Synthesis and glycosylation of proopiomelanocortin by a Cushing tumor

Abstract

ACTH derives from a larger protein, proopiomelanocortin (POMC), which is also the precursor to lipotropin, endorphin, melanotropin, and other peptides. The sequence of steps by which POMC is processed to ACTH has been delineated in detail in cultured mouse tumor cells, but little is known about these steps in human pituitary adenomas. Synthesis and glycosylation of POMC were studied in a pituitary adenoma causing Cushing disease and in adjacent tissue by incubating intact tissue explants in medium containing [35S]methionine or [3H]glucosamine. Labeled tissue was analyzed by two-dimensional gel electrophoresis, and a spot cut from these gels was analyzed by paper electrophoresis of tryptic digests. The location of the spot and the patterns of its [35S]methionine-labeled tryptic fragments resembled those found previously in other ACTH-secreting tissues and hence this spot appears to be POMC. Both the tumor and the adjacent pituitary produced this POMC, although tumor production of POMC was greater. The electrophoretic patterns of the putative POMC tryptic peptides labeled with [35S]methionine from this tumor are similar to the patterns of POMC tryptic peptides from a Nelson tumor and an ectopic ACTH- producing tumor when analyzed by the same techniques. This suggests that the excess ACTH in all three diseases arises from the same precursor POMC molecule. Two-dimensional gel electrophoresis of [3H]glucosamine-labeled tumor suggests that the first proteolytic cleavage occurs between the alpha ACTH-(1-39) and beta-lipotropin sequences and that a subsequent cleavage occurs between the N-terminal region of POMC and ACTH. Paper electrophoresis of [3H]glucosamine-labeled tryptic glycopeptides reveals at least three radioactive peaks, suggesting that the two known glycosylation sites in human POMC may be variably glycosylated.