Impact of culture conditions, culture media volumes, and glucose content on metabolic properties of renal epithelial cell cultures. Are renal cells in tissue culture hypoxic?

Abstract

When renal proximal tubular cells are brought into tissue culture, they revert from oxidative metabolism and gluconeogenesis to high rates of glycolysis. Among the factors possibly responsible for this metabolic conversion, limited oxygen availability and/or substrate supply are discussed. In order to study the role of these factors on long-term cultures, the impact of growth conditions, culture media volume, and glucose content on carbohydrate metabolism of the continuous renal cell lines LLC-PK(1) (porcine kidney) and OK (opossum kidney) was investigated. The impact of culture media volumes and glucose content, respectively, was determined by overlaying confluent monolayer cultures of LLC-PK(1) and OK cells (i) with increasing volumes of culture medium and thus increasing amounts of glucose, and (ii) with increasing culture medium volumes at constant absolute amounts of glucose by adding glucose-free medium, in order to increase volume at a constant glucose supply. Alternatively, and in order to improve cell oxygenation, LLC-PK(1) cells were also cultured in roller bottles. Cell carbohydrate metabolism was assessed by measuring rates of glucose consumption and lactate production, respectively, and by determination of specific activities of the key glycolytic enzymes hexokinase (HK), phosphofructokinase (PFK), pyruvate kinase (PK), and lactate dehydrogenase (LDH). Mitochondrial phosphate-dependent glutaminase (PDG) was assayed as marker enzyme of oxidative metabolism of glutamine. In LLC-PK(1) and OK cells, rates of glucose consumption were independent of the initial glucose concentrations and/or the culture media volumes used. Glucose was quantitatively converted to lactate, which accumulated in a 1:2 molar ratio. Lactate in culture media reached a maximum content after 24 h, and was reutilized by the cell lines thereafter. Interestingly, the rates of lactate reuptake strictly depended on culture medium volume, indicating a volume-induced stimulation of oxidative lactate metabolism. Marked changes were found for the specific activities of glycolytic enzymes. In LLC-PK(1) cells, increased glucose supply caused increases in HK, PFK, PK and LDH activities, which were superimposed to the stimulatory effects of increased media volumes. Enzyme activity showed a biphasic response, indicating that both glucose supply and culture media volumes covering the cell monolayer are factors determining glycolytic rates of LLC-PK(1) renal cells. Conversely, in OK cells glycolytic enzyme activities decreased with increasing culture media volumes at constant glucose levels. As expected, under conditions of enhanced oxygenation of LLC-PK(1) cells in roller bottle culture, glycolytic enzyme activities decreased, whereas PDG activity increased, which was paralleled by increased rates of ammonia generation. Thus, changes in nutrient supply and oxygenation of renal epithelial cell cultures by altered culture media volumes dramatically influence metabolic rates and levels of enzyme activities, respectively.