Role of TGFbeta and function of macrophages in mammary gland involution
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Mammary gland involution is separated into two distinct phases. The first, induced by milk stasis following weaning, involves epithelial cell apoptosis and is regulated by locally synthesized factors. During the second phase, the lobular-alveolar structure collapses due to the degradation of the basement membrane and extracellular matrix. Of those local factors regulating the first phase, TGFbeta3 is the first documented for a full apoptotic response. The TGFbeta signaling is mainly through SMADs signaling. To address the mechanism of TGFbeta3 inducing apoptosis in the mammary epithelium, we used transgenic techniques to express SMAD7 specifically in this tissue. In this mouse model, we have shown that over-expression of SMAD7 in the mammary gland at early involution inhibits apoptosis and delays the structural collapsing. In addition, we showed that STAT3 phosphorylation, which is required for apoptosis, is inhibited by SMAD7 over-expression. This suggests that TGFbeta3 signaling through SMAD proteins during involution is upstream of STAT3 activation. In contrast, SMAD7 expressed on its own in the TGFbeta depleted lactational state causes apoptosis. Thus SMAD7 can have opposite functions dependent on context either blocking or causing apoptosis.;Many cells are involved in the tissue remodeling associated with involution. It has been assumed that macrophages are the cells that play an essential role in this process through phagocytosis. Recently this view has been challenged with the suggestion that the apoptotic cells are removed by autophagy. Macrophages are regulated principally by the colony stimulating factor-1 (CSF-1). Mice homozygous for a null mutation in the CSF-1 gene have impaired mammary gland development. It is necessary to ablate macrophages post-developmentally in order to study their role in involution. Therefore, a conditional allele of the CSF-1R was created. We crossed these mice with a cre-deletor strain, ZP3-cre. This resulted in a null allele as assessed by analysis of the homozygous mice that showed toothlessness, low body weight and deficient tissue macrophages. Thus a suitable conditional allele for the CSF-1R was established. These floxed CSF-1R mice will provide a valuable resource and will distinguish cell types responsible for clearance of apoptotic cells during involution.
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