In vitro effect of retinol and 13-cis retinoic acid on cytotoxicity of human monocytes

Human monocytes (HM) obtained from healthy nonsmoking donors were rendered tumoricidal activity following 24 hr-incubation in vitro with retinol (RL) or 13-cis retinoic acid(13cRA). HM treated with medium only did not kill tumor cells. Maximum tumoricidal activity of HM, as measured by lysis of ¹²⁵I-iododeoxyuridine labeled C83-2C human melanoma cells, was obtained after incubation with 10^-8M of RL or 10^-6M of 13cRA for 24 hr. Tumoricidal activity rendered by incubation in vitro with RL or 13cRA cell to half of maximum activity following further incubation in vitro with medium only for 24 hr. Pretreatment of HM with 10^-8M of RL for 0 to 24 hr did not interfere the activation of macrophage activating factor (MAF) against HM and did induce the enhancement of tumoricidal activity of HM rather than that of MAF only. We conclude that RL and 13cRA could induce HM to become tumoricidal. This may help to explain greater cancer prevention of vitamin A and its derivatives.