Analytical subcellular fractionation of rat pituitary homogenates, with special reference to prolactin proteolysis by lysosomes

Identification and localization of 23,000 and glycosylated rat prolactin in subcellular fractions of rat anterior pituitary and purified secretory granules

Rat pituitary homogenates were submitted to differential and density gradient centrifugation. Subcellular fractions as well as the purified secretory granules were examined in electron microscopy, radioimmunological techniques, protease digestion, alkaline treatment and immunoblotting. The global outcome of these experiments was that: 1) the glycosylated rPRL was foremost recorded in the crude secretory granular fraction, also in the microsomal fraction and the cytosol, but virtually not in the plasma membrane fraction; 2) in purified secretory granules glycosylated rPRL appeared as an array of near Mr, such as was formerly obtained by enzymatic deglycosylation; 3) protease digestion and ice-cold alkaline treatment of the secretory granules showed that 23,000 rPRL appears in three different physicochemical states in these organelles: unsequestered within a closed system, membrane-bounded and bound state; 4) likewise treatment of microsomal vesicles showed that 23,000 and glycosylated rPRL are sequestered in these bodies, but apparently 23,000 rPRL appears as both integral membrane-bound and released from the lumen, whereas glycosylated rPRL is chiefly retained as an integral membrane protein. 5) dopamine alters the pattern of glycosylation as well in Mr as in relative percentages of the molecular variants. The systematical occurrence of the array of near Mr glycosylated rPRL is biosynthesized as a pool of proteins with a different degree of glycosylation. On the basis of our data, we speculate that selection of definite molecular variants from this pool could play an important role in the biological function of 23,000 rPRL and that oligosaccharides could perhaps target the glycosylated forms of rPRL to specific sites of action.

Ultrastructural immunocytochemical localization of secretory proteins in autophagic vacuoles of parotid acinar cells of starved rats

Previous studies have shown that reduction of mastication has marked effects on the structure and biochemistry of the rat parotid gland. Acute starvation results in the formation in the acinar cells of large autophagic vacuoles which contain lysosomal hydrolases and within which secretory granules appear to undergo degradation. In this study we used electron microscopic immunocytochemistry and antibodies to two secretory proteins, alpha-amylase and B1-immunoreactive protein, to determine whether secretory proteins are present in autophagic vacuoles of parotid acinar cells of starved rats. Small vacuoles were observed after 24-h starvation; they increased in size and number up to 72-h starvation. Both secretory proteins were present in the secretory granules and in the dense content of the autophagic vacuoles, as shown by immunogold labelling. The lighter matrix of the vacuoles was unlabelled. These findings confirm that secretory granules may fuse with lysosomal structures, where their content of secretory proteins is presumably degraded. Thus, the rat parotid appears to be similar to other secretory cells in which cellular levels of stored secretory proteins may be regulated by the process of crinophagy.

Subcellular distribution of rat pituitary homogenates by poly(ethylene glycol)-dextran countercurrent partitioning

Rat pituitary homogenates were subjected to two phase countercurrent partition in a poly(ethylene glycol)-dextran mixture using a simple apparatus with enhanced gravity to facilitate the phase separations. Assay of the fractions for organelle marker enzymes and prolactin after 17 transfers showed similar distributions for endoplasmic reticulum, lysosomes, prolactin granules and plasma membrane at the lowest dextran concentrations. Increasing the dextran concentrations had a differential effect on the various organelles. Excellent resolution of endoplasmic reticulum from the other organelles was obtained and marked organelle heterogeneity was demonstrated. Two-phase countercurrent partition thus offers an alternative approach to the subcellular fractionation of pituitary homogenates and should prove useful in separating endoplasmic reticulum from plasma membrane and other cell components.

The levels and subcellular distribution of hormones and marker enzymes in pituitaries from control subjects and patients with prolactinomas, acromegaly or functionless pituitary tumours

Pituitary homogenates were prepared from patients undergoing pituitary ablation for breast or prostatic carcinoma (controls) and from patients with either PRL- and GH-secreting pituitary adenomas or from patients with 'functionless' pituitary tumours. The principal subcellular organelles, plasma membrane, lysosomes, mitochondria, endoplasmic reticulum, cytosol and hormone-containing granules were characterized by sucrose density gradient centrifugation. Tissue from patients with prolactinomas showed hormone granules, lysosomes and endoplasmic reticulum of lighter density than controls; cytosol, mitochondria and plasma membrane were similar. PRL-secreting tumours showed a 2-fold increase in PRL content with significant reduction of LH, FSH and GH. Activities of various lysosomal enzymes, except for PRL proteolytic activity, were significantly reduced. Similar conclusions were found for GH-secreting pituitary adenomas with a striking reduction in PRL proteolytic activity. Functionless tumours showed significant amounts, though reduced compared to control tissue, of all hormones. In contrast to the hormone-secreting adenomas, the activity of the lysosomal enzyme N-acetyl-beta-glucosaminidase was significantly increased compared to control tissue.

Inappropriately low serum GH in an acromegalic: lysosomal involvement in intracellular hormone degradation

A patient with galactorrhea, amenorrhae and severe acromegaly was found to have a large pituitary adenoma. In view of the severity of the disease the serum growth hormone (GH) level (22.5 mlU/liter) was considered inappropriately low. Tissue from the adenoma was obtained during successful treatment with interstitial irradiation. (90Yttrium). Trypsin-dispersed biopsy cells in culture for 12 days secreted low amounts of GH compared to the same number of adenoma cells from 5 other unselected acromegalics or a normal pituitary. No other hormones were secreted in culture. Immunocytochemical staining was positive only with GH antisera but showed low intracellular content. This was confirmed by direct analysis of the tumor tissue which showed the GH content to be only 20% of that found in 5 normal pituitaries and 4% of that found in 8 other adenomas from acromegalics. Electron microscopy showed a striking appearance, with GH secretory granules that were sparse in number, smaller than usual, and in the main arranged around numerous intracellular profiles with double membranes and low electron density that were tentatively identified as autophagic vacuoles (secondary lysosomes). Subcellular fractionation showed the distribution of the radioimmunoassayable GH in the gradient to be coincident with the peak of the lysosomes whereas in 2 other acromegalics the GH peak was clearly separated from the lysosomes. We conclude that the simultaneous appearance in our patient of the relatively low serum GH together with a large tumor and severe acromegaly can be explained biochemically by the striking finding of crinophagy - disposal of hormone secretory granules within the somatotroph cells themselves.