Proliferative activity and tumorigenic conversion: impact on cellular metabolism in 3-D culture

Oxygen consumption, glucose, lactate, andATP concentrations, as well as glucose and lactate turnover rates, havebeen studied in a three-dimensional carcinogenesis model of differentlytransformed rat embryo fibroblasts (spontaneously immortalized Rat1 andmyc-transfected M1, and the ras-transfected,tumorigenic descendants Rat1-T1 and MR1) to determine metabolicalterations that accompany tumorigenic conversion. Variousbioluminescence techniques, thymidine labeling, measurement ofPO₂ distributions withmicroelectrodes, and determination of cellular oxygen uptake rates(c_O2)have been applied. In the ras-transfected, tumorigenic spheroidtypes, the size dependencies of some of the measured parametersexhibited sharp breaks at diameters of ~830 µm for Rat1-T1 and~970 µm for MR1 spheroids, respectively, suggesting that somefundamental change in cell metabolism occurred at these characteristicdiameters (denoted as "metabolic switch").c_O2decreased and lactate concentration increased as functions of sizebelow the characteristic diameters. Concomitantly, glucose and lactateturnover rates decreased in MR1 spheroids and increased inRat1-T1. Spheroids larger than the characteristic diameters (exhibitingcell quiescence and lactate accumulation) showed an enhancement ofc_O2with size. Systematic variations in the ATP and glucose levels in theviable cell rim were observed for Rat1-T1 spheroids only. Proliferativeactivity, c_O2,and ATP levels in small, nontumorigenic Rat1 and M1aggregates did not differ systematically from those recorded in thelargest spheroids of the corresponding ras transfectants.Unexpectedly, respiratory activity was present not only in viable butalso in the morphologically disintegrated core regions of M1aggregates. Our data suggest that myc but not rastransfection exerts major impacts on cell metabolism. Moreover, somekind of switch has been detected that triggers profound readjustment oftumor cell metabolism when proliferative activity begins tostagnate, and that is likely to initiate some other, yetunidentified energy-consuming process.