Introduction: Retinoic acid signaling pathways are disabled in human breast cancer suggesting a controlling role in normal mammary growth that might be lost in tumorigenesis. We tested a single receptor isotype, RARα1, for its role in mouse mammary gland morphogenesis and MMTV-wnt1-induced oncogenesis.
Methods: The role of RARα1 in mammary morphogenesis was tested in RARα1-knockout (KO) mice and in mammary tumorigenesis in bi-genic (RARα1/KO crossed with MMTV-wnt1) mice. We used whole mounts analysis, stem cells/progenitor quantification, mammary gland repopulation, Q-PCR, test of tumor-free survival, tumor fragments and cell transplantation.
Results: In 2 genetic backgrounds (129/Bl-6 and FVB) the neo-natal RARα1/KO-mammary epithelial tree was 2-fold larger and the pubertal tree had 2-fold more branch points and 5-fold more mature end buds, a phenotype that was predominantly epithelial cell autonomous. The stem/progenitor compartment of the RARα1/KO mammary, defined as CD24(low)/ALDH(high activity) was increased by a median 1.7 fold, but the mammary stem cell (MaSC)-containing compartment, (CD24(low)/CD29(high)), was larger (~1.5 fold) in the wt-glands, and the mammary repopulating ability of the wt-gland epithelium was ~2-fold greater. In MMTV-wnt1 transgenic glands the progenitor (CD24(low)/ALDH(high activity)) content was 2.6-fold greater than in the wt and was further increased in the RARα1/KO-wnt1 glands. The tumor-free survival of RARα1/KO-wnt1 mice was significantly (p=0.0002, Kaplan Meier) longer, the in vivo growth of RARα1/KO-wnt1 transplanted tumor fragments was significantly (p=0.01) slower and RARα1/KO-wnt1 tumors cell suspension produced tumors after much longer latency.
Conclusions: In vitamin A-replete mice, RARα1 is required to maintain normal mammary morphogenesis, but paradoxically, also efficient tumorigenesis. While its loss increases the density of the mammary epithelial tree and the content of luminal mammary progenitors, it appears to reduce the size of the MaSC-containing compartment, the mammary repopulating activity, and to delay significantly the MMTV-wnt1-mammary tumorigenesis. Whether the delay in tumorigenesis is solely due to a reduction in wnt1 target cells or due to additional mechanisms remains to be determined. These results reveal the intricate nature of the retinoid signaling pathways in mammary development and carcinogenesis and suggest that a better understanding will be needed before retinoids can join the armament of effective anti- breast cancer therapies.