Hormonal Regulation of Casein Gene Expression in Normal and Transformed Mammary Cells

Abstract

The mammary gland provides an excellent model system in which to study the mechanisms by which both steroid and peptide hormones regulate gene expression. Both glucocorticoids and prolactin are required for maximal expression of the milk protein genes, while progesterone antagonizes the inductive effects of these lactogenic hormones. As a necessary prerequisite for elucidating the mechanism of action of these hormones, we have isolated a series of recombinant cDNA and genomic DNA clones encoding three members of the casein multigene family and a fourth, abundant rat milk protein, the whey acidic protein. Complete nucleic acid sequences have been determined for all four of these milk protein mRNAs and for portions of the large and complex, split genes encoding these mRNAs. Comparison of these sequences has revealed three regions of unusual conservation among the rapidly diverging members of the casein gene family. These include the 5' noncoding regions of the mRNAs, the major sites of casein phosphorylation and calcium binding, and the signal peptide sequences. The 5' noncoding region and the signal peptide sequence each appear to be encoded by a separate exon, while the site of phosphorylation is generated by an RNA splicing event. Analysis of the 5' flanking regions of the casein genes revealed several structural features which may represent potential progesterone and glucocorticoid receptor binding sites. All 3 casein genes contain a 16-nucleotide consensus sequence which constitutes part of a palindrome and is homologous with a chicken ovalbumin gene 5' sequence involved in progesterone receptor binding and induction of ovalbumin gene expression. Preferential binding of the chicken progesterone receptor and a 5' flanking DNA fragment of the mammalian γ-casein gene has been demonstrated. Casein genes also contain short nucleotide sequences which share 90% homology with viral transcriptional enhancer core elements. These results will be briefly illustrated in the following presentation.