Tumor-suppressor function of Beclin 1 in breast cancer cells requires E-cadherin

Significance

Beclin 1, an essential autophagy protein, is important for tumor suppression in mice, as well as in human breast and ovarian cancers. However, it is not well understood how Beclin 1 acts as a tumor suppressor. By performing a genetic screen to identify genes whose loss blocks the ability of Beclin 1 to inhibit the growth of breast cancer cells and follow-up biological analyses, we have identified a mechanism by which Beclin 1 prevents breast cancer growth. We found that Beclin 1 promotes the plasma membrane localization of E-cadherin, a breast tumor-suppressor molecule that restricts tumor growth and metastases only when present at the cell surface. These findings have important implications for understanding the cell biology of human breast cancer.

Abstract

Beclin 1, an autophagy and haploinsufficient tumor-suppressor protein, is frequently monoallelically deleted in breast and ovarian cancers. However, the precise mechanisms by which Beclin 1 inhibits tumor growth remain largely unknown. To address this question, we performed a genome-wide CRISPR/Cas9 screen in MCF7 breast cancer cells to identify genes whose loss of function reverse Beclin 1-dependent inhibition of cellular proliferation. Small guide RNAs targeting CDH1 and CTNNA1, tumor-suppressor genes that encode cadherin/catenin complex members E-cadherin and alpha-catenin, respectively, were highly enriched in the screen. CRISPR/Cas9-mediated knockout of CDH1 or CTNNA1 reversed Beclin 1-dependent suppression of breast cancer cell proliferation and anchorage-independent growth. Moreover, deletion of CDH1 or CTNNA1 inhibited the tumor-suppressor effects of Beclin 1 in breast cancer xenografts. Enforced Beclin 1 expression in MCF7 cells and tumor xenografts increased cell surface localization of E-cadherin and decreased expression of mesenchymal markers and beta-catenin/Wnt target genes. Furthermore, CRISPR/Cas9-mediated knockout of BECN1 and the autophagy class III phosphatidylinositol kinase complex 2 (PI3KC3-C2) gene, UVRAG, but not PI3KC3-C1–specific ATG14 or other autophagy genes ATG13, ATG5, or ATG7, resulted in decreased E-cadherin plasma membrane and increased cytoplasmic E-cadherin localization. Taken together, these data reveal previously unrecognized cooperation between Beclin 1 and E-cadherin–mediated tumor suppression in breast cancer cells.