Rat growth hormone expression in cell hybrids.

Extinction of rat growth hormone (rGH) gene expression occurs in somatic cell hybrids of GH3 cells (rat pituitary adenoma) and L cells (mouse transformed fibroblast). We have suggested that this is due to transacting factors contributed directly or indirectly by the L cells. To study this in more detail, the cloned rGH gene was introduced into clones of these hybrid cells by microinjection or calcium phosphate transfection. GH peptides were detected 24 hr later in the microinjected cells by immunofluorescence. Production of rGH gene transcripts in the stably transformed hybrid cells was also detected by Northern hybridization. Although the overall patterns of transcripts differed from those present in GH3 cells, normal-sized rGH mRNA species were detected, whereas these had not been observed in previous studies in which the GH gene was transfected into L cells and other mouse fibroblasts. No more than one or two extra GH gene copies per cell were required to restore GH gene expression in the hybrids. Whereas GH gene expression in the hybrid cells transfected with the GH gene might reflect activation of the endogenous GH gene, this was not the case in at least one transfected hybrid cell line which had lost the endogenous gene. These results suggest: (i) the putative transactive repressors of GH gene expression in the hybrid cells do not recognize the transfected genes; and (ii) elements in the hybrid cells, unlike the case with L cells, allow for the production of normal-sized GH mRNA that can also produce GH peptides.(ABSTRACT TRUNCATED AT 250 WORDS)