Cross-section of a hypertrophic duct from involuting mammary tissue of a transgenic
mouse expressing excess IGF1 under control of a mammary specific gene promoter. Ductile
hypertrophy is characterized by a flattened epithelia which sometimes contains several
cell layers and an abnormally thick deposit of collagen fibers and stromal cells.
Hypertrophic ducts were filled with a lipid-rich secretion not seen in non-transgenic
mice. Partially involuted and remodelled alveoli can be seen to the left and the right of
the duct.
Summary Development of the mammary gland during puberty, pregnancy and lactation is
controlled by steroid and peptide hormones and growth factors. To determine the role of
insulin-like growth factors (IGFs) in this process we developed a transgenic model using
the whey acidic protein gene to direct expression of rat IGF-1 and human IGF binding
protein-3 (IGFBP-3) to mammary tissue during late pergnancy and throughout lactation.
High levels of expression of transgenic IGF-1 and IGFBP-3 were seen in lobular-alveolar
cells by in situ hybridization. There was no obvious effectr on mammary development during
pregnancy and lactation: indeed, mothers were capable of nursing their pups normally and
the only structural difference seen in the mammary gland atr peak lactation was an overall
smaller size of the alveoli. We also evaluated the role of IGF-1 and IGFBP-3 in the
remodelling of mammary tissue during involution. Compared to control animals, the process
of involution was modified in both transgenic lines. The degree of apoptotic cells was
lower in the WAP-IGF-1 and WAP-BP-3 expressing mice. In addition, there was a more
quiescent pattern of involution with residual lobular secretory ability and a mutated host
inflammatory reaction with fewer lumenal microcalicifcations. These results demonstrate
the IGF-1 and IGFBP-3 may modulate the involutionary process of the lactating mammary
gland.
Citations Neuschwander, S., Schwartz, A., Wood, T.L., Roberts, C.T., Hennighausen, L and LeRoith, D (1996) Involution of the lactating mammary gland is inhibited by the IGF system in transgenic mice. J. Clin. Invest. 97, 2225-2232.
Note
: Transgenic mice have been produced that express des(1-3) human IGFI in mammary tissue (info). A separate mini-review discusses the role of IGF1 in mammary gland development.Mammary Phenotype
Females from both the high expressing WAP-IGF1 line and the WAP-BP3 line delivered litters of normal size and were able to lactate and raise their litters repeatedly. This clearly demonstrated that mammary gland development during pregnancy and functional terminal differentiation of alveolar epithelial cells was unimpaired in the presence of excess IGF1 or BP3. Histological analyses of mammary tissue from animals at day 10 of lactation revealed that the fat pads of both the WAP-IGF1 and WAP-BP3 transgenic mice and non-transgenic littermates were filled with lobulo-alveolar structures. The alveolar cells were cuboidal with prominent cytoplasmic vacuoles and surrounded large lumina, features indicative of extensive secretion. Although mammary tissue in transgenic mice was able to synthesize and secrete sufficient milk to maintain a litter, the alveoli appeared smaller than those found in non-transgenic littermates.
The role of persistent IGF-1 and IGFBP-3 synthesis on mammary gland involution and tissue remodelling was investigated. Pups were removed from their mothers at day 10 of lactation, and mammar tissue was evaluated after 5d of involution. While extensive remodelling of mammary tissue occurred in control mice, a modified involution was seen in both transgenic mice (Figure 2). In non-transgenic mice most of the lobules and alveoli had collapsed at this point and remodelling of mammary tissue was almost completed. In stark contrast, extensive lobulo-alveolar units remained in the WAP-IGF1 mice and to a lesser extent in the WAP-BP3 mice. The retention of alveoli during involution was not uniform throughout the fat pad, but rather focal. This suggests that transgene expression was not uniform throughout the mammary tissue.
Involution of mammary tissue is characterized by extensive remodelling and apoptosis mediated through p53 independent pathways (Li et al., 1996. While the presence of apoptotic cells was widespread in non-transgenic mice, less apotosis was observed in the alveoli of both WAP-IGF1 and WAP-BP3 mice (Figure). In control mice some alveoli were still visible at day 5 of involution, but their epithelial cell layer was already disrupted and dead cells had been shedded into the lumen. Using a TUNEL assay we could show extensive cell death in remodelled alveoli of non-transgenic mice. In contrast, apoptotic cells were rare in retained alveoli in the transgenic mice.
key words IGF1, BP3, mammary development, involution, apoptosis, WAP
Contributed By
Lothar Hennighausen
Section of Developmental Biology
National Institutes of Health
Bldg. 10, Rm. 9N113
Bethesda, MD 20892
Phone: (301) 496-2716
Fax: (301) 496-0839
e-mail: mammary@nih.gov
last update: June 1998