Differential effects of retinoic acid (RA) and N-(4-hydroxyphenyl) retinamide (4-HPR) on cell growth, induction of differentiation, and changes in p34cdc2, Bcl-2, and actin expression in the human promyelocytic HL-60 leukemic cells.

Incubation of the HL-60 cells with 3 microM of RA and 4-HPR resulted in suppression of cell growth and decrease in cell viability. A significant percentage of the RA-treated cells also displayed differentiation towards neutrophils, as assayed by changes in nitroblue tetrazolium reduction (NBT) and alpha-naphthyl-acetate esterase (ANAE) activities, whereas the 4-HPR treated cells remained essentially undifferentiated. Flow cytometric analysis showed 4-HPR to cause partial cell arrest in the G2/M phase after a 3-day treatment and an additional G1 phase arrest after a 7-day treatment. With RA-treated cells, a reduction in the percentage of cells in the G1 phase was observed after 7 days of treatment. In 4-HPR-treated cells an extra peak, characteristic of cells undergoing apoptosis, was found in the cell cycle phase distribution analysis. Determination of specific protein expression changes by Western blot analysis showed that the p34cdc2 was down-regulated by both chemicals. Furthermore, RA induced bcl-2 but prevented the processing of actin, whereas 4-HPR had little effect on bcl-2 but increased the specific processing of actin. These results suggest that RA promotes neutrophil differentiation and the establishment of a semi apoptosis-resistant state, possibly through the overexpression of the bcl-2 gene. By contrast, 4-HPR may trigger apoptosis by inducing overall cyto-architectural changes and specific DNA fragmentation subsequent to increased turnover of the protein actin.