Rab7 regulates CDH1 endocytosis, circular dorsal ruffles genesis and thyroglobulin internalization in a thyroid cell line

Rab7 regulates the biogenesis of late endosomes, lysosomes and autophagosomes. It has been proposed that a functional and physical interaction exists between Rab7 and Rac1 GTPases in CDH1 endocytosis and ruffled border formation. In FRT cells over-expressing Rab7, increased expression and activity of Rac1 was observed, whereas a reduction of Rab7 expression by RNAi resulted in reduced Rac1 activity, as measured by PAK1 phosphorylation. We found that CDH1 endocytosis was extremely reduced only in Rab7 over-expressing cells but was unchanged in Rab7 silenced cells. In Rab7 under or over-expressing cells, Rab7 and LC3B-II co-localized and co-localization in large circular structures occurred only in Rab7 over-expressing cells. These large circular structures occurred in about 10% of the cell population; some of them (61%) showed co-localization of Rab7 with cortactin and f-actin and were identified as circular dorsal ruffles (CDRs), the others as mature autophagosomes. We propose that the over-expression of Rab7 is sufficient to induce CDRs. Furthermore, in FRT cells, we found that the expression of the insoluble/active form of Rab7, rather than Rab5 or Rab8, was inducible by cAMP and that cAMP-stimulated FRT cells showed increased PAK1 phosphorylation and were no longer able to endocytose CDH1. Finally, we demonstrated that Rab7 over-expressing cells are able to endocytose exogenous thyroglobulin via pinocytosis/CDRs more efficiently than control cells. We propose that the major thyroglobulin endocytosis described in thyroid autonomous adenomas due to Rab7 increased expression, occurs via CDRs.