Control of the expression and activity of the Galpha-interacting protein (GAIP) in human intestinal cells.

The Galpha-interacting protein (GAIP) is known to interact with the Galphai3 protein. It has been suggested that, depending on its expression, GAIP can be a regulator of trimeric Gi protein signaling pathways. In the present study we show that the GAIP mRNA content declines during the enterocytic differentiation of two cell lines derived from human colon adenocarcinomas: HT-29 and Caco-2. In undifferentiated HT-29 cells, when the GDP/GTP cycle on the trimeric Gi3 protein is interrupted by either pertussis toxin treatment or by the transfection of a mutant of the Galphai3 protein with no GTPase activity (Q204L), we observed a decrease in the GAIP mRNA content. As these conditions are known to impair the Gi3-dependent lysosomal-autophagic pathway existing in undifferentiated HT-29 cells, we have investigated the role of GAIP in controling the lysosomal-autophagic pathway. Overexpression of GAIP stimulated protein degradation along the macroautophagic pathway. In contrast, overexpression of GAIP did not modify the low rate of macroautophagy in cells expressing the Q204L mutant of the Galphai3 protein. These results show that GAIP regulates a major catabolic pathway and that the expression of GAIP is dependent upon the activity of the Galphai3 protein and the state of enterocytic differentiation of cells.