Mass spectrometric analysis of opioid and tachykinin neuropeptides in non-secreting and ACTH-secreting human pituitary adenomas

Abstract

In a study to test the hypothesis that defects in the metabolism of neuropeptides might be a contributing factor to human anterior pituitary tumor formation, the proenkephalin A, proopiomelanocortin (POMC), and tachykinin systems, which produce methionine enkephalin (ME), beta-endorphin (BE), and substance P (SP), respectively, were measured in patients who had a wide variety of pituitary tumors. Mass spectrometry was used to optimize the level of molecular specificity of the ME and BE analytical measurements, and radioimmunoassay was used to measure SP-like immunoreactivity (SP-li). Compared to data obtained from pituitaries from post-mortem controls, the non-secreting tumors contained a significantly lower amount of the POMC neuropeptide, BE. The lower ME level was not significant. However, two adrenocorticotrophic hormone (ACTH)-secreting tumors contained ME, BE, and SP-li amounts that were much higher than both the controls and nonsecreting tumors. These data suggest that a hypometabolism of the POMC precursor may be operating in non-secreting tumors, and that a hypermetabolism of the proenkephalin A, POMC, and tachykinin precursors may be operating in two ACTH-secreting tumors. These data demonstrate that mass spectrometry plays a critical role in the study of human pituitary tumors.

Analysis of the proteome in the human pituitary

“…The current model of pituitary tumorigenesis considers a multifactorial process that involves intrinsic defects within the gland as well as dysregulation of hypothalamic stimuli [2,5]. Previous studies of our group have focused on the quantitation of opioid and tachykinin neuropeptides in control vs. tumor pituitary tissue [6,7]. We have found significant changes in the amounts of b-endorphin (BE) and methionine enkephalin (ME) in the tumors; those changes suggest that alterations in the neuropeptide metabolism may play a role in pituitary tumorigenesis.…”

Rethinking the stalk effect: A new hypothesis explaining suprasellar tumor-induced hyperprolactinemia

“…However, it is noteworthy that one patient (G.H.) with a non-secreting tumor but increased prolactin levels, was also found to have elevated levels of substance P in this tumor, relative to normal controls [8]. Future prospective studies should investigate the relationship between tachykinins, either in the circulation or the tumor, and prolactin concentrations.…”

Peptide quantification by tandem mass spectrometry

“…POMC is enzymatically processed into ACTH and P-lipotropin (P-LPH), and P-LPH is, in turn, cleaved to form BE and yLPH (Loh, 1992). BE has been quantified in bovine (zhu & Desiderio, 1994), rat , and human Desiderio et al, 1993) pituitaries.…”

“…In the anterior pituitary, P-LPH and "BE" exist in equimolar amounts, whereas in the neurointermediate pituitary, P-LPH is almost entirely processed further to "BE" (Millan & Herz, 1985). Although the human pituitary lacks ~n many species, '<BE" is localked the neurointermediate lobe, it contains abundant "BE" (Dass, Kusmierz, & Desiderio, 1991;Desiderio et al, 1993). C. Proenkephalin,,,,,,,,,, Figure 3(B) contains the cDNA-derived amino acid sequence of preproenkephalinhuma,1.267, which contains four copies of ME (YGGFM), one copy of LE (YGGFL), and one copy each of two C-terminally-extended ME peptides (the octapeptide ME-RGL and the heptapeptide ME-RF) (Numa, 1984).…”