Wnt1 is epistatic to Id2 in inducing mammary hyperplasia, ductal side-branching, and tumors in the mouse
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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.
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.
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.
We conclude that functional Id2 expression is not required for Wnt1 to induce mammary hyperplasia and mammary tumors.
KeywordsMammary Gland Mammary Tumor Mouse Mammary Tumor Virus Mouse Mammary Gland bHLH Protein
Basic helix-loop-helix (bHLH) transcription factors such as MyoD, E12, and E47 are key regulators of gene expression and control many differentiation events during development [1, 2, 3]. These transcription factors bind E-box or E-box-like sequences as homo-or heterodimers, and control the transcription of target genes containing these sequences in their promoters. The HLH domains dimerize with each other, whereas the basic domains bind to DNA. The Id (Inhibitor of DNA binding) proteins are HLH proteins that lack a basic domain. Id proteins act as dominant inhibitors of bHLH transcription factors by blocking their ability to bind to DNA and activate gene transcription [2, 3]. Since the bHLH proteins regulate cell-type specific gene expression during cell commitment and differentiation, the formation of inactive heterodimers of bHLH proteins with Id proteins inhibits the commitment and differentiation the bHLH proteins promote.
There are 4 mammalian Id genes, which show differences in their patterns of expression and function [2, 3]. One of them, Id2, is expressed in glandular and ductal epithelium of the mouse mammary gland and has also been implicated in its development. Mammary glands of female mice that are homozygous mutant for Id2 have impaired lobulo-alveolar development . In several tissues, including colon tumors induced by activation of the Wnt pathway, the expression of Id2 is regulated by Wnt-β catenin signaling [5, 6]. It has been proposed that Wnt signaling may inhibit differentiation and promote the maintenance of a proliferative state by increasing Id2 expression, thereby leading to cancer. We have asked therefore 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.
We used heterozygous Id2 males and females on a 129/Sv background. Id2 genotyping was done by PCR (95C, 5 min; 62C, 1 min, 72C, 1 min, 95C,1 min, 30 cycles; 62C, 1 min, 72C, 5 min) using primers Id2-S (5'-tctgagcttatgtcgaatgatagc-3'), Id-2AS (5'-cgtgttctcctggtgaaatggctg-3'), and neo 1 (5'-tcgtgctttacggtatcgccgctc-3").
Hemizygous transgenic MMTV-Wnt1 males on a mixed FVB/BL6/SJL background were obtained from Yi Li in the H.Varmus laboratory. Genotyping was done by PCR (94C, 4 min; 94C, 45 sec, 55C, 30 sec, 72C, 60 sec, 30 cycles; 72C, 10 min) using Wnt1 (5'-gaacttgcttctcttctcatagcc-3') and SV40 (5'-ccacacaggcatagagtgtctgc-3') primers that produce a 350 bp product in transgenic mice.
Five mammary glands per mouse were removed and fat and muscle were dissected away. The glands were flattened between two slides and flooded with Carnoy's fixative (3:1 95% ethanol to glacial acetic acid) and fixed overnight. They were then de-fatted in 3 changes of acetone, rehydrated, stained overnight in 0.2% carmine and 0.5%KSO4, dehydrated, cleared in xylene, and mounted in Permount.
The subject animals were kept in mixed groups in autoclaved cages because Id2 -/- mice have an immunologic defect. Even with this care, 50% die before maturity . Id2 -/- mice were born in sub-Mendelian ratios, they were smaller than litter-mates, and several died of unknown causes.
The lack of Id2, a Wnt target, has severe consequences for mouse mammary gland development. During pregnancy, the Id2 mammary gland is deficient in lobulo-alveolar development. This failure of development is believed to be due to a proliferation failure rather than precocious differentiation of the mammary epithelia . We show in this work that forced expression of Wnt1 in the mammary gland is capable of overcoming this block to proliferation. Although many targets of the Wnt pathway have been identified (see http://www.stanford.edu/~rnusse/pathways/targets.html), the mechanism through which hyperplasia and side branching is promoted by Wnt1 expression in the virgin mammary gland is unknown. Our results demonstrate that Wnt1 is not operating solely through Id2 or that it is not operating through Id2 at all.
Another known Wnt target with a similar loss of function phenotype is Cyclin D1 , a protein whose expression promotes advancement through the cell cycle and whose over expression results in hyperplasia and tumors in the mammary gland . However, when Wnt1 is expressed in Cyclin D1 -/- mice, only a slight reduction in tumorigenesis is observed , suggesting that the Wnt1 pathway also does not operate primarily through Cyclin D1. Furthermore, over expression of Cyclin D1 did not promote lobulo-alveolar development in Id2-/- mice , a result that is in contrast to the dense side branching of the Wnt1 Id2-/- phenotype, suggesting that Wnt1 signaling is independent of both Cyclin D1 and Id2 in the mammary gland.
By showing that forced expression of Wnt1 in the mammary gland is capable of overcoming the block to proliferation caused by the absence of Id2, we conclude that functional Id2 expression is not required for Wnt1 to induce mammary hyperplasia and mammary tumors.
The work was supported by the Howard Hughes Medical Institute and a grant from the US ARMY Medical Research and Material Command (DAMD17-99-1-9386).
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