Effects of Retinole Acid on Plasminogen Activator and Mitogenic Responses of Cultured Mouse Cells1

Treatment of simian virus 40-transformed 3T3 cells with 1 ¡IMß-all-frans-retinoic acid (RA) resulted in a 5to 6-fold enhancement of plasminogen activator (PA) release. Intracellular PA levels rose to twice control levels. Confluent 3T3 cells were less responsive to RA. In 6 of 10 experiments, no increase in 3T3 cell PA levels was noted, although up to a 2.5-fold enhancement of PA elaboration has been observed in some experiments at a dose of 10 ¡IMRA. Simultaneous treatments of 3T3 cells with 10 JIMRA and 2.1 to 9.3 mM Ca2+, 5 to 40 ng phorbol myristate acetate per ml, or 150 to 600 ng Fraction I from lactalbumin hydrolysate (FI) protein per ml indicated that RA potentiated the PA stimulatory activities of these agents. Extracellular PA levels of RA-treated cells increased by 4 to 10 times the amount of increase observed for Ca?+, PMA, or FI alone. A potentiating activity of RA was also evident when quiescent 3T3 cells were pretreated with 10 /IM RA and then stimulated to synthesize DNA with Ca2+, PMA, or FI. For RApretreated cells, an increased percentage of nuclei was labeled with [3H]thymidine (24 hr) in response to doses of the three mitogens which were ineffective without RA pretreatment (2.4 row Ca2+, 5 ng PMA per ml, or 150 ng FI protein per ml). Additional experiments have indicated that, like platelet ex tracts, RA renders quiescent 3T3 cells competent to synthesize DNA in response to the progression factors of human plasma as defined by Pledger ef al. (Proc. Nati. Acad. Sei. U. S. A., 74: 4481-4485, 1977). Pretreatment of quiescent 3T3 cells for 6 hr with 10 fiM RA resulted in a greater than 4-fold increase in the percentage of cells which incorporated [3H]thymidine in response to a 36-hr treatment with 10% human plasma, as compared to cells treated with human plasma alone. Thus, under certain conditions, RA may have cell-activating proper ties, and caution should be exercised with regard to its sug gested use as an antitumor agent.