Forced expression of cyclin D1 does not compensate for Id2 deficiency in the mammary gland

Exocrine gland structure-function relationships

Fluid secretion by exocrine glandular organs is essential to the survival of mammals. Each glandular unit within the body is uniquely organized to carry out its own specific functions, with failure to establish these specialized structures resulting in impaired organ function. Here, we review glandular organs in terms of shared and divergent architecture. We first describe the structural organization of the diverse glandular secretory units (the end-pieces) and their fluid transporting systems (the ducts) within the mammalian system, focusing on how tissue architecture corresponds to functional output. We then highlight how defects in development of end-piece and ductal architecture impacts secretory function. Finally, we discuss how knowledge of exocrine gland structure-function relationships can be applied to the development of new diagnostics, regenerative approaches and tissue regeneration.

Side branching and luminal lineage commitment by ID2 in developing mammary glands

Mammary glands develop through primary ductal elongation and side branching to maximize the spatial area. Although primary ducts are generated by bifurcation of terminal end buds, the mechanism through which side branching occurs is still largely unclear. Here, we show that inhibitor of DNA-binding 2 (ID2) drives side branch formation through differentiation of K6+ bipotent progenitor cells into CD61+ luminal progenitor cells. Id2-null mice had side branching defects, along with developmental blockage of K6+ bipotent progenitor cells into CD61+ luminal progenitor cells. Notably, CD61+ luminal progenitor cells were found in budding and side branches, but not in terminal end buds. Hormone reconstitution studies using ovariectomized MMTV-NLS-Id2 transgenic mice revealed that ID2 is a key mediator of progesterone, which drives luminal lineage differentiation and side branching. Our results suggest that CD61 is a marker for side branches and that ID2 regulates side branch formation by inducing luminal lineage commitment from K6+ bipotent progenitor cells to CD61+ luminal progenitor cells.

Strain-specific differences in the mechanisms of progesterone regulation of murine mammary gland development

Progesterone (P) is required for normal mammary gland development, and is implicated in the etiology of mammary cancer in rodents and humans. We analyzed mammary gland developmental responses to P and estrogen (E) in two strains of mice (BALB/c and C57BL/6) that exhibit differences in ductal development at sexual maturity and alveologenesis during pregnancy. C57BL/6 mice exhibited reduced proliferative and morphological responses to P. Analysis of known mediators of sidebranching and alveologenesis revealed that reduced P-induced expression of P receptor isoform B and receptor activator of nuclear factor-kappaB ligand (RANKL), as well as altered expression and regulation of cyclin D1, CCAAT/enhancer binding protein beta, and the downstream effectors of RANKL, nuclear Id2 and p21, contribute significantly to the reduced P responsiveness of the C57BL/6 mammary gland. In contrast, E responsiveness was greater in C57BL/6 than in BALB/c glands. E may play a compensatory role in C57BL/6 alveologenesis through its effect on the induction and activation of signal transducer and activator of transcription 5a, a known regulator of RANKL. These observations suggest that in human populations with heterogeneous genetic backgrounds, individuals may respond differentially to the same hormone. Thus, genetic diversity may have a role in determining the effects of P in normal mammary development and tumorigenesis.

Utilization of pathway signatures to reveal distinct types of B lymphoma in the Emicro-myc model and human diffuse large B-cell lymphoma

The Emu-myc transgenic mouse has provided a valuable model for the study of B-cell lymphoma. Making use of gene expression analysis and, in particular, expression signatures of cell signaling pathway activation, we now show that several forms of B lymphoma can be identified in the Emu-myc mice associated with time of tumor onset. Furthermore, one form of Emu-myc tumor with pre-B character is shown to resemble human Burkitt lymphoma, whereas others exhibit more differentiated B-cell characteristics and show similarity with human diffuse large B-cell lymphoma in the pattern of gene expression, as well as oncogenic pathway activation. Importantly, we show that signatures of oncogenic pathway activity provide further dissection of the spectrum of diffuse large B-cell lymphoma, identifying a subset of patients who have very poor prognosis and could benefit from more aggressive or novel therapeutic strategies. Taken together, these studies provide insight into the complexity of the oncogenic process and a novel strategy for dissecting the heterogeneity of B lymphoma.

Receptor activator of NF-kappaB ligand regulates the proliferation of mammary epithelial cells via Id2

Receptor activator of NF-kappaB ligand (RANKL) is a key regulator for mammary gland development during pregnancy. RANKL-deficient mice display impaired development of lobulo-alveolar mammary structures. Similar mammary gland defects have been reported in mice lacking Id2. Here we report that RANKL induces the proliferation of mammary epithelial cells via Id2. RANKL triggers marked nuclear translocation of Id2 in mammary epithelial cells. In vivo studies further demonstrated the defective nuclear translocation of Id2, but the normal expression of cyclin D1, in the mammary epithelial cells of rankl-/- mice. In vitro studies with nuclear localization sequence-tagged Id2 revealed that the nuclear localization of Id2 itself is critical for the downregulation of p21 promoter activity. Moreover, RANKL stimulation failed to induce cell growth and to downregulate p21 expression in Id2-/- mammary epithelial cells. Our results indicate that the inhibitor of helix-loop-helix protein, Id2, is critical to control the proliferation of mammary epithelial cells in response to RANKL stimulation.

Wnt1 is epistatic to Id2 in inducing mammary hyperplasia, ductal side-branching, and tumors in the mouse

Background

During pregnancy, the mammary glands from Id2 mutant animals are deficient in lobulo-alveolar development. This failure of development is believed to be due to a proliferation defect.

Methods

We have asked whether functional Id2 expression is necessary for Wnt induced mammary hyperplasia, side branching, and cancer, by generating mice expressing a Wnt1 transgene in an Id2 mutant background.

Results

We show in this work that forced expression of Wnt1 in the mammary gland is capable of overcoming the block to proliferation caused by the absence of Id2. We also show that Wnt1 expression is able to cause mammary tumors in an Id2 mutant background.

Conclusions

We conclude that functional Id2 expression is not required for Wnt1 to induce mammary hyperplasia and mammary tumors.