Biosynthesis and control of secretion of corticotropic, lipotropic and melanotropic peptides

Marked elevation of serum dehydroepiandrosterone sulphate in Cushing's disease with macronodular adrenocortical hyperplasia

A 30-year-old man presented with longstanding hypercortisolism and biochemical studies typical of pituitary-dependent Cushing's disease. After unsuccessful transsphenoidal surgery, plasma ACTH transiently became undetectable and adrenal computed tomography (CT) was consistent with macronodular hyperplasia. Serum dehydroepiandrosterone sulphate (DHEA-S) exceeded 10,000 ng/ml (normal 2,000-3,350 ng/ml). Despite either transient adrenal autonomy or variable adrenocortical ACTH hyperresponsiveness, urinary cortisol normalized within eight months following pituitary irradiation alone. Serum DHEA-S fell progressively but remained mildly elevated (4,000 ng/ml). Follow-up CT showed minimal residual adrenal nodularity. Given these findings and a review of the literature, we propose that chronic cosecretion of non-ACTH proopiomelanocortinderived peptides may have stimulated both hypersecretion of DHEA-S and adrenocortical macronodularity in this patient.

The regulation of the corticomelanotropic cell activity in aves. III--Effect of various peptides on the release of MSH from dispersed, perfused duck pituitary cells. Cosecretion of ACTH with MSH

1. The melanotropin-releasing activity of arginine-vasopressin (AVP), arginine-vasotocin (AVT), oxitocin (OT), mesotocin (MT) and corticotropin-releasing factor (CRF) was studied in the duck using dispersed, perfused pituitary cells and a specific alpha-MSH RIA. 2. Log dose-response curves were obtained for all the peptides ranging from 5 to 100 ng/ml. All peptides behaved as partial agonists compared to duck median eminence extracts (DME). 3. AVT and MT displayed an alpha-MSH releasing capacity of 60% relative to DME whereas all other peptides behaved as weak agonists with less than 15% capacity relative to DME. 4. AVT and CRF when perfused together acted synergistically on alpha-MSH release yielding a dose response line whose slope approximated that of DME. 5. ACTH was cosecreted together with alpha-MSH in all situations studied with an ACTH to alpha-MSH molar ratio of about 10. 6. It is concluded that CRF and neurohypophyseal peptides may be physiological stimulators of both alpha-MSH and ACTH release in aves.

Pro-opiocortin: the multiple adrenal hormone precursor. Review

Corticotropin (ACTH) is biosynthesized in the human pituitary gland as a long polypeptide precursor (pro-opiocortin) of some 240 residues. When ACTH is secreted in response to stress, the peptides derived from the rest of this precursor, pro-gamma-melanotropin (gamma-MSH) and beta-lipotropin (beta-LPH), are also secreted (Fig. 1). This article will describe the search for a biological significance for this phenomenon.

Pro-gamma-melanocyte-stimulating hormone cleavage in adrenal gland undergoing compensatory growth

Regulation of the rapid compensatory growth seen in the remaining adrenal gland of rats following unilateral adrenalectomy is poorly understood. The role of adrenocorticotropic hormone (ACTH) is obscure as immunoneutralization of circulating ACTH does not affect the observed compensatory growth or hyperplasia. This finding, together with the fact that mechanical manipulation of one adrenal without extirpation is followed by growth only in the contralateral gland, has led to the concept of neural regulation of compensatory adrenal growth via a loop from one adrenal through the hypothalamus and back to the contralateral gland which is independent of ACTH secretion. We recently showed that peptides from the N terminal of ACTH precursor proopiocortin (POC), not containing the gamma-melanocyte-stimulating hormone (gamma-MSH) sequence, can stimulate adrenal mitogenesis and proposed that normal long-term adrenal growth and proliferation involves post-secretional proteolytic cleavage of pro-gamma-MSH [or N-POC(1-74)] to generate the mitogenic factor N-POC(1-48/49) and a C-terminal fragment N-POC(50-74), or rat gamma 3-MSH. We have now investigated this hypothesis further in rats by selectively quenching different regions of circulating POC peptides with specific antisera and observing the effect on the increases in weight, RNA and DNA normally seen in the remaining gland following unilateral adrenalectomy. Our results, reported here, suggest that neurally mediated proteolytic cleavage of the circulating inactive mitogenic precursor pro-gamma-MSH at the adrenal gland is the major mechanism of control of compensatory growth.

An investigation of N-terminal pro-opiocortin peptides in the rat pituitary

Extracts of neurointermediate lobe (NIL) and anterior lobe (AL) of the rat pituitary, and material released from perfused rat pars distalis (PD) and pars intermedia (PI) cells were gel chromatographed and monitored using three antisera, each recognizing different regions of the non-corticotropin (ACTH)-lipotropin (LPH) portion of pro-opiocortin (POC). Two peaks (termed N-POC I) which emerged close to the elution position of rat beta-LPH were detected. The first peak was reduced significantly in the PI. Two smaller N-POC fragments which eluted near beta-endorphin were detected only in extracts and secretions of intermediate lobe tissue. One peak cross-reacted in the gamma 3-melanotropin (MSH) assay (N-POC III) whereas the other peak possessed amino (N)-terminal N-POC immunoreactivity (N-POC II). The results demonstrated differences in the distribution and nature of N-POC peptides released and extracted from the PD and PI of the rat pituitary, and suggest that the enzymic processing of N-POC is different in the two pituitary lobes.

Role of vasopressin in the ACTH response to isoprenaline

The present study investigated whether or not the beta-sympathomimetic amine isoprenaline, given systemically to conscious rats, influences corticotrophin (ACTH) release and if so, what could be the role of vasopressin in this response. Isoprenaline (i.m.) elevated plasma ACTH-like immunoreactivity (ACTHi) in a time- and dose-dependent manner. The highest dose of isoprenaline used (240 microgram/kg) raised plasma ACTHi about six fold. Most of the ACTHi co-migrated with porcine ACTH-(1-39) on Sephadex G-50 column chromatography. The beta-receptor antagonist propranolol abolished the increase in plasma ACTHi induced by isoprenaline, as did dexamethasone pretreatment. The increase in plasma ACTHi following isoprenaline (120 microgram/kg) injection was diminished by about 35% in rats congenitally lacking vasopressin (Brattleboro rats), when compared to normal rats. The vasopressin analogue, [1-deaminopenicillamine, 2-(O-methyl)tyrosine]-arginine-vasopressin, almost completely prevented the rise in plasma ACTHi provoked by i.v. injection of arginine vasopressin and diminished by about 40% the isoprenaline-(120 microgram/kg)-caused ACTHi release. However, this vasopressin analogue had no effect in Brattleboro rats. These results indicate that isoprenaline, given systemically, stimulates the release of pituitary ACTH and this response appears to be mediated in part by vasopressin acting as an ACTH-releasing factor.

Purification and characterization of amino-terminal pro-opiocortin peptides from human pituitary glands

Two cystine-containing peptides isolated from human pituitaries were partially sequenced. The peptides are fragments derived from the amino-terminal of pro-opiocortin (NPOC) and have been chemically characterized as NPOC 1-28 (which lacks gamma-MSH) and NPOC 2-59. Their availability enables us to investigate new putative biological roles for the amino-terminal conserved sequence of pro-opiocortin.