Tibolone exerts progestational inhibition of matrix metalloproteinase expression in human endometrial stromal cells

Tibolone and its metabolites were evaluated on matrix metalloproteinase (MMP) expression in human endometrial stromal cells (HESCs) under the hypothesis that these steroids would act as progestins on MMP-1, -2, and -3 expression. After 7 days of priming and 24 h experimental incubation of confluent cultured HESCs, 10−7 M medroxyprogesterone acetate (P) reduced MMP-1 to 49 ± 34% (p < 0.05) and MMP-3 to 33 ± 22% of basal levels (mean ± S.E.M., p < 0.05, n = 5). Although HESCs were unaffected by 10−8 M estradiol (E), E + P reduced MMP-1 and MMP-3 levels an additional 2.5-fold from P alone. Tibolone and Δ-4 tibolone were equivalent to E + P in inhibiting MMP-1 and MMP-3 output, whereas 10−6 M of 3α-OH or 3β-OH tibolone was required to elicit significant inhibition of both MMPs (p < 0.05). By contrast, none of the treatments affected HESC-secreted MMP-2 output. The ELISA results were confirmed by Western blotting and by substrate gel zymography. Quantitative RT-PCR demonstrated corresponding changes in MMP-1 and MMP-3 mRNA levels. Inhibition of MMP-1 and MMP-3 expression by tibolone and Δ-4 tibolone is consistent with the metabolism of tibolone to Δ-4 tibolone, and subsequent binding of Δ-4 tibolone to the progesterone receptor. Since 3α-OH and 3β-OH tibolone bind exclusively to the estrogen receptor, their inhibition of MMP-1 and MMP-3 suggests metabolism by HESCs to Δ-4 tibolone. These observations help to explain the paradox that the endometrium becomes atrophic after tibolone administration despite the persistence in the circulation of 3α-OH and 3β-OH tibolone, but not tibolone or Δ-4 tibolone.