regulatory element binding protein and dietary lipid regulation of fatty acid synthesis in the mammary epithelium.

Claudins are a large family of membrane proteins whose classic function is to regulate the permeability of tight junctions in epithelia. They are tetraspanins, with four alpha-helices crossing the membrane, two extracellular loops, a short cytoplasmic N-terminus and a longer and more variable C-terminus. The extracellular ends of the helices are known to undergo side-to-side (cis) interactions that allow the formation of claudin polymers in the plane of the membrane. The extracellular loops also engage in head-to-head (trans) interactions thought to mediate the formation of tight junctions. However, claudins are also present in intracellular structures, thought to be vesicles, with less well-characterized functions. Here, we briefly review our current understanding of claudin structure and function followed by an examination of changes in claudin mRNA and protein expression and localization through mammary gland development. Claudins-1, 3, 4, 7, and 8 are the five most prominent members of the claudin family in the mouse mammary gland, with varied abundance and intracellular localization during the different stages of post-pubertal development. Claudin-1 is clearly localized to tight junctions in mammary ducts in non-pregnant non-lactating animals. Cytoplasmic puncta that stain for claudin-7 are present throughout development. During pregnancy claudin-3 is localized both to the tight junction and basolaterally while claudin-4 is found only in sparse puncta. In the lactating mouse both claudin-3 and claudin-8 are localized at the tight junction where they may be important in forming the paracellular barrier. At involution and under challenge by lipopolysaccharide claudins −1, −3, and −4 are significantly upregulated. Claudin-3 is still colocalized with tight junction molecules but is also distributed through the cytoplasm as is claudin-4. These largely descriptive data provide the essential framework for future mechanistic studies of the function and regulation of mammary epithelial cell claudins.Electronic supplementary materialThe online version of this article (doi:10.1007/s10911-017-9379-6) contains supplementary material, which is available to authorized users.

BackgroundThe Six1 homeobox gene is highly expressed in the embryonic mammary gland, continues to be expressed in early postnatal mammary development, but is lost when the mammary gland differentiates during pregnancy. However, Six1 is re-expressed in breast cancers, suggesting that its re-instatement in the adult mammary gland may contribute to breast tumorigenesis via initiating a developmental process out of context. Indeed, recent studies demonstrate that Six1 overexpression in the adult mouse mammary gland is sufficient for initiating invasive carcinomas, and that its overexpression in xenograft models of mammary cancer leads to metastasis. These data demonstrate that Six1 is causally involved in both breast tumorigenesis and metastasis, thus raising the possibility that it may be a viable therapeutic target. However, because Six1 is highly expressed in the developing mammary gland, and because it has been implicated in the expansion of mammary stem cells, targeting Six1 as an anti-cancer therapy may have unwanted side effects in the breast.ResultsWe sought to determine the role of Six1 in mammary development using two independent mouse models. To study the effect of Six1 loss in early mammary development when Six1 is normally expressed, Six1-/- embryonic mammary glands were transplanted into Rag1-/- mice. In addition, to determine whether Six1 downregulation is required during later stages of development to allow for proper differentiation, we overexpressed Six1 during adulthood using an inducible, mammary-specific transgenic mouse model. Morphogenesis of the mammary gland occurred normally in animals transplanted with Six1-/- embryonic mammary glands, likely through the redundant functions of other Six family members such as Six2 and Six4, whose expression was increased in response to Six1 loss. Surprisingly, inappropriate expression of Six1 in the adult mammary gland, when levels are normally low to absent, did not inhibit normal mammary differentiation during pregnancy or lactation.ConclusionsSix1 is not critical for normal mammary gland development, since neither loss nor inappropriate overexpression of Six1 adversely affects normal mammary gland development or function. However, as both Six2 and Six4 levels are increased in Six1-/- mammary glands, we postulate that these Six family members are functionally redundant in the gland, as is true of many homeobox gene families. This data, in conjunction with recent findings that Six1 is capable of promoting breast cancer initiation and progression, suggest that Six1 may serve as a reasonable chemotherapeutic target in a clinical setting, particularly for those women diagnosed with breast cancer in their childbearing years.

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