A serine-proteinase inhibitor (Boc-D-Phe-Pro-Arg-H) inhibits the secretion of adrenocorticotropin- and beta-endorphin-immunoreactive peptides in vitro

Oligopeptides interfering with calcium channels inhibit prolactin and growth hormone release by cultured anterior pituitary cells of the rat

A number of oligopeptides, protected at their N termini and possessing an aldehyde residue at their C terminal amino acids, are able to inhibit 45Ca2+ influx into anterior pituitary cells grown in monolayer culture and depolarized with high extracellular potassium concentration. In addition, the same oligopeptides interfere with hormone release, especially with that produced by lactotrophs. Our findings imply that oligopeptides may represent a new class of calcium channel ligands, and the pituitary cells are sensitive targets for them.

Presence of ACTH and beta-endorphin immunoreactive molecules in the freshwater snail Planorbarius corneus (L.) (Gastropoda, Pulmonata) and their possible role in phagocytosis

The presence of ACTH and beta-endorphin immunoreactive molecules in the cell-free hemolymph and in the hemocytes of the freshwater snail Planorbarius corneus were demonstrated by immunocytochemistry and RIA tests. Only spreading phagocytic hemocytes were positive, in contrast with other hemocytes devoid of phagocytic activity, i.e., round hemocytes. These data were confirmed by flow cytometry. Another cell type with marked phagocytic activity, i.e., digestive cells of digestive gland, were also positive to anti-ACTH. Corticotropin-releasing factor immunoreactive molecules were found in the cell-free hemolymph and hemocytes, by RIA. Our data suggest that cells with phagocytic activity, the oldest immune response, may represent a suitable model to unravel the tangled web of the common ancestor of the immune and the neuroendocrine systems.

Proteinases and their inhibitors in cells and tissues

A large body of evidence has been assembled to indicate the substantial importance of proteolytic processes in various physiological functions. It has recently become clear too that endo-acting peptide bond hydrolases provisionally characterized and classified at present as serine, cysteine, aspartic and metallo together with unknown catalytic mechanism proteinases sometimes act in cascades. They are controlled by natural proteinase inhibitors present in cells and body fluids. In the first part of the present monograph the author was concerned to present an overview on the morphological and physiological approach to localization, surveying reaction principles and methods suitable for visualization of proteolytic enzymes and their natural and synthetic inhibitors. In the second part the roles played by proteinases have been summarized from the point of view of cell biology. The selection of earlier and recent data reviewed on the involvement of proteolysis in the behavior of individual cells reveals that enzymes, whether they be exogeneous or intrinsic, can be effective and sensitive modulators of cellular growth and morphology. There exists a close correlation between malignant growth and degradation of cells. It appears likely that as yet unknown or at least so far inadequately characterized factors that influence the survival or the death of cells may turn out to be proteinases. The causal role of extracellular proteolysis in cancer cell metastases, in stopping cancer cell growth and in cytolysis remains for further investigated. Ovulation, fertilization and implantation are basic biological functions in which proteolytic enzymes play a key role. The emergence of new approaches in reproductive biology and a growing factual basis will inevitably necessitate a reevaluation of present knowledge of proteolytic processes involved. The molecular aspects of intracellular protein catabolism have been discussed in terms of the inhibition of lysosomal and/or non-lysosomal protein breakdown. Peptide and protein hormone biosynthesis and inactivation are still at the centre of interest in cell biology, and a number of proteinases have been implicated in both processes. A number of conjectures partly based on the author's own work have been discussed which suggest the possibility of the involvement of proteolysis in exocytosis and endocytosis. The author's optimistic conclusion is that through the common action of biochemists, cell biologists, cytochemists, and pharmacologists the mystery of cellular proteolysis is beginning to be solved.

Calcium ion uptake, somatotropin release, and fine structure of somatotrophs in cultures of the rat anterior pituitary upon the action of an oligopeptide (Boc-Gln-Leu-Lysinal)

Cultured cells from the anterior pituitary glands of adult rats were treated with the tripeptide aldehyde proteinase inhibitor, Boc-Gln-Leu-Lys-H. The oligopeptide had a profound releasing effect on growth hormone, whereas the prolactin release remained unchanged at 10(-3) mol/l drug concentration after an incubation for 2 h. In the presence of the oligopeptide a time- and dose-dependent calcium influx into cultured cells has been shown which was proved to be almost completely antagonized with magnesium ions but not with Nifedipine. In addition, radioactive calcium ions could be detected in a number of cells by light microscopic autoradiography when cultures were treated with Boc-Gln-Leu-Lys-H for short periods. The selective Gel action of the oligopeptide on growth hormone producing cells has been demonstrated also in fine structural investigations: multigranular and single exocytotic profiles have been observed. Accordingly, we have postulated that Boc-Gln-Leu-Lysinal mimics the effects of the known ionophores. Its mode of action needs, however, further studies especially on isolated somatotrophs.

The tripeptide aldehyde, Boc-DPhe-Phe-Lysinal, is a novel Ca2+ channel inhibitor in pituitary cells

The effect of Boc-DPhe-Phe-Lysinal (Boc-DPPL) on the 45Ca2+ uptake of rat anterior pituitary monolayer cultures was investigated. The compound decreased the basal Ca2+ uptake at 3 x 10(-4) mol/l. The 45Ca2+ uptake stimulated by potassium-induced depolarization was more sensitive to Boc-DPPL inhibition, a slight decrease was seen with 3 x 10(-6) mol/l and there was a half maximal inhibition at 3 x 10(-5) mol/l. Boc-DPPL is known to inhibit pituitary hormone release in similar concentrations, an effect might also be due to its calcium antagonist property.

Is calcium or cyclic AMP involved in the inhibitory effect on pituitary hormone secretion of the tripeptide aldehyde proteinase inhibitors?

The mechanism by which tripeptide aldehyde proteinase inhibitors decrease prolactin (PRL) and growth hormone (GH) secretion was studied. Agents known to modify the intracellular levels of cyclic adenosine monophosphate (cAMP) or cytosolic free calcium were used in monolayer cultures of the rat anterior pituitary gland. The phosphodiesterase inhibitor isobutyl-methylxanthine (IBMX), 8-bromo-cAMP and forskolin all stimulated PRL release. Boc-D-Phe-Pro-arginal (Boc-DPPA) at 1 mmol/l concentration was a potent inhibitor of basal PRL release and significantly decreased the effect of 8-Br-cAMP, forskolin or IBMX (0.5 mmol/l). Forskolin (1 mumol/l) stimulated ACTH, PRL and GH release and all these effects were decreased by 100 mumol/l of Boc-D-Phe-Phe-lysinal (Boc-DPPL). Neither tripeptide aldehyde affected the forskolin-induced rise in intracellular cAMP. Growth hormone releasing factor (hpGRF, 1 nmol/l) stimulated both GH release and intracellular cAMP generation; Boc-DPPL (100 mumol/l) significantly decreased stimulated GH release without affecting cAMP accumulation. Increasing medium K+ to 10 times normal level stimulated PRL release presumably by enhancing Ca2+ entry into the cells and 1 mmol/l Boc-DPPA decreased high potassium-stimulated PRL release. The ionophore A-23187 stimulated PRL release at 10 mumol/l but not at 1 mumol/l. At 1 mumol/l A-23187 prevented the Boc-DPPA-induced inhibition of PRL release. These findings suggest that the tripeptide aldehyde proteinase inhibitors inhibit PRL and GH release at a site beyond cAMP formation.

Dissimilar responsiveness of cultured corticotrophs and melanotrophs to tripeptide aldehydes

Cultured cells from adult rat anterior pituitaries or intermediate lobes were treated with the proteinase inhibitor tripeptide aldehydes BOC-DPhe-Pro-Arg-H (Boc-fPRH) and DPhe-Pro-Arg-H (fPRH), ovine corticotropin-releasing factor (oCRF), and bromocriptine. One millimolar fPRH stimulated basal, and slightly enhanced oCRF-induced ACTH release by melanotrophs in short-term experiments. The basal release of alpha-MSH was also stimulated by the drug. In long-term experiments, fPRH elevated markedly both the release and the intracellular level of ACTH; BOC-fPRH caused an increased alpha-MSH release. Tritiated fPRH had no preference for POMC-producing cells and BOC-fPRH or fPRH were harmless to the cell morphology. In anterior pituitary cell cultures, fPRH diminished slightly basal and oCRF-induced ACTH release. Bromocriptine was ineffective on corticotrophs, however, in melanotrophs it inhibited ACTH release markedly with or without fPRH in the medium. The dissimilar responsiveness of the corticotrophs and melanotrophs to the peptide aldehydes may be interpreted in terms of their differing membrane receptors or intracellular mechanism of stimulus-secretion coupling.

Various proteinase inhibitors decrease prolactin and growth hormone release by anterior pituitary cells

Proteinase inhibitors were tested for their ability to inhibit prolactin (PRL) and growth hormone (GH) release by cultured anterior pituitary cells of the rat. Inhibitors of microbial origin (chymostatin, elastatinal, leupeptin) had either no or a moderate effect on hormone release while some tripeptide aldehydes, especially those with lysine at their C terminus, inhibited markedly PRL and to a lesser extent GH release. Boc-DPhe-Phe-lysinal was the most effective on lactotrophs inhibiting PRL release more than 50% at 10(-4) M. The site(s) of action of tripeptide aldehydes remain to be elucidated.

Inhibition of growth hormone and prolactin secretion by a serine proteinase inhibitor

The action of the tripeptide aldehyde t-butyloxycarbonyl-DPhe-Pro-Arg-H (boc-fPR-H), belonging to a family of serine proteinase inhibitors, on the release of immunoreactive prolactin (iPRL) and growth hormone (iGH) has been studied. In rat anterior pituitary cell cultures and pituitary quarters 1 mM boc-fPR-H inhibited basal iPRL and iGH release. Thyroliberin-induced iPRL release by cultured cells was also markedly inhibited with a concomitant accumulation of intra-cellular iPRL. During the short- and long-term exposure of cells to boc-fPR-H there no changes in total cell protein contents and in activities of some lysosomal marker enzymes. A wide scale of unchanged parameters characteristic for cellular metabolism indicated that the tripeptide aldehyde has no cytotoxic effect. The marked inhibition of basal as well as stimulated hormone release in the presence of the enzyme inhibitor might suggest that at least a portion of the hormones is released via a proteolytic enzyme-dependent process.

Lysosomal enzyme activities in hypo- and hypersecretory anterior pituitary cells. A combined immunocytochemical and enzyme cytochemical study

The involvement of lysosomes in ACTH and prolactin secretion was studied. Lysosomes were visualized in the anterior pituitary by their non-specific esterase (gold thioacetic acid technique) or acid phosphatase (Gomori technique) activity. Corticotrophs and mammotrophs were identified by postembedding immunocytochemistry for their respective hormones. Corticotrophs were rendered hypersecretory by bilateral adrenalectomy (7 or 12 days prior to examination), hyposecretory by dexamethasone administration. Prolactin secretion was enhanced by 17-beta-estradiol, prolactin release was inhibited by bromoergocriptine administration. Long-term hypersecretion of ACTH was accompanied by the presence of numerous autophagic vacuoles often containing secretory granules in the corticotrophs. Lysosomal enzyme-containing tubules and small lysosomes were abundant in the cytoplasm near the cell membrane, among the mature secretory granules. Feed-back inhibition of ACTH release by dexamethasone resulted in the extension of enzyme-containing tubules, continuous with cisternae and small lysosomes anywhere in the cytoplasm and in the appearance of numerous crinophagic vacuoles. A higher frequency of tubular lysosomes was described at the periphery of mammotrophs stimulated by 17-beta-estradiol. Bromoergocriptine caused a high incidence of characteristic crinophagic vacuoles in the prolactin cells. The concept of crinophagy has been extended to the corticotrophs. Morphological phenomena were attributed to the traffic and increased turnover of membranes, ligands and cytoplasmic organelles during stimulated secretion.