. The cell model will be acutely dispersed bovine parathyroid cells. We will measure the content of ceramide and sphingosine in parathyroid cells as measures of the activity of the pathway. Pharmacologic agents that either block sphingomyelinase or alternate metabolic pathways for ceramide will be used to study the role of this pathway in controlling PTH secretion from parathyroid cells. Sphingosine, ceramide and exogenous enzyme will be added as well to examine if the effects of the endogenous pathway can be reproduced. All of these agents will be used to try to define the role of this pathway in the response of PTH secretion to high extracellular calcium, 1,25 dihydroxycholecalciferol and changes in cellular DG content. The proposed studies will also measure PKC activity to examine if the actions of the sphingomyelinase pathway are mediated by PKC. These proposed studies are relevant to the understanding of disordered PTH secretion in primary and secondary hyperparathyroidism. The sphingomyelinase pathway may have as yet unrecognized importance in other cells and tissues as well.
| Bielawska, A; Greenberg, M S; Perry, D et al. (1996) (1S,2R)-D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol as an inhibitor of ceramidase. J Biol Chem 271:12646-54 |
| McKay, C; Miller, A (1996) Relationship among cellular diacylglycerol, sphingosine formation, protein kinase C activity, and parathyroid hormone secretion from dispersed bovine parathyroid cells. Endocrinology 137:2473-9 |
