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A Heterologous Hormone Response Element Enhances Expression of Rat β-Casein Promoter-Driven Chloramphenicol Acetyltransferase Fusion Genes in the Mammary Gland of Transgenic Mice

Department of Cell Biology, Baylor College of Medicine Houston, Texas 77030

Address requests for reprints to: Dr. Jeffrey M. Rosen, Department of Cell Biology, Baylor College of Medicine, One Baylor Plaza, Houston, Texas 77030.

Abstract

Previous studies have demonstrated that the entire rat β-casein (RβC) gene and a –524/+490 RβC fragment-chloramphenicol acetyltransferase (CAT) fusion gene are expressed preferentially in the mammary gland of transgenic mice in a developmentally regulated fashion. However, transgene expression was infrequent, less than 1% of that observed for the endogenous gene, and varied as much as 500-fold, presumably due to the site of chromosomal integration. To determine whether a heterologous hormone-responsive enhancer could be used to increase both the level and frequency of expression in the mammary gland, a fragment derived from the mouse mammary tumor virus long terminal repeat containing four hormone response elements (HREs) was inserted into the RβC promoter at a site not known to contain transcriptional regulatory elements. Transgenic mice generated which carried HRE-enhanced RβC-CAT fusion genes expressed CAT activity in the mammary glands of all founder lines examined at levels that were on average 13- fold greater than for lines generated with similar constructs not carrying HREs. In the highest expressing line, the level of HRE-enhanced transgene expression was found to be developmentally regulated, increasing 14-fold in the mammary gland from virgin to day 10 of lactation. In this line, expression was also observed in the thymus and spleen; however, the level of CAT activity was 4-fold lower than in the mammary gland and was not developmentally regulated. In adrenalectomized mice, the administration of dexamethasone stimulated CAT expression in the mammary gland but not in the thymus and spleen. These studies demonstrate that in the context of the RβC promoter, the HRE functions in the mammary gland to increase both the frequency and level of transgene expression.

FOOTNOTES

This work was supported in part by USDA Grant 88–37266–3951.

Received for publication June 18, 1991. Revision received July 23, 1991. Accepted for publication July 26, 1991.