Immunohistochemical expression of protein kinase C type III in human pituitary adenomas.

Immunohistochemical expression of the three major isozymes of protein kinase C--Types I, II, and III--was studied in 32 cases of human pituitary adenomas, and the results were compared in detail with their clinical data. Immunoreactivity for the Type I and Type II isozymes was negative in tumor cells of all pituitary adenomas. Moderate to strong cytoplasmic immunoreactivity for the Type III isozyme was constantly seen in acromegaly, Cushing's disease, and nonfunctioning adenomas, which indicated overexpression of the isozyme, since only slight cytoplasmic immunoreactivity was observed in the normal human anterior pituitary cells. Among 13 prolactinomas, 5 cases showed positive immunoreactivity for Type III in all tumor cells, whereas 8 cases showed negative immunoreactivity for the isozyme in all or more than 85% of tumor cells. The sizes of the tumors in this protein kinase C Type III negative group of prolactinomas tended to be smaller than those of the Type III positive prolactinomas. Also, the negative immunoreactivity for Type III was predominantly observed in those cases where prolactinomas were relatively well controlled by continuous oral dosage of dopamine agonists before operation. These results suggest that protein kinase C Type III is closely involved in human pituitary adenomas. The exceptional negativity for the isozyme in prolactinomas may be relevant to the suppression of tumor growth by dopamine agonists.