REGULATION OF PROTHYMOSIN-ALPHA MESSENGER-RNA LEVELS IN RAT PITUITARY-TUMOR CELLS

Prothymosin alpha (PTA) mRNA and histone H4 (H4) mRNA levels were stud ied in various experimental conditions that affected GH1 pituitary tum or cell proliferation. Cell proliferation and progression through the cell cycle was assessed by counting cells, H-3-thymidine incorporation and flow cytometry. PTA mRNA levels were decreased in a time-dependen t fashion following serum deprivation; when the cells were induced to grow by serum refeeding, PTA mRNA expression was greatly stimulated. I nterestingly, after caprylic acid treatment (2.5 mM for 24 h) that arr ested cells in the G0/G1 phase of the cell cycle, PTA mRNA and H4 mRNA levels were almost undetectable; conversely, following caprylic acid withdrawal, PTA mRNA and H4 mRNA expression were greatly stimulated. F urthermore, cells cultured in T3-deprived serum, which was found to de crease GH1 cell proliferation, had low levels of PTA and H4 mRNAs. Thi s effect was reversed by the addition of nanomolar concentrations of T 3 to the culture. On the other hand, IGF-1 addition to the culture did not substantially modify PTA mRNA levels. The present data clearly in dicate that PTA mRNA expression is tied to the proliferating activity of GH1 cells and, thus, could be used as a marker of the action that v arious agents have on GH1 cell proliferation.