aPKCζ cortical loading is associated with Lgl cytoplasmic release and tumor growth in Drosophila and human epithelia

Atypical protein kinase C (aPKC) and Lethal giant larvae (Lgl) regulate apical–basal polarity in Drosophila and mammalian epithelia. At the apical domain, aPKC phosphorylates and displaces Lgl that, in turn, maintains aPKC inactive at the basolateral region. The mutual exclusion of these two proteins seems to be crucial for the correct epithelial structure and function. Here we show that a cortical aPKC loading induces Lgl cytoplasmic release and massive overgrowth in Drosophila imaginal epithelia, whereas a cytoplasmic expression does not alter proliferation and epithelial overall structure. As two aPKC isoforms (ι and ζ) exist in humans and we previously showed that Drosophila Lgl is the functional homologue of the Human giant larvae-1 (Hugl-1) protein, we argued if the same mechanism of mutual exclusion could be impaired in human epithelial disorders and investigated aPKCι, aPKCζ and Hugl-1 localization in cancers deriving from ovarian surface epithelium. Both in mucinous and serous histotypes, aPKCζ showed an apical-to-cortical redistribution and Hugl-1 showed a membrane-to-cytoplasm release, perfectly recapitulating the Drosophila model. Although several recent works support a causative role for aPKCι overexpression in human carcinomas, our results suggest a key role for aPKCζ in apical–basal polarity loosening, a mechanism that seems to be driven by changes in protein localization rather than in protein abundance.