Up-regulation of ret by reserpine in the adult rat adrenal medulla

The receptor tyrosine kinase, ret, is activated by glial cell line-derived neurotrophic factor, neurturin and related ligands that bind to glycosylphosphatidylinositol-tailed receptors GFRalpha1-4. Ret expression is developmentally regulated and detectable only at very low levels in adult adrenal medulla. However, mutations of ret that cause constitutive activation or alter signal transduction give rise to adrenal medullary hyperplasia and pheochromocytomas in humans with hereditary multiple endocrine neoplasia (MEN) syndromes 2A and 2B and in animal models. These discordant observations pose the conundrum of how a molecule barely detectable in the adult adrenal can contribute to development of adrenal medullary pathology that typically occurs in adults. We recently reported that depolarization and phorbol esters that activate protein kinase C act synergistically with neurturin to up-regulate ret protein and mRNA expression in adult rat chromaffin cell cultures. Those findings suggested that ret expression in vivo is not static and might be regulated in part by neurally derived signals. We show here that the anti-hypertensive agent reserpine, which is known to cause a reflex increase in trans-synaptic stimulation of chromaffin cells, increases expression of ret mRNA and protein in adult rat adrenal medullary tissue in vivo. Elevated ret protein levels are detectable both by immunoblots and immunohistochemistry, which shows immunoreactive ret in chromaffin cells and neurons after reserpine administration. The finding that ret expression is subject to up-regulation by environmental signals in vivo suggests that epigenetic factors might influence the development of adrenal medullary disease by affecting the expression of ret. It is known that long-term administration of reserpine leads to the development of adrenal medullary hyperplasia and pheochromocytomas in rats. Our findings suggest potential utility of the rat model for studying the roles of ret in the adrenal medulla and the mechanisms of its involvement in MEN 2 and other pheochromocytoma syndromes.