Changes in growth, pO₂ and pH after exposure to oxamate: As multicellular spheroids

The basic metabolism and PO₂ and pH gradients in spheroids were characterized and, in some cases, changed by the addition of the pyruvate analogue oxamate. Two human tumour spheroid types, colon adenocarcinoma HT29 and malignant glioma U118MG, were applied as models. Microelectrode measurements in HT29 spheroids showed steep PO₂ gradients with large differences between surface and center, ΔPO₂, and low central P02 values. The HT29 spheroids had rather flat pH gradients. The U118MG spheroids had less steep PO₂ gradients but steeper pH gradients. Determinations of 1-¹⁴C and 2-₁₄C pyruvate oxidation rates, for characterization of the oxidative glucose breakdown as well as of lactate dehydrogenase kinetics, showed consistent results with the microelectrode measurements in that there was high oxidative metabolism in the HT29 spheroids whereas the U118MG spheroids relied more on glycolysis. Western blot investigations of the LDH isoenzyme composition showed different isoenzyme patterns in the two spheroid types with a lack of LDH1 in U118MG spheroids. Addition of 40 mM of oxamate gave decreased 1-¹⁴C- and 2-¹⁴C-pyruvate oxidation rates in the HT29 cells and inhibition of LDH activity in the U118MG cells. Oxamate increased the central PO₂ values in HT29 spheroids and the central pH values in U118MG spheroids. One example of experimental therapy was applied and oxamate acted as a radiation sensitizer in the U118MG system and as a radioprotective substance in the HT29 system. This has to be analysed in more detail.