Corticotropin-releasing factor stimulates phospholipid methylation and corticotropin secretion in mouse pituitary tumor cells

60 YEARS OF POMC: Biosynthesis, trafficking, and secretion of pro-opiomelanocortin-derived peptides

Pro-opiomelanocortin (POMC) is a prohormone that encodes multiple smaller peptide hormones within its structure. These peptide hormones can be generated by cleavage of POMC at basic residue cleavage sites by prohormone-converting enzymes in the regulated secretory pathway (RSP) of POMC-synthesizing endocrine cells and neurons. The peptides are stored inside the cells in dense-core secretory granules until released in a stimulus-dependent manner. The complexity of the regulation of the biosynthesis, trafficking, and secretion of POMC and its peptides reflects an impressive level of control over many factors involved in the ultimate role of POMC-expressing cells, that is, to produce a range of different biologically active peptide hormones ready for action when signaled by the body. From the discovery of POMC as the precursor to adrenocorticotropic hormone (ACTH) and β-lipotropin in the late 1970s to our current knowledge, the understanding of POMC physiology remains a monumental body of work that has provided insight into many aspects of molecular endocrinology. In this article, we describe the intracellular trafficking of POMC in endocrine cells, its sorting into dense-core secretory granules and transport of these granules to the RSP. Additionally, we review the enzymes involved in the maturation of POMC to its various peptides and the mechanisms involved in the differential processing of POMC in different cell types. Finally, we highlight studies pertaining to the regulation of ACTH secretion in the anterior and intermediate pituitary and POMC neurons of the hypothalamus.

Major role of cathepsin L for producing the peptide hormones ACTH, beta-endorphin, and alpha-MSH, illustrated by protease gene knockout and expression

The pituitary hormones adrenocorticotropic hormone (ACTH), beta-endorphin, and alpha-melanocyte stimulating hormone (alpha-MSH) are synthesized by proteolytic processing of their common proopiomelanocortin (POMC) precursor. Key findings from this study show that cathepsin L functions as a major proteolytic enzyme for the production of POMC-derived peptide hormones in secretory vesicles. Specifically, cathepsin L knock-out mice showed major decreases in ACTH, beta-endorphin, and alpha-MSH that were reduced to 23, 18, and 7% of wild-type controls (100%) in pituitary. These decreased peptide levels were accompanied by increased levels of POMC consistent with proteolysis of POMC by cathepsin L. Immunofluorescence microscopy showed colocalization of cathepsin L with beta-endorphin and alpha-MSH in the intermediate pituitary and with ACTH in the anterior pituitary. In contrast, cathepsin L was only partially colocalized with the lysosomal marker Lamp-1 in pituitary, consistent with its extralysosomal function in secretory vesicles. Expression of cathepsin L in pituitary AtT-20 cells resulted in increased ACTH and beta-endorphin in the regulated secretory pathway. Furthermore, treatment of AtT-20 cells with CLIK-148, a specific inhibitor of cathepsin L, resulted in reduced production of ACTH and accumulation of POMC. These findings demonstrate a prominent role for cathepsin L in the production of ACTH, beta-endorphin, and alpha-MSH peptide hormones in the regulated secretory pathway.

Bridging the gap between protein carboxyl methylation and phospholipid methylation to understand glucose-stimulated insulin secretion from the pancreatic beta cell

Recent findings have implicated post-translational modifications at C-terminal cysteines [e.g., methylation] of specific proteins [e.g., G-proteins] in glucose-stimulated insulin secretion [GSIS]. Furthermore, methylation at the C-terminal leucine of the catalytic subunit of protein phosphatase 2A [PP2Ac] has also been shown to be relevant for GSIS. In addition to these two classes of protein methyl transferases, a novel class of glucose-activated phospholipid methyl transferases have also been identified in the beta cell. These enzymes catalyze three successive methylations of phosphatidylethanolamine to yield phosphatidylcholine. The "newly formed" phosphatidylcholine is felt to induce alterations in the membrane fluidity, which might favor vesicular fusion with the plasma membrane for the exocytosis of insulin. The objectives of this commentary are to: (i) review the existing evidence on the regulation, by glucose and other insulin secretagogues, of post-translational carboxylmethylation [CML] of specific proteins in the beta cell; (ii) discuss the experimental evidence, which implicates regulation, by glucose and other insulin secretagogues, of phosphatidylethanolamine methylation in the islet beta cell; (iii) propose a model for potential cross-talk between the protein and lipid methylation pathways in the regulation of GSIS and (iv) highlight potential avenues for future research, including the development of specific pharmacological inhibitors to further decipher regulatory roles for these methylation reactions in islet beta cell function.

Membrane phospholipid methylation is associated with surfactant secretion in rabbit type II alveolar epithelial cells

We investigated the involvement of membrane phospholipid methylation in receptor-mediated secretion of surfactant in adult rabbit type II alveolar epithelial cells (type II pneumocyte). Phospholipid methyltransferase activity was found in type II pneumocyte microsomes. Cell cultures of adult rabbit type II pneumocytes were then used to assay methyltransferase activity in the presence of the beta-adrenergic agonist, terbutaline, and the methyltransferase inhibitor, 3-deazaadenosine. Terbutaline predictably stimulated adenylate cyclase activity and surfactant secretion. It was also found to stimulate incorporation of methyl groups into phosphatidylcholine and to increase beta-adrenergic receptor availability as assayed by binding of dihydroalprenolol (DHA). Surfactant secretion, as well as adenylate cyclase activity, were stimulated by terbutaline and were inhibited by 3-deazaadenosine. 3-Deazaadenosine did not inhibit DHA binding. These results suggest that phospholipid methylation plays a role in stimulus-secretion coupling in adult rabbit type II pneumocytes.

Biochemical responses of the luminal epithelium and uterine sensitization

In our discussion of responses of the uterine epithelium to the blastocyst and their relationship to sensitization to deciduogenic stimuli, we have attempted to identify pieces of information that could be arranged into possible mechanisms. These choices and the limits of space resulted in a great deal of information that was not mentioned: changes in the charge and composition of the glycocalyx of the luminal epithelium, the morphology of the apical surface of the epithelium, steroid hormone receptor levels, epithelial protein synthesis, and most of the information obtained from species other than rats and mice. Despite whatever distortion that may have resulted from our reductions and simplifications of the mechanisms of implantation, it should be clear that blastocyst implantation involves communication not only between blastocyst and uterus, but also between various uterine tissues. Our understanding of implantation will be improved if we can identify the responses of individual tissues and the mechanisms of this communication. It seems likely that development and loss of uterine sensitivity controlled by progesterone and estradiol will depend upon separate biochemical responses in specific uterine tissues.

Inhibitors of the cytochrome P-450 enzymes block the secretagogue-induced release of corticotropin in mouse pituitary tumor cells

A mouse pituitary tumor cell line (AtT-20) releases corticotropin (ACTH) in response to a number of secretagogues, including corticotropin-releasing factor (CRF), beta-adrenergic agents, N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate (Bt2 cAMP), and potassium. The stimulation of ACTH secretion induced by the secretagogues can be blocked by inhibitors of the enzymes that generate (phospholipase A2) and metabolize (lipoxygenase and epoxygenase) arachidonic acid. The phospholipase A2 blockers mepacrine and p-bromophenacylbromide inhibited the ACTH release induced by secretagogues. The lipoxygenase inhibitors nordihydroguaieretic acid, butylated hydroxytoluene, and icosatetraynoic acid abolished the ACTH secretion induced by secretagogues, whereas indomethacin, a cycloxygenase inhibitor, did not. Blockers of the cytochrome P-450 epoxygenase, such as SKF 525A and piperonyl butoxide, compounds that have different molecular structures, also suppressed secretagogue-induced ACTH release. These findings suggest that metabolites of arachidonic acid formed via the epoxygenase and/or the lipoxygenase pathway are involved in the stimulation of ACTH release caused by secretagogues.

Cellular mechanisms regulating adrenocorticotropin release

Using a tumor cell line of the mouse anterior pituitary (AtT-20/D16-16), the various factors regulating ACTH release and the intracellular mechanisms mediating this response were investigated. CRF, catecholamines and VIP stimulate ACTH release whereas glucocorticoids and SRIF block secretion. Glucocorticoids block both ACTH synthesis and release. SRIF acts through multiple mechanisms to prevent stimulated ACTH release. Cyclic AMP and Ca++ are important second messengers in the receptor mediated release of ACTH but other mediators may also be involved. The interaction of these various CRF-like substances and inhibitors of ACTH release may result in a fine-tune regulation of corticotroph activity Such regulation may be important in the organism response to stress.