PROTHYMOSIN-ALPHA MESSENGER-RNA IS EXPRESSED IN COMPETENT AND PROLIFERATING RAT-THYROID CELLS (FRTL-5) BUT IS NOT SUFFICIENT TO ELICIT CELLPROGRESSION THROUGH THE CELL-CYCLE

Using flow cytometry we observed the effects that different hormonal t reatments had on the progression of rat thyroid (FRTL-5) cells through the cell cycle. The absence of hormones or the addition of TSH (6 mU/ ml) did not induce DNA synthesis; however, the addition of IGF-I (30 n g/ml) promoted cell proliferation. The number of cells recruited by IG F-I was lower than when IGF-I and TSH were used. We therefore conclude d that we had a model with three different types of cells: (1) quiesce nt cells, cells cultured in the absence of hormones, considered to be G(0)-arrested cells, (2) competent cells, TSH-treated cells that did n ot proliferate (being arrested in a cycle phase different from G(0)) a nd (3) actively proliferating cells, cells treated with TSH plus IGF-I . Prothymosin alpha (PTA) mRNA levels were almost undetectable in cell s cultured without hormones at all times studied, i.e. 8, 14 and 24 h. On the contrary, TSH and/or IGF-I greatly increased PTA mRNA. These d ata indicate that G(0)-arrested quiescent cells do not express PTA mRN A and that PTA mRNA is induced when FRTL-5 cells are committed to prol iferate by the addition of TSH, in spite of being arrested by the lack of IGF-I. We therefore conclude that PTA mRNA expression may be an ev ent that is necessary for cells to proliferate, but that it is not suf ficient for the promotion of cell progression through the cell cycle.