Regulation of mitochondrial hexokinase in cultured HT 29 human cancer cells. An ultrastructural and biochemical study

Abstract

The involvement of the mitochondrial bound hexokinase in aerobic glycolysis was investigated in two subpopulations of the HT 29 human colon cancer cell line: a poorly differentiated one with high aerobic lactate production (referred as undifferentiated or standard cells), and an enterocyte-like differentiated one with lower lactate production (referred as differentiated or Glc − cells). After mild digitonin treatment, 85% of the total cellular hexokinase activity remained in the particulate fraction in both cell types. In both cases mitochondria appeared to be tightly coupled but the Glc − cells exhibited a significantly higher oxidation rate in the presence of glucose. Electron microscopy of freeze-fractured cells revealed the absence of contacts between the two limiting mitochondrial membranes in the highly glycolytic standard cells, whereas the contacts were present in the Glc − cells. Furthermore, we investigated the functional relationship between bound hexokinase (as hexokinase-porin complex) and the inner compartment of mitochondria isolated from standard and Glc − HT 29 cells. In contrast to the differentiated cells the hexokinase in undifferentiated standard cells was not functionally coupled to the oxidative phosphorylation. This suggests that the high rate of lactate formation in neoplastic cells is not caused by an increase of particulate hexokinase activity but rather by a disregulation of the hexokinase-porin complex caused by the absence of contact sites between the two mitochondrial membranes. In agreement with this interpretation, the hexokinase-porin complex could be completely removed by digitonin treatment in standard HT 29 cells, while this was not possible in mitochondria from Glc − cells.