AMPK β1 Deletion Reduces Appetite, Preventing Obesity and Hepatic Insulin Resistance

Abstract

The AMP-activated protein kinase (AMPK) is an ␣␤␥ heterotrimer that regulates appetite and fuel metabolism. We have generated AMPK ␤1 ؊/؊ mice on a C57Bl/6 background that are viable, fertile, survived greater than 2 years, and display no visible brain developmental defects. These mice have a 90% reduction in hepatic AMPK activity due to loss of the catalytic ␣ subunits, with modest reductions of activity detected in the hypothalamus and white adipose tissue and no change in skeletal muscle or heart. On a low fat or an obesity-inducing high fat diet, ␤1 ؊/؊ mice had reduced food intake, reduced adiposity, and reduced total body mass. Metabolic rate, physical activity, adipose tissue lipolysis, and lipogenesis were similar to wild type littermates. The reduced appetite and body mass of ␤1 ؊/؊ mice were associated with protection from high fat diet-induced hyperinsulinemia, hepatic steatosis, and insulin resistance. We demonstrate that the loss of ␤1 reduces food intake and protects against the deleterious effects of an obesity-inducing diet.The AMP-activated protein kinase (AMPK) 6 is an evolutionarily conserved serine/threonine protein kinase that functions as a metabolic regulatory enzyme at both the cellular and whole body level (1). AMPK is activated in response to physiological processes that raise intracellular levels of AMP, such as exercise and hypoxia. It restores cellular energy balance by switching off ATP-consuming anabolic pathways and switching on ATPgenerating catabolic pathways by direct phosphorylation of downstream targets. Modulation of AMPK activity by hormones adds an additional layer of control, allowing cellular energy supply and demand to be balanced with the energy requirements of the whole organism (2).AMPK functions as an ␣␤␥ heterotrimer. Different isoforms for each of the subunits exist (␣1, ␣2, ␤1, ␤2, ␥1, ␥2, and ␥3) as well as some splice variants, allowing more than 12 heterotrimeric combinations to be generated that may mediate unique tissue-specific functions (3, 4). The 63-kDa AMPK ␣ subunits, designated ␣1 and ␣2, contain a serine/threonine protein kinase catalytic domain that is activated by phosphorylation of Thr-172 in the activation loop (5, 6). We, and others have shown that the C terminus of the ␤ subunits are essential for AMPK heterotrimer assembly by anchoring the ␣ and ␥ subunits (7,8). The ␤1 and ␤2 subunits show 82% identity from residue 73 to 270, but only 43% identity for the N-terminal residues 1-72 (9). The ␤1 subunit is N-terminally myristoylated and is phosphorylated on multiple serines (10); however, the physiological importance of these phosphorylation sites is poorly understood. Northern blot analysis of human tissues revealed that AMPK ␤1 expression is highest in the liver and brain and low in kidney and skeletal muscle, whereas ␤2 is most highly expressed in skeletal muscle with lower expression in kidney, liver, and lung (11).Hepatic AMPK is thought to play important roles in regulating lipid metabolism, glucose homeostasis, and insulin sensitivity (1)....

AMPK β1 reduces tumor progression and improves survival in p53 null mice

“…Genetic deletion of p53 results in the development of a T‐cell lymphoma at approximately 6 months of age (Jacks et al ., 1994). In contrast, AMPK β1 null mice have a normal lifespan with no signs of elevated tumorigenesis (Dzamko et al ., 2010). The AMPK β1−/− mice were born at the expected Mendelian ratio (Fig.…”

“…Proliferation data from the ACC DKI MEFs or cells treated with lipogenesis inhibitors corroborated these results, suggesting that AMPK control of lipogenesis, through ACC inhibition, is important for limiting cellular proliferation. These data are in agreement with previous studies indicating that hepatocytes (Dzamko et al ., 2010) and macrophages (Fullerton et al ., 2015) from AMPK β1‐deficient mice have increased lipid synthesis and that activation of AMPK using A769662 or metformin lowers lipogenesis through an AMPK β1‐dependent pathway requiring the phosphorylation of ACC (Fullerton et al ., 2013). These data are also consistent with recent studies indicating that inhibiting lipogenesis, by mimicking AMPK phosphorylation of ACC using small molecules (Svensson et al ., 2016) or through chemical inhibition of fatty acid synthase (Menendez and Lupu, 2007; Orita et al ., 2008), is effective for limiting cellular proliferation.…”

“…p53+/− mice on a C57BL/6 background were purchased from The Jackson Laboratory (B6.129S2‐Trp53 tm1Tyj/J , stock 002101; Bar Harbor, ME, USA). AMPK β1 null mice (Dzamko et al ., 2010) were generated on a C57Bl6 background as described and p53 null mice had been backcrossed to a C57Bl6 background for at least 10 generations. Heterozygous mice (p53+/−) were interbred to generate p53−/− and p53+/+ littermates.…”

“…Heterozygous mice (p53+/−) were interbred to generate p53−/− and p53+/+ littermates. The p53−/− mice were then crossed with the AMPK β1−/− mice (Dzamko et al ., 2010) to generate the p53+/− AMPK β1+/− mice. These mice were then interbred to generate the p53−/− AMPK β1−/− mice on a C57Bl6 background.…”

“…The AMPK β1 isoform plays a vital role in regulating AMPK heterotrimer formation and enzyme activity in mouse liver and hematological cells such as macrophages and T cells where it increases fatty acid oxidation, suppresses fatty acid synthesis, and inhibits inflammatory pathways (Blagih et al ., 2015; Dzamko et al ., 2010; Fullerton et al ., 2015; Galic et al ., 2011). It is also the primary subunit which is targeted by small‐molecule direct AMPK activators such as A769662 (Cool et al ., 2006; Sanders et al ., 2007; Scott et al ., 2014), MT‐63‐78 (Zadra et al ., 2014), and salicylate (Hawley et al ., 2012), all of which have been shown to reduce cancer cell proliferation in some (Huang et al ., 2008; O'Brien et al ., 2015; Zadra et al ., 2014), but not all studies (Vincent et al ., 2015).…”