INHIBITION OF CELLULAR GROWTH AND INDUCTION OF APOPTOSIS IN PITUITARY-ADENOMA CELL-LINES BY THE PROTEIN-KINASE-C INHIBITOR HYPERICIN - POTENTIAL THERAPEUTIC APPLICATION

Protein kinase C (PKC) is an enzyme involved in the regulation of cell ular growth, proliferation, and differentiation in a number of tissues including the anterior pituitary, in which it is also believed to pla y a role in hormone secretion. Protein kinase C activity and expressio n have been found to be greater in adenomatous pituitary cells than in normal human and rat pituitary cells and higher in invasive pituitary tumor cells than in noninvasive ones. Inhibition of PKC activity has been shown in a variety of tumor cells to inhibit growth in a dose-rel ated fashion. The purpose of the current study was to determine whethe r hypericin, a potent inhibitor of PKC activity that may be administer ed clinically, alters the growth and proliferation in established pitu itary adenoma Lines and to determine if inhibition of PKC activity ind uces apoptosis, as reported in some other tumor cell types. Two establ ished pituitary adenoma cell lines, AtT-20 and GH(4)C(1), were treated with hypericin in tissue culture for defined periods following passag e. Inhibition of growth was found to be dose dependent in all three ce ll lines in low micromolar concentrations of hypericin, as determined by viable cell counts, methylthiotetrazole assay, and [H-3]thymidine u ptake studies. Concentrations of hypericin as low as 100 nM also induc ed apoptosis in these established lines, whereas treatment of normal h uman fibroblasts with a concentration of 10 mu M failed to induce apop tosis. The potential use of hypericin in the therapy of pituitary aden omas warrants additional in vitro investigations with the aim of later moving toward therapeutic trials in selected patients in whom surgica l or medical therapy has failed.