piRNA-independent Function of PIWIL1 as a Novel co-activator for Anaphase Promoting Complex/Cyclosome (APC/C) to Drive Pancreatic Cancer Metastasis

Piwi proteins are normally restricted in germ cells to suppress transposons through association with piRNAs, but are also frequently activated in many types of human cancers. Of great puzzle is the lack of induction of corresponding piRNAs in cancer cells, as we document here in human pancreatic ductal adenocarcinomas (PDAC), implying that such germline-specific proteins are somehow hijacked to promote tumorigenesis through different mode of action. To pursue the underlying mechanism, we now demonstrate that in the absence of piRNAs, the aberrantly induced human PIWIL1 in PDAC functions as an oncoprotein by activating the Anaphase Promoting Complex/Cyclosome (APC/C), which then targets a critical cell adhesion-related protein Pinin to enhance PDAC metastasis. This is in contrast to piRNA-dependent PIWIL1 ubiquitination and removal by APC/C during late spermiogenesis. These findings thus unveil an piRNA-dependent mechanism to switch PIWIL1 from a substrate in spermatids to a co-activator of APC/C in human cancer cells. Small RNA profiling of cancer cell line BxPC-3, with NaIO4 oxidization or not. Oxidation with NaIO4 is used to enrich piRNAs as they are protected from periodate oxidation by their terminal 2'-O-methyl modification while RNAs with a terminal 2′,3′-hydroxyl are modified and depleted during cDNA library preparation. Mouse testis is positive control.