Counteracting Roles of AMP Deaminase and AMP Kinase in the Development of Fatty Liver

Abstract

Fatty liver (hepatic steatosis) is associated with nucleotide turnover, loss of ATP and generation of adenosine monophosphate (AMP). It is well known that in fatty liver, activity of the AMP-activated kinase (AMPK) is reduced and that its stimulation can prevent hepatic steatosis by both enhancing fat oxidation and reducing lipogenesis. Here we show that another AMP dependent enzyme, AMPD2, has opposing effects on fatty acid oxidation when compared to AMPK. In human hepatocytres, AMPD2 activation –either by overexpression or by lowering intracellular phosphate levels with fructose- is associated with a significant reduction in AMPK activity. Likewise, silencing of AMPK spontaneously increases AMPD activity, demonstrating that these enzymes counter-regulate each other. Furthermore, we show that a downstream product of AMP metabolism through AMPD2, uric acid, can inhibit AMPK activity in human hepatocytes. Finally, we show that fructose-induced fat accumulation in hepatocytes is due to a dominant stimulation of AMPD2 despite stimulating AMPK. In this regard, AMPD2-deficient hepatocytes demonstrate a further activation of AMPK after fructose exposure in association with increased fatty acid oxidation, and conversely silencing AMPK enhances AMPD-dependent fat accumulation. In vivo, we show that sucrose fed rats also develop fatty liver that is blocked by metformin in association with both a reduction in AMPD activity and an increase in AMPK activity. In summary, AMPD and AMPK are both important in hepatic fat accumulation and counter-regulate each other. We present the novel finding that uric acid inhibits AMPK kinase activity in fructose-fed hepatocytes thus providing new insights into the pathogenesis of fatty liver.

Evolutionary history and metabolic insights of ancient mammalian uricases

“…S4) showed a tendency to have greater levels of phosphorylated AMP kinase, and its target protein ACC, in response to fructose compared with control cells. These results are consistent with earlier work showing that uric acid is an inhibitor of AMP kinase (26). These results also demonstrate that triglyceride accumulation is correlated with the enzymatic efficiency of uricases and that, whereas ancient uricases are capable of managing uric acid levels in our assay conditions, more derived versions such as recombinant human uricase are not able to manage uric acid.…”

“…Transduction and generation of stable clones expressing uricases was performed as previously described (26). The coding sequences of the various uricases were inserted into the pLVXIRES Puro vector by direct cloning using EcoRI and NotI sites.…”

New Insights on the Risk for Cardiovascular Disease in African Americans

“…108-112 Stimulation of this pathway results in inhibition of AMP kinase while at the same time inducing mitochondrial oxidative stress and intracellular uric acid generation, resulting in both fat accumulation (from stimulation of fat synthesis and blockade of fatty acid oxidation) and increased gluconeogenesis. [109][110][111][112] The ability of fructose to cause obesity is not simply from its caloric value but rather from its capacity to induce leptin resistance (thereby blocking satiety responses) while at the same time reducing ATP production by blocking fat oxidation. 109,[113][114][115] Laboratory animals fed fructose develop features of metabolic syndrome and, if prone, will even develop diabetes.…”

“…[109][110][111][112] The ability of fructose to cause obesity is not simply from its caloric value but rather from its capacity to induce leptin resistance (thereby blocking satiety responses) while at the same time reducing ATP production by blocking fat oxidation. 109,[113][114][115] Laboratory animals fed fructose develop features of metabolic syndrome and, if prone, will even develop diabetes. 116,117 Features of metabolic syndrome can be induced even with caloric restriction provided the sugar (or fructose) content is high, 116 demonstrating that it is not overnutrition but rather the presence of fructose that is responsible for the development of metabolic syndrome.…”

“…Indeed, recent studies suggest that humans may be more sensitive to the effects of fructose than most mammals because uric acid mediates some of the metabolic features. [109][110][111] Humans have higher uric acid levels than most mammals as a result of a mutation in uricase, 125 an enzyme that degrades uric acid. When uricase is inhibited in rats, even small concentrations of fructose-containing sugars can induce features of metabolic syndrome.…”