Leukemia inhibitory factor (LIF) is a pleiotropic cytokine which is essential for implantation in rodents and is expressed during the progesterone-dominated secretory phase of the menstrual cycle in the human endometrium. However, the effect of progestin on the transcriptional regulation of the LIF promoter has not been studied so far. In the present study, we used a luciferase reporter plasmid bearing 666 bp of the human LIF promoter (hLIF666-Luc) to investigate the effects of progestin on the transcriptional regulation of LIF in SKUT-1B uterine tumor cells. Jurkat T-lymphoma cells were used for comparison. Since both cell lines are devoid of functional progesterone receptors (PR), we co-transfected the cells with hLIF666-Luc and an expression vector for the human PR form B (PR-B) or A (PR-A). Addition of the progesterone agonist MPA (medroxy-progesterone acetate, 2.5 x 10(-7) M) resulted in induction of LIF transcription only in SKUT-1B cells, while it had no effect in Jurkat cells. Both PR forms were effective in inducing the LIF promoter in SKUT-1B cells when activated by MPA. However, the induction through PR-A was inhibited more efficiently by the progestin antagonist RU 486. We next investigated the stimulatory effect of MPA in SKUT-1B cells on deletion constructs (h274LIF-Luc, h148LIF-Luc and H82LIF-Luc) and found that it is maintained on these fragments. Thus, 82 bp are sufficient to mediate this effect. Our results show that the human LIF promoter is active in uterine tumor cells, and that it is differentially regulated by progestin in cells of uterine and lymphoid origin.