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Somatolactin (SL) is a pituitary hormone belonging to the growth hormone/prolactin superfamily, with recognizable homologues in all fish taxa examined to date. Although sequences from most fish share reasonably high sequence identity, several more highly divergent SLs have been reported. Goldfish SL and a second SL protein found in rainbow trout (rtSLP) are remarkably different from each other and also dissimilar to other SLs. It has been unclear whether rtSLP is a recent paralogue restricted to rainbow trout, or reflects a more ancient duplication of the SL gene, and whether it is related to the goldfish sequence. Here we report the cloning of two different zebrafish SL cDNAs, which share only 57.5% nucleotide and 47.7% deduced amino acid identities. One copy, designated zebrafish SLalpha (zfSLalpha), displays a typical range of sequence similarity to most other SLs. The other copy, zebrafish SLbeta (zfSLbeta), shows low identity to most other SLs; surprisingly, it is most similar to the divergent SL sequence from goldfish. The mRNAs of zfSLalpha and zfSLbeta were expressed specifically in two distinct regions of the pars intermedia in zebrafish. Cells expressing zfSLalpha are located at the posterior pars intermedia, bordering the neurohypophysis, whereas zfSLbeta is expressed in the anterior part of the pars intermedia, bordering the pars distalis. Phylogenetic analyses indicate that zfSLbeta, goldfish SL and rtSLP all belong to the SL hormone family; however, along with the genes from eel and catfish, these divergent sequences form a group that is clearly distinct from all other SLs. These results suggest the presence of two distinct SL families, SLalpha and SLbeta, which may trace back to a teleost genome duplication prior to divergence of the cyprinids and salmonids.

The biosynthesis of 17beta-estradiol (E(2)) in human placenta involves the actions of aromatase and 17beta-hydroxysteroid dehydrogenase type 1 (17HSD1). Aromatase, an enzyme complex comprised of P450aromatase (P450arom) and NADH-cytochrome P450 reductase, converts androgens to estrogens, whereas 17HSD1 catalyzes the reduction of estrone to E(2). In the present study, the effects of retinoic acids (RAs) on P450arom and 17HSD1 expression in placental cells were investigated. Treatment with all-trans-RA (at-RA) or 9cis-RA increased E(2) production in JEG-3 choriocarcinoma cells and cytotrophoblast (CTB) cells isolated from normal early placentas. Meanwhile, the activity of aromatase and expression of P450arom mRNA were induced by at-RA in JEG-3 cells. Northern blot analysis showed that the effect on P450arom mRNA expression occurs in a dose- and time-dependent fashion. Similar to at-RA and 9cis-RA, Ro40-6055, the retinoic acid receptor alpha (RARalpha)-selective activator, increased the expression of P450arom and 17HSD1 mRNA in JEG-3 cells. On the other hand, Ro41-5253 (Ro41), the RARalpha-selective antagonist, blocked the stimulatory effect of RAs on P450arom expression. Surprisingly, Ro41 induced the activity and mRNA expression of 17HSD1 in JEG-3 cells, which is in contrast to the expected inhibitory effect and, moreover, remarkably potentiated the induction by at-RA and 9cis-RA. However, reporter gene analysis revealed that the influence of Ro41 on the transcription of the HSD17B1 gene, which encodes 17HSD1, is considerably milder in JEG-3 cells, and it only additively enhanced the effect of at-RA. Finally, it was found that at-RA and 9cis-RA increased the expression of P450arom and 17HSD1 mRNA in CTB cells, but to a lesser extent. The data suggest that RAs may play a role in promoting the biosynthesis of E(2 )in the placenta. In addition, Ro41 has divergent effects on gene expression in JEG-3 cells.