Transplantation of GH3 rat pituitary tumor cells that express both PRL and GH to female Wistar-Furth rats results in tumors that secrete only GH. We have used in vivo passage of GH3 cells as a model system to study specific repression of PRL. RNA blot hybridization revealed that PRL message was repressed 95% in cells transplanted to host animals compared to that in GH3 cells in culture. In contrast, there was little change in GH message in the transplanted cells, and there was a 4-fold increase in insulin-like growth factor-I transcript levels. When the transplanted cells were returned to cell culture, PRL mRNA levels increased rapidly, reaching levels similar to those in GH3 cells within 72 h. Gene transfer studies demonstrated a low level PRL promoter utilization in GH3 cells after in vivo passage, when endogenous PRL was repressed. Transfection of the transplanted cells maintained in culture for 96 h, when endogenous PRL was expressed, demonstrated increased PRL promoter activity. Messenger RNA levels for the transcription factor Pit-1 were equivalent in GH3 cells and cells after in vivo passage, and the presence of Pit-1 protein in extracts from transplanted cells was demonstrated by Western blot analysis. Electrophoretic gel mobility shift assays indicated that protein interactions with the PRL promoter were very different for extracts prepared from cells in which PRL was repressed compared to those from cells maintained in culture until PRL expression had recovered. Competition for protein binding with a Pit-l DNA element and immunoclearing with an antibody directed to Pit-l implicated the involvement of that protein in the DNA-protein complexes. A change in the mobility shift pattern occurred in advance of the recovery of endogenous PRL expression. Taken together, these results are consistent with transcriptional mechanisms of repression of PRL gene expression in the transplanted GH3 cells.
ASJC Scopus subject areas
- Molecular Biology