Endometrial interleukin-6 in vitro is not regulated directly by female steroid hormones, but by pro-inflammatory cytokines and hypoxia

Endometrial interleukin-6 (IL-6) mRNA has been reported to be suppressed in the mid-secretory phase in patients with recurrent early spontaneous abortions. This prompted our study concerning the regulation of endometrial IL-6 in cell culture models of endometrial epithelial and stromal cells. Steroid-dependent secretion of IL-6 was analysed by 17beta-estradiol (10(-8) mol/l) or progesterone (10(-6) mol/l) treatment and withdrawal (n = 8). Regulation by pro-inflammatory cytokines was studied in co-cultures of endometrial cells with human blood peripheral mononuclear cells (PBMC; n = 5) and by stimulation with IL-1beta, IL-6 and tumour necrosis factor alpha (TNFalpha), secreted by PBMCs at high concentrations. Regulation by hypoxia was assessed by culture of endometrial cells in 2% oxygen for 6 and 24 h (n = 5). IL-6 mRNA and protein levels were analysed by RT-PCR and enzyme-linked immunosorbent assays respectively. Endometrial IL-6 was not directly affected by 17beta-estradiol and/or progesterone. Co-culturing endometrial cells with PBMCs led to an increase of stromal but not epithelial IL-6 mRNA levels. In stromal cells, IL-6 secretion increased 2-10-fold if stimulated with 10 ng/ml recombinant IL-1beta or TNFalpha (P < 0.05). Hypoxia stimulated IL-6 secretion in epithelial cells up to 2-fold and in stromal cells up to 48-fold (P < 0.05). In conclusion, IL-6 expression in stromal and epithelial cells in vitro is regulated differently by pro-inflammatory cytokines and hypoxia. These results suggest a tight and specific network of control for this important cytokine within different endometrial compartments.