What is the Cellular Cascade from Glucose to the Transcription Machinery
In order to explain the glucose responsiveness of tissues, the following hypotheses have been considered: (i) the presence of glucose might modify the nuclear amount of transcription factors (including modifications of cellular localization); and (ii) transcription factors undergo post-translational modifications in the presence of glucose, leading to a modification of interactions with the basic transcription machinery or with co-activator proteins. Post-translational modifications of a transcription factor might involve allosteric modification of the protein by binding of the glucose-related signal metabolite, modifications through phosphorylation/dephosphorylation mechanisms by modulating the activity of a nuclear protein kinase/phosphatase, or both. In this context, it has been shown that protein phosphatase inhibitors prevent the effect of glucose on FAS, ACC and S14 gene expression (Sudo and Mariash, 1994; Daniel et al., 1996; Foretz et al., 1998), suggesting that the glucose stimulatory effect could involve a dephosphorylation mechanism.
The AMP-activated protein kinase (AMPK) provides a potential candidate for a protein kinase involved in the regulation of glucose-activated genes. AMPK is a serine/threonine kinase acting as a metabolic 'master switch' by phosphorylating key enzymes involved in cholesterol and fatty acid metabolism. Indeed, AMPK phosphorylates and inactivates ACC and 3-hydroxy-3-methylglutaryl-CoA reductase, resulting in the inhibition of both lipogenesis and cholesterol synthesis (Carling et al., 1987). AMPK is activated by stresses that deplete ATP and increase AMP within the cell, such as hypoxia or muscle contraction. AMP activates AMPK by two mechanisms: (i) allosteric activation of AMPK; and (ii) stimulation of an AMPK kinase, leading to phosphorylation of AMPK (Hardie et al., 1998).
A significant clue regarding a possible role for AMPK in the regulation of gene transcription came from the finding that it is structurally and functionally related to the yeast protein kinase complex Snf1 (sucrose non-fermenting) (Woods et al., 1994). In yeast, the transcription of a number of genes is repressed by high concentrations of glucose (Gancedo, 1998). The kinase activity of Snf1 is essential for the derepression ofthese genes in yeast grown under conditions of glucose limitation. AMPK and Snf1 both form heterotrimeric complexes consisting of one catalytic subunit and two regulatory subunits (Hardie and Carling, 1997; Hardie et al., 1998). The amino acid sequences of the mammalian AMPK subunits are highly homologous to their counterparts in the Snf1 complex, and the two kinases show functional similarities (Woods et al., 1994).
These findings led several groups to speculate that AMPK may be involved in the regulation of gene transcription by glucose in mammals. Evidence that this may be the case came from studies in which AMPK in hepatocytes was activated by incubation with 5-amino-imidazolecarboxamide (AICA) riboside, a cell-permeable activator of AMPK, leading to inhibition of the glucose-induced expression of FAS, L-PK and S14 (Foretz etal., 1998;Leclerc etal., 1998). A similar inhibition of FAS, L-PK and S14 gene expression was obtained when AMPK activity was increased in cultured hepatocytes by the overexpression of a constitutively active form of AMPK (Woods et al., 2000). These results imply that AMPK is involved in the repression of genes induced by glucose in hepatocytes.
Since an increase in AMPK activity inhibits glucose-activated gene transcription, a decrease in AMPK activity could be part of a mechanism involved in the stimulation ofgene transcription by glucose. However, inhibition of AMPK activity using a dominant-negative form of the kinase has no detectable effect on any of the glucose-induced genes (Woods et al., 2000). In addition, changing the glucose concentration in the medium of cultured hepatocytes from 5 to 25 mM has no inhibitory effects on AMPK activity (Foretz et al., 1998). Taken together, these results suggest that in liver, glucose does not exert its effects on gene expression by directly inhibiting AMPK.
In contrast, the activity of AMPK increases in b-cell lines in response to a decrease in the extracellular glucose concentration from 30 to 3 mM (Salt et al, 1998; da Silva Xavier et al, 2000). This is paralleled by an increased promoter activity of a transfected L-PK promoter. In MIN6 cells cultured at 3 mM glucose, inhibiting the AMPK activity in both cytoplasm and nucleus by injection of an a2 AMPK antibody mimics the effect of a high glucose concentration on the L-PK promoter activity (da Silva Xavier etal., 2000). Thus, it is feasible that in cells that are particularly sensitive to a low concentration of glucose (such as b-cells), inhibition of AMPK activity during periods of increasing glucose concentrations is part of the mechanism leading to increased gene transcription.
In conclusion, AMPK inhibits the expression of genes encoding lipogenic and glycolytic enzymes in the liver. However, the inhibitory effect of AMPK does not affect induction ofgene expression by glucose. Since other genes that are not controlled by glucose are also inhibited by AMPK (Hubert et al., 2000; Lochhead et al., 2000; Zhou et al, 2000; Zheng et al, 2001; MacLean et al., 2002), this suggests that AMPK could have a general inhibitory effect on genes encoding enzymes involved in ATP-consuming pathways.
Post a comment