Lactoferrin expression in mammary epithelial cells is mediated by changes in cell shape and actin cytoskeleton

Abstract

Lactoferrin is a secreted iron binding protein which is expressed during normal functional development of mammary epithelium. Murine mammary epithelial cell lines competent for milk protein expression were used to identify microenvironmental factors that regulate lactoferrin expression. While lactoferrin was not expressed in adherent monolayer cultures under standard subconfluent conditions on plastic, lactoferrin mRNA and protein steadily accumulated when the cells aggregated to form spheroids on a reconstituted basement membrane gel. However, unlike other milk proteins such as beta-casein, lactoferrin expression was also induced at high cell density in the absence of exogenously added basement membrane or prolactin. These results led us to examine whether changes in cell growth, cell-cell interactions and/or cell shape were responsible for regulation of lactoferrin gene expression. Rounded, non-proliferating cells in suspension in serum-free medium expressed lactoferrin even as single cells. Conversely, lactoferrin expression could be inhibited in non-proliferative cells in serum-free medium by maintaining them in contact with an air-dried extracellular matrix which caused the cells to retain flat, spread morphologies. These findings indicated that cessation of cell growth was not sufficient, that cell-cell interactions were not required, and that cell culture conditions which minimize cell spreading may be important in maintaining lactoferrin expression. Additional data supporting this latter concept were generated by treating spread cells with cytochalasin D. The resulting disruption of microfilament assembly induced both cell rounding and lactoferrin expression. Shape-dependent regulation of lactoferrin mRNA was both transcriptional and post-transcriptional. Surprisingly, treatment of rounded cells with a transcription inhibitor, actinomycin D, produced a stabilization of lactoferrin mRNA, suggesting that transcription of an unstable factor is required for degradation of lactoferrin mRNA. Importantly, lactoferrin mRNA expression was regulated similarly in early passage normal human mammary epithelial cells. In vivo, the changing extracellular matrix components of the mammary gland during different stages of normal and abnormal growth and differentiation may provide different physical constraints on the configurations of cell surface molecules. These physical constraints may be communicated to the cell interior through mechanical changes in the cytoskeleton. Unlike beta-casein whose expression is upregulated by specific integrin-mediated signals, lactoferrin may be representative of a class of proteins synthesized in the mammary gland using basal transcriptional and translational machinery. The suppression of lactoferrin expression that is observed in monolayer culture and in malignant tissues may reflect inappropriate cell shapes and cytoskeletal structures that are manifested under these conditions.