5-bromodeoxyuridine-induced amplification of prolactin gene in GH cells is an extrachromosomal event

Treatment of a 5-bromodeoxyuridine-resistant (brdUrdr) and prolactin-nonproducing (Prl-) subclone of GH cells with this drug led to amplification of the prolactin (Prl) gene and induced Prl synthesis. Withdrawal of the drug treatment reversed both of these processes. In normal rats, the increased Prl synthesis observed during late pregnancy and lactation does not seem to be mediated via amplification of the gene. Amplification of the Prl gene and induction of Prl synthesis can also be observed in the Prl-, brdUrd-sensitive (brdUrds) GH cell strain. Prl gene amplification thus does not seem to be associated with the mechanism that confers the brdUrdr phenotype to these cells. brdUrd-induced amplification of the Prl gene can be identified with the low molecular weight, extrachromosomal, supernatant DNA fraction, isolated by Hirt's method. Southern blot analysis of Hirt's supernatant DNA (undigested) from brdUrd-treated cells generated a distinct band following hybridization with [32P]pDNAPrl-insert. The size of this band is greater than 23 kb but smaller than chromosomal DNA. Growth hormone (Gh) and albumin (Alb) gene sequences can be detected in the chromosomal DNA preparation but are absent in the extrachromosomal DNA prepared from Hirt's supernatant. The levels of Gh and Alb sequences are unaffected by brdUrd treatment of these cells. Results presented here suggest that in rat pituitary glands as well as in GH cells, hormonally controlled increased Prl synthesis is not caused by gene amplification. However, the brdUrd-induced expression of the Prl gene seems to be linked to the mechanism of drug-induced amplification of the Prl gene, mediated via an extrachromosomal event.