Endogenous and exogenous pituitary-specific promoters are differentially controlled.

We have engineered GH3 cells with reporter genes under control of the growth hormone and prolactin promoters and measured protein production. The results indicate very low level production of reporter proteins from the cells regardless of the promoter used to drive expression. This was surprising in light of the observation that the cells still produced high levels of endogenous growth hormone and prolactin. Chinese hamster ovary (CHO) cells were engineered to express the Pit-1 transactivator. Transfection of reporter genes under control of the prolactin promoter demonstrated a clear enhancement of expression levels compared to the same promoter in parental CHO cells. Pit-1 expression is not sufficient, however, for high level, stable expression from the growth hormone promoter. These results indicate that the growth hormone and prolactin promoters are not sufficient for high level, stable expression even in normally permissive cells and suggest that Pit-1 alone is not sufficient for strong promoter activity from the integrated plasmids.