Abstract
The RNA interference (RNAi) machinery is an essential component of the cell, regulating miRNA biogenesis and function. RNAi complexes were thought to localize either in the nucleus, such as the microprocessor, or in the cytoplasm, such as the RNA-induced silencing complex (RISC). We recently revealed that the core microprocessor components DROSHA and DGCR8, as well as the main components of RISC, including Ago2, also associate with the apical adherens junctions of well-differentiated cultured epithelial cells. Here, we demonstrate that the localization of the core RNAi components is specific and predominant at apical areas of cell-cell contact of human normal colon epithelial tissues and normal primary colon epithelial cells. Importantly, the apical junctional localization of RNAi proteins is disrupted or lost in human colon tumors and in poorly differentiated colon cancer cell lines, correlating with the dysregulation of the adherens junction component PLEKHA7. We show that the restoration of PLEKHA7 expression at adherens junctions of aggressively tumorigenic colon cancer cells restores the junctional localization of RNAi components and suppresses cancer cell growth in vitro and in vivo. In summary, this work identifies the apical junctional localization of the RNAi machinery as a key feature of the differentiated colonic epithelium, with a putative tumor suppressing function.
Highlights
Colorectal cancer is the third most prevalent and second deadliest form of cancer [1]
Nuclear localization of DROSHA and DGCR8 and cytoplasmic localization of Ago2 is observed in these samples, their apical localization seems predominant and highlights the apical areas of colonic crypts. These observations are in agreement with our findings both in Caco2 cells and in primary colon epithelial cells (Figure 1) and demonstrate that the core components of the RNA interference (RNAi) machinery primarily localize at the apical adherens junctions of well-differentiated human colonic epithelial tissues
The loss of junctional localization of RNAi components in the colon cancer cell lines that we examined cannot be attributed to their downregulation, as they are still robustly expressed in these cells, in at least similar levels with Caco2 cells (Figure 3E)
Summary
Colorectal cancer is the third most prevalent and second deadliest form of cancer [1]. We demonstrated that the apical adherens junctions can suppress pro-tumorigenic cell transformation through PLEKHA7, whereas the basolateral cell-cell junction complexes promote it, in the absence of PLEKHA7 [10,21,22] In agreement with these findings, we and others have shown that PLEKHA7 is broadly mis-localized or downregulated in human breast and kidney tumors, E-cadherin is still broadly expressed at areas of cell-cell contact [10,23]. Nuclear localization of DROSHA and DGCR8 and cytoplasmic localization of Ago is observed in these samples, their apical localization seems predominant and highlights the apical areas of colonic crypts These observations are in agreement with our findings both in Caco cells and in primary colon epithelial cells (Figure 1) and demonstrate that the core components of the RNAi machinery primarily localize at the apical adherens junctions of well-differentiated human colonic epithelial tissues
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