Plasminogen Activators of the Enzyme Characterization and Pituitary Gland: Hormonal Modulation

We studied plasminogen activator (PA) of the rat pituitary gland in organ and cell monolayer culture. Both anterior and intermediate lobes contain, synthesize and secrete a mixture consisting of the two known types of PA: urokinase and so-called tissue PA. Both enzymes were formed essentially by all PA secreting cells, and PA was identified specifically in mammotrophs, corticotrophs, and luteinizing hormone containing gonadotrophs. Pituitary PA production was modulated on exposure to a variety of biological effectors: anterior lobe PA secretion was stimulated by agents that raised intracellular cAMP concentration; this process depended on de novo enzyme synthesis. Enzyme production was repressed by androgens and glucocorticoids. When anterior lobe cultures were maintained in plasminogen-free media, the extracellular, secreted forms of ACTH consisted almost exclusively of the high molecular weight forms (31,000 and 23,000); the smaller forms (13,000 and 4,500) were also found in the extracellular medium of cultures supplemented with plasminogen. In contrast, the size distribution of intracellular ACTH species was unaffected by the presence of plasminogen. These results resemble those previously obtained with pancreatic islets and are consistent with the possibility that plasmin, generated by PA secretion, participates in prohormone processing. PA synthesis in intermediate lobe explants was stimulated by exposure to dibutyryl cAMP, and repressed by hydrocortisone. In accordance with the dopaminergic control of intermediate lobe function in some vertebrates, apomorphine strongly repressed PA synthesis in intermediate, but not anterior lobe cultures. A growing body of evidence indicates that plasminogen activator (PA) 1 production is a pervasive cellular mechanism for initiating localized proteolysis (33). This process is common to a broad spectrum of cell types, is correlated with a variety of physiological phenomena and, in most cases, is modulated by specific hormones. PA is particularly conspicuous in endocrine tissue including the thyroid (J.-D. Vassalli, unpublished observations), parathyroid (5), and pancreatic islets; in the latter, the rate of PA synthesis is determined by extracellular glucose concentration and enzyme production may provide a mechanism for prohormone processing (46). Recent studies of adrenocorticotropin (ACTH) structure and synthesis in amphibian (8, 22) and mammalian pituitary (9, 26, 35) have shown that the predominant 4,500-mol-wt form of corticotropin is derived from a larger (31,000 mol wt) ~Abbreviations used in this paper. ACTH, adrenocorticotropin; dbcAMP, dibutyryl cyclic AMP; LH, luteinizing hormone; a-MSH, c~-melanotropic-stimulating hormone; PA, plasminogen activator. precursor that carries the antigenic determinants for both ACTH and B-lipotropin: corticotropin is apparently generated by proteolytic processing at sites consisting of two adjacent positively charged amino acids. Such sites are common to many proteins, including the prohormones for insulin (4, 38), ACTH (27), and parathyroid hormone (15) among others (18, 34), and indicate that processing is probably catalyzed by an enzyme with trypsin-like specificity. This inference is strengthened by the finding that brief trypsinization can convert both proinsulin (38) and "big" ACTH to the respective hormonal derivatives (12); however, trypsin itself cannot normally mediate prohormone conversion since it is produced only in the exocrine pancreas, and another enzyme must therefore be responsible for hormone processing. A number of considerations prompted us to examine PA production in the pituitary gland: firstly, as already noted, PA is synthesized abundantly in other endocrines that secrete peptide hormones, but not in the adrenal cortex where only steroid hormones are produced (M. McConnell and E. Reich, THE JOURNAL OF C£LL BIOLOGY VOLUME 97 October 1983 1029-1037 © The Rockefeller University Press • 0021-9525183/10/1029/09 $I .0