Hormones Are Not Enough

Abstract

Cytokines are critical to the development of distinct lineages of CD4 + T helper (Th) cells. For example, interleukin-12 (IL-12) activates signal transducer and activator of transcription 4 (STAT4) and promotes Th1 differentiation, whereas IL-4 and IL-13 activate STAT6, leading to the development of Th2 cells. Suppressor of cytokine signaling (SOCS) proteins negatively regulate cytokine receptor signaling and thus help determine the fate of the T cell. SOCS3 inhibits IL-12 signaling, whereas IL-4 signaling is negatively regulated by SOCS5. Another feature of Th cell development is that while one set of cytokines (Th1- or Th2-promoting) is produced, production of the opposing set of cytokines is inhibited. Mammary epithelial cells (MECs) are dramatically increased in number during pregnancy and develop into either luminal cells, which produce milk, or myoepithelial cells, which form ducts. Differentiation of MECs is classically thought to be regulated by hormones such as estrogen and progesterone, but Khaled et al . investigated roles for cytokines, cytokine receptors, and their associated transcription factors based on this group’s previous finding that STAT6 is highly abundant in mammary glands during development. The authors used reverse transcription polymerase chain reaction (RT-PCR) assays, Western blotting, and immunohistochemistry to demonstrate that phosphorylation of STAT6 became detectable in the luminal and ductal cells of mouse mammary glands 5 days after the onset of gestation and persisted until the onset of lactation. The mammary glands of pregnant Stat6 –/– mice had fewer side branches and alveolar buds than those of pregnant wild-type mice, and, during the course of gestation, Stat6 –/– mice had delayed mammary gland development. Western blotting showed that a number of signaling pathways involved in cell proliferation and survival were perturbed in the mammary glands of pregnant Stat6 –/– mice, including the activation of extracellular signal-regulated kinase (ERK) and Akt. RT-PCR assays showed that, early in gestation, expression of the Il4 and Il13 genes was lower in the mammary glands of Stat6 –/– mice than in glands from wild-type mice. This led the authors to examine mammary gland development in Il4 –/– / Il13 –/– double-knockout mice, which was also delayed compared with that in wild-type control mice. Conversely, mammary gland development in Socs5 –/– mice (which have enhanced IL-4 and IL-13 signaling) was accelerated compared with that in wild-type mice. Western blotting analysis of a mouse mammary epithelial cell line (KIM-2) that undergoes hormone-induced differentiation in vitro showed that these cells responded to IL-4 and IL-13 with increased phosphorylation of STAT6. Differentiation of KIM-2 cells to the luminal cell lineage was characterized by the early down-regulation of genes encoding Th1 cytokines, such as IL-12, and an accompanying increase in the expression of genes encoding the Th2 cytokines IL-4 and IL-13, as determined by quantitative RT-PCR. This demonstration that Th2 cytokines are necessary for the development of epithelial cells in the mammary gland complicates the accepted scheme that hormones are the primary regulators involved. W. T. Khaled, E. K. C. Read, S. E. Nicholson, F. O. Baxter, A. J. Brennan, P. J. Came, N. Sprigg, A. N. J. McKenzie, C. J. Watson, The IL-4/IL-13/Stat6 signalling pathway promotes luminal mammary epithelial cell development. Development 134 , 2739-2750 (2007). [Abstract] [Full Text]