Human growth hormone gene expression in rat but not human non-pituitary cells after stable gene transfer

Tissue-specific expression of the rat growth hormone (rGH) gene requires binding of a pituitary-specific factor. Binding of this factor has been used to explain tissue-specific expression of the human growth hormone (hGH-N) gene in transfected rat pituitary (GC) tumour cells. Neither rat fibroblast (R2) nor human cervical carcinoma (HeLa) cells contain the rat pituitary-specific factor. Thus, no expression of hGH-N or rGH would be expected in these cells. R2 cell lines containing stably integrated hGH-N or rGH genes were generated. Expression of hGH-N but not rGH was detected. By contrast, stably transfected HeLa cells did not express the endogenous or transfected hGH-N genes. However, an hGH-N transcript was detected when hGH-N gene expression was directed by a viral promoter. This suggests that the block in expression occurs at the level of transcription and not mRNA stability. Hybrid genes containing 496 base pairs (bp) of hGH-N or 234 bp of rGH 5'-flanking DNA, including promoter sequences, fused to the bacterial gene coding for chloramphenicol acetyltransferase were used to stably transfect R2 cells. The hybrid hGH-N gene was more active than a promoterless construction in these cells. By contrast, the hybrid rGH gene was not. These data suggest that the hGH-N gene can be activated by rat transcription factors other than those found in pituitary cells.

Human chorionic somatomammotropin gene expression in primary placental cell cultures

Mono-nucleated cytotrophoblasts (cytoTBs) were prepared by Percoll gradient fractionation of enzymically disaggregated human placental tissue. These cells were plated in monolayer culture in the presence of fetal calf and calf serum. Within 2-24 h, the cytoTBs aggregate, and by 48 h, they are clearly fused into multinucleated syncytia. The presence of human chorionic somatomammotropin (hCS) and chorionic gonadotropin (hCG) in the cells after 48 h was determined by immunohistochemistry. To assess whether hCS is synthesized in our cultures we examined hCS mRNA accumulation with time. The presence of hCS mRNA was detected at the time of plating but full length transcripts were seen only at later times indicating synthesis in culture. However, preparations at the time of plating contain fragments of syncytiotrophoblast (syncytioTB) generated by enzymic or mechanical disaggregation. These fragments could fuse with the cytoTBs. The inclusion of these fragments makes analysis of placental hormones by protein detection an unreliable assay for synthesis. Analysis of mRNA levels support hCS synthesis in culture and correlates with aggregation and fusion of cytoTBs. Thus, the fused cells in culture mimic the cellular site of hCS synthesis in vivo, the syncytioTB.

Human chorionic somatomammotropin and growth hormone gene expression in rat pituitary tumour cells is dependent on proximal promoter sequences

Human placental chorionic somatomammotropin (hCS-A or hCS-B) and pituitary growth hormone (hGH-N) are related by structure and function. The hCS-A gene is expressed in rat pituitary tumour (GC) cells after gene transfer. Deletion of hCS-A 5'-flanking DNA reveals repressor activity upstream of nucleotide -132, and a region essential for expression in GC cells between nucleotides -94 and -61. The sequences in this region differ from the equivalent hGH-N gene DNA by one nucleotide, and include the binding site (-92 to -65) for a pituitary-specific factor (GHF-1), required for hGH-N expression in GC cells. Exchange of hGH-N with hCS-A gene DNA in this region maintains expression in GC cells. By contrast, modification of these sequences blocks expression. These data indicate that proximal promoter sequences, equivalent to those bound by GHF-1 on the hGH-N gene, are required for hCS-A expression in GC cells.