A Heterotrimeric G-protein Controls Autophagic Sequestration in the Human Colon Cancer Cell Line HT-29 (*)

Abstract

Human colon cancer HT-29 cells exhibit a differentiation-dependent autophagic-lysosomal pathway that is responsible for the degradation of a pool of newly synthesized N-linked glycoproteins in undifferentiated cells. In the present study, we have investigated the molecular control of this degradative pathway in undifferentiated HT-29 cells. For this purpose, we have modulated the function and expression of the heterotrimeric G-proteins (G_s and G_i) in these cells. After pertussis toxin treatment which ADP-ribosylates heterotrimeric G_i-proteins, we observed an inhibition of autophagic sequestration and the complete restoration of the passage of N-linked glycoproteins through the Golgi complex. In contrast, autophagic sequestration was not reduced by cholera toxin, which acts on heterotrimeric G_s-proteins. Further insights on the nature of the pertussis toxin-sensitive α subunit controlling autophagic sequestration were obtained by cDNA transfections of α_i subunits. Overexpression of the α subunit increased autophagic sequestration and degradation in undifferentiated cells, whereas overexpression of the α subunit, the only other pertussis toxin-sensitive α subunit expressed in HT-29 cells, did not alter the rate of autophagy.