ENHANCEMENT OF CISPLATIN AND ETOPOSIDE CYTOTOXICITY AFTER ALL-TRANS-RETINOIC-ACID-INDUCED CELLULAR-DIFFERENTIATION OF A MURINE EMBRYONAL CARCINOMA CELL-LINE

The potential of a combination of differentiation induction and chemot herapy was analyzed. Treatment of the murine embryonal carcinoma (EC) cell line PCC4 in vitro with all-trans-retinoic acid (RA) was followed by exposure to cisplatin (CDDP) or etoposide (VP-16). The expression of EC-cell-specific markers decreased upon 96 hr exposure to 10(-9), 1 0(-8), 10(-7) and 10(-6) M RA, indicating a loss of embryonal phenotyp e of the cells, whereas expression of markers specific for cytokeratin s and neurofilaments was increased after this treatment. These data su ggest early somatic differentiation of PCC4 cells upon treatment with RA. Cellular growth rate of PCC4 cells was not affected by preincubati on with RA at 10(-9) M for 96 hr, but was reduced at 10(-8) and 10(-7) M RA and inhibited at 10(-6) M RA. Culture of PCC4 cells in the prese nce of 10(-7) M RA for 96 hr led to accumulation in G1 of the cell cyc le, whereas at 10(-8) M RA cell-cycle distribution was not affected. R A-treated and -untreated PCC4 cells were compared for CDDP and VP-16 s ensitivity. Pre-treatment with 10(-9), 10(-8) and 10(-7) M RA increase d CDDP sensitivity, resulting in a 1.9-, 2.7- and 2.6-fold decrease in the concentrations inhibiting survival by 50% (IC50s) respectively. P re-treatment with 10(-8) and 10(-7) M RA increased VP-16 sensitivity 2 .5- and 3.0-fold, respectively. Enhanced CDDP sensitivity at RA concen trations not affecting cell-cycle distribution was not attributable to changes in cellular platinum (Pt) accumulation, or to changes of Pt-D NA binding. Enhanced VP-16 sensitivity also occurred while topoisomera se-II activity was unchanged and topoisomerase-I activity was increase d 2-fold. Furthermore, there was a 2.0- to 4.5-fold increase in the ce llular concentration of VP-16 after 10(-8) and 10(-7) M RA pre-treatme nt. In conclusion, this study shows that in PCC4 cells the rationale f or combining RA and VP- 16 or CDDP is 2-fold: RA induces differentiati on and increases CDDP and VP-16 sensitivity. (C) 1993 Wiley-Liss, Inc.