Earlier studies reported allelic deletion of the essential autophagy regulator BECN1 in breast cancers implicating BECN1 loss, and likely defective autophagy, in tumorigenesis. Recent studies have questioned the tumor suppressive role of autophagy, as autophagy-related gene (Atg) defects generally suppress tumorigenesis in well-characterized mouse tumor models. We now report that, while it delays or does not alter mammary tumorigenesis driven by Palb2 loss or ERBB2 and PyMT overexpression, monoallelic Becn1 loss promotes mammary tumor development in 2 specific contexts, namely following parity and in association with wingless-type MMTV integration site family, member 1 (WNT1) activation. Our studies demonstrate that Becn1 heterozygosity, which results in immature mammary epithelial cell expansion and aberrant TNFRSF11A/TNR11/RANK (tumor necrosis factor receptor superfamily, member 11a, NFKB activator) signaling, promotes mammary tumorigenesis in multiparous FVB/N mice and in cooperation with the progenitor cell-transforming WNT1 oncogene. Similar to our Becn1(+/-);MMTV-Wnt1 mouse model, low BECN1 expression and an activated WNT pathway gene signature correlate with the triple-negative subtype, TNFRSF11A axis activation and poor prognosis in human breast cancers. Our results suggest that BECN1 may have nonautophagy-related roles in mammary development, provide insight in the seemingly paradoxical roles of BECN1 in tumorigenesis, and constitute the basis for further studies on the pathophysiology and treatment of clinically aggressive triple negative breast cancers (TNBCs).
Keywords: 8-O-dG, 8-oxo-7, 8-dihydroguanine; ATG, autophagy-related; BECN1, Beclin 1, autophagy-related; BSA, bovine serum albumin; Beclin 1; CASP3, caspase 3; CD24, cluster of differentiation 24; DAPI, 4′, 6-diamidino-2-phenylindole; DFS, disease-free survival; DMEM, Dulbecco's modified Eagle's medium; E, 17b-estradiol; EGF, epidermal growth factor; EGFP, enhanced green fluorescent protein; EGFR/ERBB1, epidermal growth factor receptor; EM, electron microscopy; EMT, epithelial-to-mesenchymal transition; ERBB2, v-erb-b2 avian erythroblastic leukemia viral oncogene homolog 2; ESR1, estrogen receptor 1; FACS, fluorescence activated cell sorting; FGF2/bFGF, fibroblast growth factor 2 (basic); GSEA, gene set enrichment analysis; H&E, hematoxylin &, eosin; HR, hormone receptor; IF, immunofluorescence; IHC, immunohistochemistry; IL, interleukin; ITGB1/CD29, Integrin, beta 1 (fibronectin receptor beta polypeptide, antigen CD29 includes MDF2, MSK12); ITGB3/CD61, integrin, beta 3 (platelet glycoprotein IIIa, antigen CD61); KRT, keratin; Keratin 6; LIN−, lineage negative (CD31− CD45− LY76−); LY76/TER119, lymphocyte antigen 76; MAP1LC3B/LC3B, microtubule-associated protein 1 light chain 3 beta; MEC, mammary epithelial cell; MEGM, mammary epithelial growth medium; MGs, mammary glands; MKI67, marker of proliferation Ki-67; MMTV, mouse mammary tumor virus; MaPC, mammary progenitor cell; MaSC, mammary stem cell; NFKB; NFKB/NFkB, nuclear factor of kappa light polypeptide gene enhancer in B-cells; PBS, phosphate-buffered saline; PECAM1/CD31, platelet/endothelial cell adhesion molecule 1; PGR, progesterone receptor; PI, propidium iodide; PTPRC/CD45, protein tyrosine phosphatase, receptor type, C; RELA/P65, v-rel avian reticuloendotheliosis viral oncogene homolog a; ROS, reactive oxygen species; SD, standard deviation; SNPs, single nucleotide polymorphisms; SQSTM1/p62, sequestosome1; TEBs, terminal end buds; TNBC; TNBCs, triple-negative breast cancers; TNF, tumor necrosis factor; TNF11; TNFRSF11A; TNFRSF11A/TNR11/RANK, tumor necrosis factor receptor superfamily, member 11a, NFKB activator; TNFSF11; TNFSF11/TNF11/RANKL, tumor necrosis factor (ligand) superfamily, member 11; TNR11; TP53 (TRP53 in mice), tumor protein p53 (transformation related protein 53 in mice); WNT1; WNT1, wingless-Type MMTV integration site family, member 1; basal-like breast cancer; iMMECs, immortalized mouse mammary epithelial cells; p-KRT8/p-K8, phosphorylated Keratin 8; parity.