Engulfment Protein GULP Is Regulator of Transforming Growth Factor-β Response in Ovarian Cells

Transforming growth factor beta (TGF-beta) is a key regulatory molecule with pleiotropic effects on cell growth, migration, and invasion. As a result, impairment of proper TGF-beta signaling is central to tumorigenesis and metastasis. The TGF-beta receptor V (TGFBRV or LRP1) has been shown to be responsible for TGF-beta-mediated cell growth inhibition in Chinese hamster ovary (CHO) cells. The LRP1 adapter protein GULP mediates internalization of the various LRP1-specific ligands, and we hypothesize that GULP acts as a novel regulator of TGF-beta signaling in ovarian cells. CHO cells that overexpress exogenous GULP (FL) demonstrate enhancement in growth inhibition, migration, and invasion from TGF-beta treatment, whereas cells that lack GULP (AS) show impairment of growth inhibition and decreased migration and invasion. The enhanced TGF-beta response in FL cells was confirmed by a prolonged TGF-beta-induced SMAD3 phosphorylation, whereas a shortening of the phosphorylation event is observed in AS cells. Mechanistically, the presence of GULP retains the TGF-beta in a signaling-competent early endosome for enhanced signaling. To address this mechanism in a physiological setting, TGF-beta insensitive ovarian adenocarcinoma cells (HEY) have a very low GULP expression level, similar to the observation made in a wide selection of human ovarian adenocarcinomas. Transfection of GULP into the HEY cells restored the TGF-beta responsiveness, as measured by SMAD3 phosphorylation and impairment of cell growth. Because GULP expression positively regulates TGF-beta signaling leading to growth inhibition, this may represent an attractive target to achieve TGF-beta responsiveness in ovarian cells.