RARα1 control of mammary gland ductal morphogenesis and wnt1-tumorigenesis

Abstract

IntroductionRetinoic 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 (retinoic acid receptor isotype alpha, isoform 1), for its role in mouse mammary gland morphogenesis and mouse mammary tumor virus (MMTV)-wingless-related MMTV integration site 1 (wnt1)-induced oncogenesis.MethodsThe 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, quantitative polymerase chain reaction (Q-PCR), test of tumor-free survival, tumor fragments and cell transplantation.ResultsIn two genetic backgrounds (129/Bl-6 and FVB) the neo-natal RARα1/KO-mammary epithelial tree was two-fold larger and the pubertal tree had two-fold more branch points and five-fold more mature end buds, a phenotype that was predominantly epithelial cell autonomous. The stem/progenitor compartment of the RARα1/KO mammary, defined as CD24low/ALDHhigh activity was increased by a median 1.7-fold, but the mammary stem cell (MaSC)-containing compartment, (CD24low/CD29high), was larger (approximately 1.5-fold) in the wild type (wt)-glands, and the mammary repopulating ability of the wt-gland epithelium was approximately two-fold greater. In MMTV-wnt1 transgenic glands the progenitor (CD24low/ALDHhigh 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.ConclusionsIn 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.

ATRA transcriptionally induces nSMase2 through CBP/p300-mediated histone acetylation

“…Collectively, these data agree with previous studies in MCF7 and T47D cells (9,22,23); moreover, the role that nSMase2 plays in ATRA-induced growth arrest (5) is consistent with the well-established antiproliferative role of RAR- (23,29,40,41). Additionally, as numerous studies have found that loss of RAR- signaling leads to alterations in mammary acini formation both in vivo and in vitro (27,(42)(43)(44), this suggests that the role of nSMase2 in ATRA-induced growth mRNA levels. Collectively, these results identify two novel regulators of nSMase2 and place nSMase2 as part of the CBP and p300 responses.…”

Epigenetic regulation by RARα maintains ligand-independent transcriptional activity

“…Studies from MMTV-wnt1 animal models demonstrated that agonist activated RARα inhibited mammary tumor formation and growth, but that the loss of RARα delayed mammary tumorigenesis (14), thus supporting roles of RARα as both a tumor suppressor and protooncogene. Current models of nuclear receptor action suggest that the different functions of RARα are dictated by cofactor recruitment, resulting in epigenetic changes, e.g.…”

Protein tyrosine phosphatase 1B restrains mammary alveologenesis and secretory differentiation

“…2C). Several studies have suggested that high aldehyde dehydrogenase (ALDH) activity is a property of stem and/or progenitor cells in human and mouse mammary tissues (Ginestier et al, 2007;Cohn et al, 2010;Eirew et al, 2012). Using the Aldefluor assay, we found a higher ALDH activity in MECs from Ptp1b -/-than Ptp1b +/+ mice (supplementary material Fig.…”