The present study was carried out to examine the effect of activin A on cell-cycle progression induced by insulin-like growth factor-I (IGF-I) in Balb/c 3T3 cells. When activin A was added together with IGF-I to competent cells primed with epidermal growth factor (primed competent cells), both [3H]thymidine incorporation and nuclear labelling induced by IGF-I were inhibited. The inhibition was concentration-dependent and the maximum inhibition was obtained with 1 nmol activin A/l. To ascertain the time in which activin A exerted its inhibitory action, we divided 12 h, the time required for primed competent cells to progress towards the S phase, into four periods and activin A was added during each of the four periods. It was effective when added during either the second (3 to 6 h) or the third period (6 to 9 h) but it did not affect cell-cycle progression when added during the first (0 to 3 h) or the last period (9 to 12 h). We then examined whether activin A affected intracellular events elicited by IGF-I. It did not affect either autophosphorylation of the IGF-I receptor or calcium entry induced by IGF-I. Likewise, it did not cause any change in the radioactivity of 1,2-diacylglycerol (DAG) in cells prelabelled with [3H]myristate while the increase in the mass of DAG induced by IGF-I was markedly reduced by activin A. The dose-response relationship for the activin A-mediated reduction of DAG mass correlated well with that for the activin A-mediated reduction of DNA synthesis. Activin A was effective in reducing DAG mass even when added 3 h after the addition of IGF-I. These results indicate that activin A attenuates cell-cycle progression in the middle of the G1 phase. The results also raise the possibility that reduction of DAG mass may account for the inhibitory effect of activin A on cell-cycle progression.
Journal of Endocrinology (1993) 137, 99–105
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