Differential transcriptional and translational regulations of calbindin-D9k by steroid hormones and their receptors in the uterus of immature mice.

Calbindin-D(9k) (CaBP-9k) is a cytosolic calcium binding protein mainly expressed in the duodenum, placenta and uterus, and intestinal CaBP-9k is regulated by 1, 25-dyhydroxyvitamin D3. However, despite the presence of vitamin D receptors, uterine CaBP-9k is not under the control of vitamin D, but seems to be regulated by sex steroids. This steroids-dependent regulation of CaBP-9k is not only limited to a tissue-specific manner but also extends to a species-specific manner. In this study, we examined the regulation of CaBP-9k gene at the transcriptional and translational levels, and also localized CaBP-9k protein in the uterus of immature mice. Treatment with progesterone (P4) resulted in the induction of CaBP-9k mRNA, and a co-treatment with estrogen (E2) plus P4 evoked a synergic effect on its mRNA level in this tissue. Interestingly, the translation of CaBP-9k protein was enhanced by E2, while no difference was observed at the transcriptional level. Not only P4 but also E2 itself induced an increase of CaBP-9k protein, and co-treatment with E2 and P4 showed a similar effect on its protein level in the uterus of immature mice. The CaBP-9k protein was localized in the glandular epithelium of stroma in the uterus of immature mice at diestrus, indicating that the expression of CaBP-9k protein is differentially regulated by sex steroids. A potential mechanism of synergic effect of P4 and E2 may be E2 action in the increase of progesterone receptor (PR), with up-regulated PR increasing P4-induced CaBP-9k expression. This complicated relationship between CaBP-9k and steroid receptors suggests that P4 regulates CaBP-9k gene in the uterus of immature mice, in addition, E2 also can affect the expression of CaBP-9k through the regulation of PR. The expression levels of ERalpha and PR were further examined in this tissue. E2 stimulated the expression levels of ERalpha and PR mRNAs and P4 inhibited the expression of these transcripts at an early time point (12 h) and increased them at 24 and 48 h, while co-treatments with both steroids increased transcripts of ERalpha and PR at 24 h. In conclusion, P4 and PR may be dominant factors in the regulation of CaBP-9k. Also, E2 and ERalpha can influence the expression of the CaBP-9k gene via an indirect pathway in the uterus of immature mice.