EXPRESSION OF D-TYPE CYCLINS IN NORMAL AND NEOPLASTIC RAT PITUITARY

The D-type cyclins (D1, D2, and D3) are involved in progression throug h the G1 phase of the cell cycle and are induced as part of the delaye d early response to growth factor stimulation. Tu better understand th e role of D-type cyclins in pituitary cell function and the regulatory role of growth factors in the cell. cycle, we analyzed the expression and regulation of D-type cyclins in normal and neoplastic rat pituita ry cells. Immunocytochemical and RT-PCR analyses showed expression of all three D-type cyclins in the normal pituitary, with higher percenta ges of positive cells by immunocytochemistry in the nuclei of normal p ituitaries (D1, 20-30%; D2, 50-60%; D3, 70-80%), compared with GH(3) c ells. In the normal pituitary, there were significantly higher levels of cyclins D2 and DS in PRL, GI-I, LH, and TSH cells, compared with AC TH cells. Cyclin DI protein was not detected in GH(3) cells, while D2 was present in less than 1 percent and D3 in 10-15 percent of GH(3) ce lls. There were low levels of cyclin D1 and D2 messenger RNA expressio n in GH(3) cells, by RT-PCR. When dissociated rat pituitary cells were cultured in the presence of basic fibroblast growth factor (5.6 nM) f or 3 days, cyclin D2 was up-regulated X-fold in normal PRL cells (cont rol, 33 +/- 1%; treated, 68 +/- 2%). Similarly, bFGF treatment stimula ted cyclin D3 expression 3-fold in GH(3) cells (control, 15 +/- 1%; tr eated, 44 +/- 1%). Treatment of GH(3) cells with 5-aza-2'-deoxgcytidin e, which induces gene demethylation, produced marked increases in cycl in D2 and D3 expression. Transfection of mouse cyclin D1 complementary DNA, driven by a cytomegalovirus promoter inter GH(3) cells, led to e ctopic cyclin D1 expression; and there was a slight stimulation of cel l proliferation and increased apoptosis in GH(3) cells. These results indicate that there is a differential expression of various D-type cyc lins in different types of normal pituitary cells and between normal p ituitary and GH(3) cells. Growth factors, such as bFGF and demethylati on increased D-type cyclin expression, whereas ectopic overexpression of cyclin D1 stimulates cell proliferation and increases apoptosis in GH(3) pituitary tumor cells.