Reply to: “Direct actions of GLP-1 analogues on AMP-activated protein kinase activity are distinct from cyclic AMP accumulation”

Glucagon-like peptide-1 reduces hepatic lipogenesis via activation of AMP-activated protein kinaseJournal of HepatologyVol. 54Issue 6PreviewGlucagon-like peptide-1 (GLP-1), a gut-derived peptide degraded by dipeptidyl peptidase-4 (DPP4), stimulates insulin secretion in response to nutrients, yet its direct effect on the liver is controversial. We investigated the effects of GLP-1 on hepatic fat and glucose metabolism and elucidated its mechanism of action. Full-Text PDF Direct actions of GLP-1 analogues on AMP-activated protein kinase activity are distinct from cyclic AMP accumulationJournal of HepatologyVol. 58Issue 3PreviewRegarding the publication by Ben-Shlomo et al. [1], the authors present data supporting the role of the incretin hormone glucagon-like peptide 1 (GLP-1) for decreased hepatic steatosis. They use a rat model with dipeptidyl peptidase 4 (DPP4) deficiency which decreases GLP-1 degradation and raises circulating endogenous GLP-1 levels by more than 3-fold. They observe a reduction in liver triglyceride in concert with decreased hepatic transcript levels of lipogenic genes. These findings are consistent with data from other laboratories using GLP-1 and related analogues, as well as DPP4 inhibition, in rodent models of hepatic steatosis [2–5]. Full-Text PDF Open Access We thank Dr Samson for her valuable comments. First of all, we would like to emphasize that, in our study [[1]Ben-Shlomo S. Zvibel I. Shnell M. Shlomai A. Chepurko E. Halpern Z. et al.Glucagon-like peptide-1 reduces hepatic lipogenesis via activation of AMP-activated protein kinase.J Hepatol. 2011; 54: 1214-1223Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar], we have mechanistically focused on the association between GLP-1 and phosphorylated AMP-activated kinase (pAMPK). Indeed, by using a dominant negative AMPK mutant adenovirus, we have proved that AMPK directly mediates GLP-1 action on lipogenesis in hepatocytes [[1]Ben-Shlomo S. Zvibel I. Shnell M. Shlomai A. Chepurko E. Halpern Z. et al.Glucagon-like peptide-1 reduces hepatic lipogenesis via activation of AMP-activated protein kinase.J Hepatol. 2011; 54: 1214-1223Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar]. We have by no means intended to decipher the connection between the cAMP and AMPK pathways, and in this respect, we have relied upon recent published peer review literature. We do agree with the authors’ claim that we should not have used phrases such as “cAMP/AMPK” pathways, without discussing this possible pathway in the Discussion section of our article. In most cells, intracellular cAMP signaling is mediated by two groups of effectors that bind to cAMP: protein kinase A and exchange proteins directly activated by cAMP (Epac) [[2]de Rooij J. Zwartkruis F.J. Verheijen M.H. Cool R.H. Nijman S.M. Wittinghofer A. et al.Epac is a Rap1 guanine-nucleotide-exchange factor directly activated by cyclic AMP.Nature. 1998; 396: 474-477Crossref PubMed Scopus (1634) Google Scholar]. Accumulating data point to a pathway linking cAMP and AMPK activation via Epac and, therefore, we disagree with the authors’ statement that “There is no evidence that cAMP has any direct role in the regulation of AMPK activity”. The first evidence linking cAMP and AMPK has emerged from studies in adipocytes [3Yin W. Mu J. Birnbaum M.J. Role of AMP-activated protein kinase in cyclic AMP-dependent lipolysis in 3T3-L1 adipocytes.J Biol Chem. 2003; 278: 43074-43080Crossref PubMed Scopus (241) Google Scholar, 4Omar B. Zmuda-Trzebiatowska E. Manganiello V. Göransson O. Degerman E. Regulation of AMP-activated protein kinase by cAMP in adipocytes: roles for phosphodiesterases, protein kinase B, protein kinase A, Epac and lipolysis.Cell Signal. 2009; 21: 760-766Crossref PubMed Scopus (109) Google Scholar]. Yin et al. have demonstrated in 3T3- L1 adipocytes that cAMP induction by forskolin promotes AMPK phosphorylation through activation of Thr-172 residues in a dose and time dependent manner [[3]Yin W. Mu J. Birnbaum M.J. Role of AMP-activated protein kinase in cyclic AMP-dependent lipolysis in 3T3-L1 adipocytes.J Biol Chem. 2003; 278: 43074-43080Crossref PubMed Scopus (241) Google Scholar]. Later, Fu et al. have described in hepatocytes a unique pathway in which taurocholate promotes bile canaliculi network formation [[5]Fu D. Wakabayashi Y. Lippincott-Schwartz J. Arias I.M. Bile acid stimulates hepatocyte polarization through a cAMP-Epac-MEK-LKB1-AMPK pathway.Proc Natl Acad Sci U S A. 2011; 108: 1403-1408Crossref PubMed Scopus (114) Google Scholar]. In this study, combining molecular and pharmacologic approaches, the authors have delineated the pathway in a step-wise manner and have found that taurocholate increases cAMP, leading to activation of Epac and its downstream molecules Rap1 and MEK. This signaling pathway has been directly related to liver kinase B1 (LKB1) and AMPK activation. Importantly, pharmacologic PKA inhibition does not block the described cascade [[5]Fu D. Wakabayashi Y. Lippincott-Schwartz J. Arias I.M. Bile acid stimulates hepatocyte polarization through a cAMP-Epac-MEK-LKB1-AMPK pathway.Proc Natl Acad Sci U S A. 2011; 108: 1403-1408Crossref PubMed Scopus (114) Google Scholar]. Another recent study investigating the anti-diabetogenic properties of resveratrol has found that this molecule acts as a phosphodiesterase inhibitor, enhancing cAMP levels and activating AMPK in an Epac1 dependent manner [[6]Park S.J. Ahmad F. Philp A. Baar K. Williams T. Luo H. et al.Resveratrol ameliorates aging-related metabolic phenotypes by inhibiting cAMP phosphodiesterases.Cell. 2012; 148: 421-433Abstract Full Text Full Text PDF PubMed Scopus (1059) Google Scholar]. Again, this pathway is not dependent on PKA activation. Taken together, the cited literature does show that cAMP can lead to activation of AMPK. The authors claim that cAMP and AMPK have different roles in hepatocytes; however, both molecules indicate a low energy state in the cell. Indeed, while our article describes an anti-lipogenic role for GLP-1, a known effect of both signals, we have found no effect of GLP-1 with regard to gluconeogenesis, a process regulated in an opposite manner by the two signals. With regard to the presence of pancreatic GLP-1R on hepatocytes, it is indeed a controversial issue. However, several groups have demonstrated the presence of this receptor on hepatocytes, including human hepatocytes, and have also shown that GLP-1 increases cAMP in hepatocytes [7Gupta N.A. Mells J. Dunham R.M. Grakoui A. Handy J. Saxena N.K. et al.Glucagon-like peptide-1 receptor is present on human hepatocytes and has a direct role in decreasing hepatic steatosis in vitro by modulating elements of the insulin signaling pathway.Hepatology. 2010; 51: 1584-1592Crossref PubMed Scopus (358) Google Scholar, 8Ding X. Saxena N.K. Lin S. Gupta N.A. Anania F.A. Exendin-4, a glucagon-like protein-1 (GLP-1) receptor agonist, reverses hepatic steatosis in ob/ob mice.Hepatology. 2006; 43: 173-181Crossref PubMed Scopus (443) Google Scholar, 9Svegliati-Baroni G. Saccomanno S. Rychlicki C. Agostinelli L. De Minicis S. Candelaresi C. et al.Glucagon-like peptide-1 receptor activation stimulates hepatic lipid oxidation and restores hepatic signalling alteration induced by a high-fat diet in nonalcoholic steatohepatitis.Liver Int. 2011; 31: 1285-1297Crossref PubMed Scopus (319) Google Scholar]. One of the articles showing expression of GLP-1R on hepatocytes is cited by the authors themselves [[9]Svegliati-Baroni G. Saccomanno S. Rychlicki C. Agostinelli L. De Minicis S. Candelaresi C. et al.Glucagon-like peptide-1 receptor activation stimulates hepatic lipid oxidation and restores hepatic signalling alteration induced by a high-fat diet in nonalcoholic steatohepatitis.Liver Int. 2011; 31: 1285-1297Crossref PubMed Scopus (319) Google Scholar]. The authors declared that they do not have anything to disclose regarding conflict of interest with respect to this manuscript.