A novel cell culture model for studying differentiation and apoptosis in the mouse mammary gland

Background: This paper describes the derivation and characterization of a n ovel, conditionally immortal mammary epithelial cell line named KIM-2. Thes e cells were derived from mid-pregnant mammary glands of a mouse harbouring one to two copies of a transgene comprised of the ovine beta -lactoglobuli n milk protein gene promoter, driving expression of a temperature-sensitive variant of simian virus-40 (SV40) large T antigen (T-Ag). Results: KIM-2 cells have a characteristic luminal epithelial cell morpholo gy and a stable, nontransformed phenotype at the semipermissive temperature of 37 degreesC. In contrast, at the permissive temperature of 33 degreesC the cells have an elongated spindle-like morphology and become transformed after prolonged culture. Differentiation of KIM-2 cells at 37 degreesC, in response to lactogenic hormones, results in the formation of polarized dome -like structures with tight junctions. This is accompanied by expression of the milk protein genes that encode beta -casein and whey acidic protein (W AP), and activation of the prolactin signalling molecule, signal transducer and activator of transcription (STAT)5. Fully differentiated KIM-2 culture s at 37 degreesC become dependent on lactogenic hormones for survival and u ndergo extensive apoptosis upon hormone withdrawal, as indicated by nuclear morphology and flow cytometric analysis. KIM-2 cells can be genetically mo dified by stable transfection and clonal lines isolated that retain the cha racteristics of untransfected cells. Conclusion: KIM-2 cells are a valuable addition, therefore, to currently av ailable lines of mammary epithelial cells. Their capacity for extensive dif ferentiation in the absence of exogenously added basement membrane, and abi lity to undergo apoptosis in response to physiological signals will provide an invaluable model system for the study of signal transduction pathways a nd transcriptional regulatory mechanisms that control differentiation and i nvolution in the mammary gland.