CONSTITUTIVE EXPRESSION OF A TRUNCATED INT3 GENE IN MOUSE MAMMARY EPITHELIUM IMPAIRS DIFFERENTIATION AND FUNCTIONAL-DEVELOPMENT

INT3 is interrupted by retroviral DNA insertion in approximately 18% o f primary Czech mouse mammary tumors induced by mouse mammary tumor vi rus. One consequence of these insertions is the production of a 2.4-ki lobase, tumor-specific RNA transcript encoding the entire intracellula r domain of the Int3 protein which is initiated from the 3' long termi nal repeat promoter of the inserted viral genome. Female mice (FVB-3) transgenic for a genomic fragment comprised of this truncated region o f INT3 express the 2.4-kilobase truncated INT3 transcript and exhibit focal mammary tumors at 100% penetrance. INT3 is a member of a family of genes, highly conserved through evolution and characterized by Dros ophila melanogaster Notch and Caenorhabditis elegans lin-12, the funct ion of which relates to cell fate determination. Upon transfection int o the appropriate hosts, expression vectors of truncated Notch and lin -12, representing their respective cytoplasmic domains, have been demo nstrated to effect their complete gene function with respect to cell f ate determination. This suggests that the extracellular portion of the se proteins function only to regulate activity. Reciprocal transplanta tion of transgenic FVB-3 and normal mammary tissue to the epithelium-d ivested fat pads of the respective donor females demonstrates that FVB -3 mammary epithelium is unable to grow and/or to functionally differe ntiate. However, normal epithelium grows and fully differentiates in t ransgenic FVB-3 fat pads, indicating that the dysfunction of FVB-3 mam mary glands is due to a deficiency inherent in their epithelium. Elect ron microscopy reveals that transgenic INT3 epithelial cells do not fo rm intercellular junctional complexes in the developing subadult mamma ry gland. The hormonal stimulation of pregnancy overcomes the deficien cy for ductal growth so apparent in the virgin gland such that pregnan t FVB-3 glands produce complete ductal systems. Nevertheless, during p regnancy, FVB-3 mammary cells fail to form secretory lobules and to pr oduce milk. Examination of INT3 expression by immunocytochemistry and reverse transcriptase-PCR show that INT3 is expressed constitutively i n mammary stroma and epithelia at all stages of postpubertal mammary e volution. These results indicate that deregulated expression of a trun cated Int3 in mammary epithelial cells limits their capacity to perfor m the cell fate decisions required for morphogenesis and functional di fferentiation.