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Mammalian pyruvate kinase (PK), a key glycolytic enzyme, has two genes named PKL and PKM, which produce the L- and R-type isoenzymes by means of alternative promoters, and the M1-and M2-types by mutually exclusive alternative splicing respectively. The expression of these genes is tissue-specific and under developmental, dietary and hormonal control. The L-type isoenzyme (L-PK) gene contains multiple regulatory elements necessary for regulation in the 5' flanking region, up to position -170. Both L-II and L-III elements are required for stimulation of L-PK gene transcription by carbohydrates such as glucose and fructose, although the L-III element is itself responsive to carbohydrates. The L-II element is also responsible for the gene regulation by polyunsaturated fatty acids. Nuclear factor-1 proteins and hepatocyte nuclear factor 4, which bind to the L-II element, may also be involved in carbohydrate and polyunsaturated fatty acid regulation of the L-PK gene respectively. However, the L-III-element-binding protein that is involved in carbohydrate regulation remains to be clarified, although involvement by an upstream stimulating factor has been proposed. Available evidence suggests that the carbohydrate signalling pathway to the L-PK gene includes a glucose metabolite, possibly glucose 6-phosphate or xylulose 5-phosphate, as well as phosphorylation and dephosphorylation mechanisms. In addition, at least five regulatory elements have been identified in the 5' flanking region of the PKM gene up to position -279. Sp1-family proteins bind to two proximal elements, but the binding of proteins to other elements have not yet been clarified. Glucose may stimulate the transcription of the PKM gene via hexosamine derivatives. Sp1 may be involved in this regulation via its dephosphorylation, although the carbohydrate response element has not been determined precisely in the PKM gene. Thus glucose stimulates transcription of the PKM gene by the mechanism which is probably different from the L-PK gene.

No abstract

Transcription of the rat fatty acid synthase (FAS) gene in the rat liver can be regulated by feeding a high carbohydrate diet. A carbohydrate response element (ChoRE) located on the rat FAS gene promoter has been identified. Using multiple copies of the ChoRE as the bait in a yeast one-hybrid system, a rat liver cDNA library was screened, and the cDNA of ChoRE-binding proteins was cloned. A positive clone that encodes a basic helix-loop-helix protein, enhancer of split-and hairy-related protein-2 (SHARP-2), was obtained. Northern blot analysis revealed that the levels of SHARP-2 mRNA increase when a high carbohydrate diet is fed to normal rats or when insulin is administered to diabetic rats. In primary cultured rat hepatocytes, insulin rapidly induced an accumulation of SHARP-2 mRNA even in the absence of glucose. A time course for the increase in SHARP-2 mRNA levels indicated that it followed by those of FAS and L-type pyruvate kinase mRNAs and that the initial time course of SHARP-2 mRNA was similar to changes in the levels of glucokinase mRNA and phosphoenolpyruvate carboxykinase mRNA. Although wortmannin, LY294002, and actinomycin D blocked the increase in SHARP-2 mRNA levels by insulin, rapamycin, staurosporine, PD98059, okadaic acid, and 8-bromocyclic AMP had no effect. In addition, nuclear run-on assay revealed that transcription of the rat SHARP-2 gene was induced by insulin. Thus, we conclude that insulin induces the transcription of the rat SHARP-2 gene via a phosphoinositide 3-kinase pathway.

The effects of nutrients and hormones on transcriptional and post‐transcriptional regulation of fatty acid synthase in rat liver were investigated following cDNA cloning. When fasted rats were fed a carbohydrate/protein diet, the transcriptional rate was greatly increased even in 1 h. The transcriptional rate, mRNA concentration and enzyme induction reached maximum levels in 4 h, 8–16 h and 48 h, respectively. Although dietary carbohydrate increased each level more than protein did, both carbohydrate and protein were required to reach a high level. Corn oil feeding markedly decreased the transcriptional rate. In diabetic rats, the transcriptional rate, mRNA concentration and enzyme induction were very low in comparison with the normal. By treating the diabetic rats with insulin, however, the transcriptional rate was increased 5‐fold in 1 h and 15‐fold in 6 h, preceding a great increase in the mRNA and enzyme levels. On the other hand, fructose feeding or triiodothyronine treatment of diabetic rats abundantly increased the mRNA concentration and somewhat increased the transcriptional rate. Thus, it is suggested that insulin mainly stimulates the transcription of the fatty acid synthase gene, whereas triiodothyronine and fructose mainly increase the mRNA stability.