Differential expression of human placental growth hormone variant and chorionic somatomammotropin genes in choriocarcinoma cells treated with methotrexate

Placental lactogen is expressed but is not translated into protein in breast cancer

Introduction

Several studies reported that the pregnancy-specific hormone placental lactogen (hPL) is expressed at both mRNA and protein levels in breast cancer. The overall objective was to establish hPL, the product of the CSH1 and CSH2 genes, as a biomarker for breast cancer.

Methods

CSH expression was determined at the mRNA level in breast cancer cell lines (BCC) and primary carcinomas by real-time and conventional PCR and the products verified as CSH1 by sequencing. Expression of hPL protein was examined by western blots and immuno-histochemistry, using commercial and custom-made polyclonal and monoclonal antibodies.

Results

Variable levels of CSH mRNA were detected in several BCC, and in some primary tumors. We detected a protein, slightly larger than recombinant hPL by western blotting using several antibodies, leading us to postulate that it represents an hPL variant (‘hPL’). Furthermore, some monoclonal antibodies detected ‘hPL’ by immunohistochemistry in breast carcinomas but not in normal breast. However, further examination revealed that these antibodies were non-specific, as efficient suppression of CSH mRNA by shRNA did not abolish the ‘hPL’ band. Custom-made monoclonal antibodies against recombinant hPL detected hPL of the correct size in placental lysate and hPL-overexpressing BCC, but not in unmodified cells or primary carcinomas. hPL protein was detected only when mRNA was increased several thousand fold.

Conclusions

We call into question previous reports of hPL expression in breast cancer which relied on mRNA levels as surrogates for protein and/or used improperly validated antibodies to measure hPL protein levels. Our data suggests that an inhibitory mechanism(s) prevents translation of CSH mRNA in breast cancer when not highly expressed. The mechanism by which translation of CSH mRNA is inhibited is intriguing and should be further investigated.

Transcription factor FOXF1 regulates growth hormone variant gene expression

Deletion analysis of the human growth hormone variant (GHV) promoter in transient transfection studies in BeWo choriocarcinoma and HepG2 cells indicated that the region extending from nt -158/+57 retained full transcriptional activity. DNase I footprint analysis of the fragment revealed a protected region at nt -82/-77, which is in a putative FOXF1/FOXF2 binding site. Supershift assays using an antiserum to human FOXF1 demonstrated that the protected region binds FOXF1. Overexpression of FOXF1 in BeWo and HepG2 cells induced the GHV promoter, whereas overexpression of FOXF2 was without effect. Mutagenesis of the FOXF1/FOXF2 site reduced basal promoter activity by 50-60% and markedly attenuated transactivation of the promoter by FOXF1. These studies indicate that FOXF1 induces GHV expression by interaction with a FOXF1/FOXF2 cis-element in the proximal promoter.

Human chorionic somatomammotropin enhancer function is mediated by cooperative binding of TEF-1 and CSEF-1 to multiple, low-affinity binding sites

The human chorionic somatomammotropin gene enhancer (CSEn) is composed of multiple enhansons (Enh) that share sequence similarities with those of the simian virus, SV40 enhancer (SVEn). The sequence homology includes two GT-IIC-like (Enh1 and Enh4) and three SphI/II-like enhansons (Enh2, Enh3, and Enh5). We previously showed that transcription enhancer factor 1 (TEF-1) and a 30-kDa placental-specific factor, chorionic somatomammotropin enhancer factor 1 (CSEF-1), bind to Enh4, which plays an essential role in enhancer function. In this study, we demonstrate that TEF-1 and CSEF-1 bind specifically to all the other GT-IIC- and SphI/II-like elements within CSEn with a broad range of binding affinities that vary between 0.005 and 0.15 that of Enh4. Each individual concatenated enhanson was able to stimulate hCS promoter activity in an orientation-independent manner in choriocarcinoma cells (BeWo) with an observed stimulation that was directly proportional to its relative binding affinity for TEF-1 and CSEF-1. These results indicate that CSEn function results from the cooperative interaction of TEF-1 and/or CSEF-1 binding to multiple, low-affinity GT-IIC- and SphI/II-like enhansons within the enhancer.

Ethanol and mitotic inhibitors promote differentiation of trophoblastic cells

Chronic ethanol abuse during pregnancy can cause fetal injury. A contributing factor in this fetal injury may be the effect of ethanol on the placenta. Ethanol treatment increases human chorionic gonadotropin (hCG) production by cultured human placental trophoblasts. In this study, we show that ethanol treatment reduces total DNA and total protein while stimulating hCG production in term trophoblasts. Ethanol treatment inhibits growth in rapidly proliferating trophoblastic cells from a first trimester placenta and JEG-3 choriocarcinoma cells. In both cell types, the normal increases in total DNA were inhibited in an ethanol dose-dependent manner. Normal increases in total protein were inhibited as well. In contrast, hCG production, an indicator of differentiation, was stimulated by ethanol treatment. Treatment of JEG-3 cells with antimitogenic agents, methotrexate (MTX) or cytosine arabinoside (Ara-C), inhibited cell growth as indicated by decreased total DNA and total protein accumulation. Similar to that with ethanol treatment, inhibition of cell proliferation was accompanied by increases in hCG production. Taken together, these data suggest that one mechanism by which ethanol increases hCG production in human placental trophoblasts may involve alterations in cellular growth and/or differentiation; such alterations may also occur in other proliferating cells in the growing fetus.

Nuclease sensitivity of the human growth hormone-chorionic somatomammotropin locus in pituitary and placenta suggest different mechanisms for tissue-specific regulation

The five human growth hormone (GH) and chorionic somatomammotropin (CS) genes are located at a single locus on chromosome 17. These genes share extensive nucleotide sequence similarity (approximately 94%) even in their flanking DNA, yet GH-N is expressed efficiently in the pituitary under the control of the pituitary-specific factor GHF-1/Pit-1 and the remaining CS-A, CS-B, CS-L and GH-V genes are transcriptionally active in the placenta. Despite this specificity in vivo, a truncated CS-A promoter can bind GHF-1/Pit-1 and allow CS-A promoter activity in pituitary cells in vitro. With a view to assessing whether the placental genes of the GH/CS locus possess a different chromatin structure in the pituitary and are, thus, less transcriptionally active than the GH-N gene, we have compared the DNAase I sensitivity of GH/CS in isolated pituitary and placenta cell nuclei. Our data indicate that these genes are equally sensitive in isolated human pituitary nuclei. By contrast, the CS-A, CS-B and CS-L genes were significantly (P < 0.05) more sensitive than the GH-N gene in isolated human placenta nuclei. Although just not significant, the GH-V gene was slightly more sensitive than the GH-N gene. This pattern was also seen with nuclei from human choriocarcinoma BeWo and JEG-3 cells, which express low and extremely low levels of CS RNA, respectively, but was distinct from the pattern observed in the non placental human cervical carcinoma HeLa cell line. These data indicate that the inactivity of the CS genes in the pituitary does not correlate with a 'closed' chromatin structure. However, they are consistent with a role for a more 'open' chromatin conformation in placenta-specific expression, but not necessarily high levels of transcriptional activity.