N-(4-Hydroxyphenyl)retinamide (HPR) is a synthetic retinoid with anticancer properties and lower toxicity than all-trans retinoic acid (RA). We have studied the effects of HPR on apoptosis and differentiation in the human promyelocytic leukemia HL-60 cell line. In addition, we have tested the hypothesis that vimentin expression after HPR and RA, taken as indirect evidence of the mechanisms of action of the two retinoids, may be different. Quantitative evaluation of the percentage of apoptotic cells was carried out on a cell by cell basis by the flow cytometric DNA-content in situ-terminal-deoxynucleotydil-transferase (TdT assay). HPR was found to clearly induce apoptosis, while RA: instead, induced differentiation without apoptosis. These data confirm previous observations. Vimentin protein content was evaluated by flow cytometry with use of monoclonal antibodies simultaneously with DNA content. We found that HPR treated apoptotic cells were characterized by negative vimentin expression, while the HPR treated non apoptotic cells had about the same level of vimentin as the RA treated cells. These latter findings suggest that HPR may induce a functional effect (apoptosis) by a mechanism of action different from that of RA. Further work is necessary to clarify this finding.

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