Proopiolipomelanocortin Peptides in Normal Pituitary, Pituitary Tumor, and Plasma of Normal and Cushing's Horses*

Abstract

Using RIAs for six regions within proopiolipomelanocortin (proOLMC), gel filtration, and electrophoresis, we studied pituitary peptides in a normal horse and one with Cushing's disease caused by a pars intermedia adenoma. Almost all immunoreactive (IR) ACTH (78%) was 4,500 mol wt (4.5K) ACTH in normal pars distalis, but it was almost 100% corticotropin-like intermediate lobe peptide (CLIP) in normal pars intermedia. alpha MSH and beta MSH were found mainly in pars intermedia: equal concentrations of the beta MSH precursors, beta-lipotropin (beta LPH) and gamma LPH, were found in pars distalis. Most IR-beta-endorphin (IR-beta END) was found as beta END in pars intermedia, but roughly equal concentrations of beta END and its precursor, beta LPH, were found in pars distalis. A 33K molecule containing IR-ACTH, IR-gamma 3MSH, and IR-beta END, presumed to be proOLMC, and a variety of 15-27K presumed biosynthetic intermediates were found in both normal pars distalis and pars intermedia. The pars intermedia adenoma causing Cushing's syndrome contained high IR-peptide concentrations. Several differences in precursors were noted, including the presence of three larger presumed precursors (38.5K, 47K, and 63K) that had both ACTH and beta END immunoreactivities and both deletions and additions of 15-27K intermediates. The Cushing's horse's plasma peptides reflected tumor concentrations; 4.5K ACTH was modestly elevated, but the concentrations of CLIP, alpha MSH, beta MSH, gamma LPH, and beta END were dramatically increased. About 20% of plasma IR-ACTH and 5% of IR-beta MSH and IR-beta END were found as high molecular weight forms. Normal processing of horse proOLMC appears to be similar to that in other species, but may be altered in pars intermedia tumors of horses with Cushing's disease, the plasma of which contains disproportionately increased concentrations of pars intermedia proOLMC peptides.

Immunocytochemical Demonstration of Proopiomelanocortin-derived Peptides in Pituitary Adenomas of the Pars Intermedia in Horses

“…'9,21,33 The overall processing of peptides in adenomas of the pars intermedia appears to be similar to that in the normal equine pars intermedia. 19,22,33 This is consistent with the finding of similar patterns of immunostaining in the normal and neoplastic pars intermedia of horses in this study.…”

“…Our immunocytochemical findings illustrate the differences between adenomas of the pars intermedia in horses and corticotroph adenomas of the pars distalis (also pars intermedia in dogs) that result in the classical Cushing's disease in human L0,20,21,23,33 Increased levels of plasma ACTH were believed to be caused by both increased biosynthetic capacity of the tumor cells and by an increase in the total number of ACTH-secreting cells. 33 In the present study, nodular or diffuse adrenal cortical hyperplasia was evident only in four horses with an adenoma of the pars intermedia (Table 1). This observation is consistent with the finding that most horses with pituitary adenomas had plasma cortisol levels in the normal range (unpublished observations) and had weak immunostaining for ACTH.…”

Correlation of Pituitary Histomorphometry with Adrenocorticotrophic Hormone Response to Domperidone Administration in the Diagnosis of Equine Pituitary Pars Intermedia Dysfunction

“…19 Hypertrophy or proliferation of PI melanotrophs is thought to result in increased POMC production. 21,23,30 In this study of donated horses with and without clinical signs of PPID, a few horses had histologically normal pituitary glands (grade 1) and a few had pituitary adenomas (grade 5), but most horses had focal hypertrophy or hyperplasia, diffuse adenomatous hyperplasia, or microadenomas. Similar pathologic findings were previously described in horses and ponies with or without clinical evidence of PPID.…”

Human plasma epidermal growth factor/beta-urogastrone is associated with blood platelets.

“…After incubation was complete, phase separation of antibody-bound from free peptide was achieved by adding 100 jI radioimmunoassay diluent containing 4 Al goat antirabbit immunoglobulin G serum (Calbiochem-Behring Corp., American Hoechst Corp., San Diego, CA), incubating the mixture for 3 h at 4°C, adding 1.6 ml ice-cold radioimmunoassay diluent containing 2.5% (wt/vol) bovine serum albumin (BSA), but not Triton X-100, and centrifuging for 20 min at 6,000 g at 4°C (20,22). The supernates were decanted and the precipitates, which contained the antibody-bound hEGF/UG, were counted in a y-scintillation spectrometer.…”