THE CHARACTERIZATION OF THE ACTH PRODUCED BY A PRIMARY PITUITARY TUMOUR IN A PATIENT WITH CUSHING'S DISEASE

Abstract

The nature of ACTH present in pituitary tumours associated with Cushing's disease has not been previously characterized nor correlated with the electron microscopic appearance. The present report describes the culture of tumour tissue obtained from a patient with a pituitary tumour associated with Cushing's disease, and the characterization of the ACTH content of tumour and media by bioassay, immunoassay, and Sephadex G-50 gel filtration. Electron microscopic studies were also performed. The pathological diagnosis was pituitary adenoma with basophilic PAS-positive granules. Electron microscopy showed uniformity of size and shape of the tumour cells, the presence of secretory granules of varying size and density, and disorganized tubular and vacuolar arrays of endoplasmic reticulum. The bioreactive ACTH content of the tumour was 0-29 mug/g, which is markedly below that seen in the normal pituitary, but within the range reported for ectopic ACTH-producing tumours. Immunoreactive ACTH when measured by a C-terminal antibody was five-fold higher than when measured by an N-terminal antibody; the latter gave a value of 1-24 mjg/g. On Sephadex G-50 gel filtration, 9% of the N-terminal immunoreactivity was present in the Vo fraction ('big' ACTH). This latter fraction had a greater percentage of bioreactivity (28%) than previously reported for this molecular species. Analysis of the tumour culture medium revealed a variation in molecular size similar to that seen in the tumour, although the percentage of ACTH of large molecular size was greater, suggesting increased secretion of a possible 'prohormone' by the tumour. Plasma ACTH was characterized by a 2:1 ratio of immunoreactivity (N-terminal) to bioreactivity, and a 4:1 ratio of C-terminal/N-terminal immunoreactivity. This report also appears to be the first of successful short-term tissue culture of a primary ACTH-producing tumour. The granule size was considerably larger than that reported for normal pituitary ACTH-containing cells.