Cytoimmunochemical study of pyruvate kinase isoenzymes in rat liver

Hormonal regulation of L-type pyruvate kinase in rat liver cells in culture

An immortalized rat liver cell line (RLC) expresses two isozymes of pyruvate kinase, the adult liver or L-type isozyme and an M-type isozyme presumed to be the M2-type. In RLC cells incubated in serum-free medium, the addition of 0.1 microM insulin maintained the initial level of L-type pyruvate kinase when it was high and induced the L-type isozyme when it was low. The addition of 1.0 mM dibutyryl cAMP and 0.5 mM theophylline decreased the L-type isozyme, even in the presence of insulin. The amount of M2-type isozyme was relatively constant under the conditions used. Regulation of the amount of L-type pyruvate kinase by both insulin and cAMP occurred primarily through changes in the rate of L-pyruvate kinase protein synthesis and translatable mRNA levels. These results are consistent with the in vivo observations that both insulin and glucagon regulate the rate of L-pyruvate kinase gene transcription and that cAMP is the dominant regulator of L-pyruvate kinase gene expression.

Insulin dependence of M2 pyruvate kinase in primary culture of human liver

In the basal state, the presence of L pyruvate kinase (LPK) was constantly observed in primary human liver cell cultures initiated from explants, when cells were examined by immunofluorescence and double labelling. After short-term insulin incubation, M pyruvate kinase (MPK) appeared. Therefore, both LPK and MPK were located in the same cells. We previously obtained the same results in isolated rat hepatocytes in which we demonstrated that short-term regulation of M2PK by insulin was a function of dose and/or incubation time. The present work established that similar conditions govern the regulation of this isozyme in vitro in human hepatocytes.