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Zinc-α2-Glycoprotein Modulates AKT-Dependent Insulin Signaling in Human Adipocytes by Activation of the PP2A Phosphatase.

Ceperuelo-Mallafré V, Ejarque M, Duran X, Pachón G, Vázquez-Carballo A, Roche K, Núñez-Roa C, Garrido-Sánchez L, Tinahones FJ, Vendrell J, Fernández-Veledo S - PLoS ONE (2015)

Bottom Line: ZAG treatment correlated with an increase in PP2A activity.Silencing of the PP2A catalytic subunit abrogated the negative effect of ZAG on insulin-stimulated AKT phosphorylation and glucose uptake but not on GLUT4 expression and basal glucose uptake.ZAG circulating levels were unchanged in a lean patient cohort stratified for HOMA-IR.

View Article: PubMed Central - PubMed

Affiliation: Hospital Universitari de Tarragona Joan XXIII, Institut d´Investigació Sanitària Pere Virgili, Universitat Rovira i Virgili, Tarragona, Spain; Centro de Investigación Biomédica en Red de Diabetes y Enfermedades Metabólicas Asociadas, Instituto de Salud Carlos III, Madrid, Spain.

ABSTRACT

Objective: Evidence from mouse models suggests that zinc-α2-glycoprotein (ZAG) is a novel anti-obesity adipokine. In humans, however, data are controversial and its physiological role in adipose tissue (AT) remains unknown. Here we explored the molecular mechanisms by which ZAG regulates carbohydrate metabolism in human adipocytes.

Methods: ZAG action on glucose uptake and insulin action was analyzed. β1 and β2-adrenoreceptor (AR) antagonists and siRNA targeting PP2A phosphatase were used to examine the mechanisms by which ZAG modulates insulin sensitivity. Plasma levels of ZAG were measured in a lean patient cohort stratified for HOMA-IR.

Results: ZAG treatment increased basal glucose uptake, correlating with an increase in GLUT expression, but induced insulin resistance in adipocytes. Pretreatment of adipocytes with propranolol and a specific β1-AR antagonist demonstrated that ZAG effects on basal glucose uptake and GLUT4 expression are mediated via β1-AR, whereas inhibition of insulin action is dependent on β2-AR activation. ZAG treatment correlated with an increase in PP2A activity. Silencing of the PP2A catalytic subunit abrogated the negative effect of ZAG on insulin-stimulated AKT phosphorylation and glucose uptake but not on GLUT4 expression and basal glucose uptake. ZAG circulating levels were unchanged in a lean patient cohort stratified for HOMA-IR. Neither glucose nor insulin was associated with plasma ZAG.

Conclusions: ZAG inhibits insulin-induced glucose uptake in human adipocytes by impairing insulin signaling at the level of AKT in a β2-AR- and PP2A-dependent manner.

No MeSH data available.


Related in: MedlinePlus

ZAG increases basal glucose uptake but impairs insulin-induced glucose uptake in human subcutaneous adipocytes by acting as a β1/2-AR agonist.ZAG enhanced GLUT4 gene exression and basal glucose uptake via β1-AR. In addition, ZAG might also activate PPA2 via β2-AR, inhibiting insulin-induced AKT phosphorylation and, in consequence, insulin-induced glucose uptake. Although some prospective cohort studies point to ZAG expression in AT as a useful biomarker to predict insulin sensitivity, our data establish ZAG as a negative modulator of insulin action.
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pone.0129644.g005: ZAG increases basal glucose uptake but impairs insulin-induced glucose uptake in human subcutaneous adipocytes by acting as a β1/2-AR agonist.ZAG enhanced GLUT4 gene exression and basal glucose uptake via β1-AR. In addition, ZAG might also activate PPA2 via β2-AR, inhibiting insulin-induced AKT phosphorylation and, in consequence, insulin-induced glucose uptake. Although some prospective cohort studies point to ZAG expression in AT as a useful biomarker to predict insulin sensitivity, our data establish ZAG as a negative modulator of insulin action.

Mentions: Although controversial, it has been also suggested that ZAG might inversely reflect the status of insulin sensitivity in obesity. This notion is supported by the positive association found between the expression of ZAG in AT and some key components of the insulin signaling pathway, such as IRS1 [14] and GLUT4 [16]. However, results regarding circulating ZAG levels and insulin resistance indices such as HOMA-IR are conflicting [12, 16, 20]. Moreover, not only obesity but also cachexia and HIV-associated lipodystrophy, where ZAG expression in AT has been inversely associated to fat mass, are related with insulin resistant states [26]. Thus, some caution should be exercised when extrapolating information from observational clinical studies in pathological conditions in an attempt to understand the physiological function of a protein. Indeed, the inverse association between circulating ZAG levels and HOMA in obesity, found by some authors [19], was not observed in a cohort of lean patients classified according to HOMA-IR (this work). Our study clearly shows that treatment of human adipocytes with ZAG increases basal glucose uptake but induces an insulin resistant state, similar to other catabolic adipokines such as TNF-alpha [28]. Analogous to other stress conditions related to insulin resistance [21, 22], ZAG produces paradoxical effects in human subcutaneous adipocytes. Accordingly, ZAG increases glucose uptake, correlating with an increase in GLUT1 and GLUT4 expression, but also activates PP2A, inhibiting insulin-induced AKT phosphorylation and in turn, glucose uptake (Fig 5). This detrimental effect is also observed in other insulin sensitive cells such as myocytes and hepatocytes, suggesting a non-tissue-specific effect of ZAG. Although previous studies by Tisdale et al. have reported that the effects of ZAG on body weight and insulin sensitivity in ob/ob mice might be mediated through β3- or possibly β2-ARs [37], a recent report demonstrates that ZAG does not behave as a typical β3/2-AR agonist [36]. Our own results would suggest that ZAG elicits its inhibitory effects on insulin signaling in human fat cells by acting as a β2-AR agonist, whereas basal glucose responses are controlled by ZAG signaling through β1-AR (Fig 5). These findings are in agreement with the important metabolic differences described for rodent and human AT, where β3-AR would represent a minor player in human white AT [30]. It is well known that β-adrenergic agonists can stimulate glucose uptake in several tissues independently of insulin [38, 39]. Insulin and β2-adrenoreceptor pathways seem not to be additive and utilize similar mechanisms to increase glucose uptake, however, some differences have been revealed in specific insulin-signaling intermediates [39]. In agreement with the paradoxical effects observed with ZAG, other GPCR agonists distinct to those activating β-ARs, such as endothelin-1, stimulates glucose uptake but also desensitizes cells to acute insulin treatment [40]. Our study also provides compelling evidence for the participation of the Ser/Thr phosphatase PP2A as an effector molecule in the ZAG signaling cascade. PP2A has been primarily described as a negative regulator of the insulin signaling pathway since it impairs AKT activation [32]. Nevertheless, certain transduction pathways activated by some insulin-sensitizing adipokines are also elicited by PP2A activation [25, 41], suggesting that PP2A specificity depends on its regulatory subunits.


Zinc-α2-Glycoprotein Modulates AKT-Dependent Insulin Signaling in Human Adipocytes by Activation of the PP2A Phosphatase.

Ceperuelo-Mallafré V, Ejarque M, Duran X, Pachón G, Vázquez-Carballo A, Roche K, Núñez-Roa C, Garrido-Sánchez L, Tinahones FJ, Vendrell J, Fernández-Veledo S - PLoS ONE (2015)

ZAG increases basal glucose uptake but impairs insulin-induced glucose uptake in human subcutaneous adipocytes by acting as a β1/2-AR agonist.ZAG enhanced GLUT4 gene exression and basal glucose uptake via β1-AR. In addition, ZAG might also activate PPA2 via β2-AR, inhibiting insulin-induced AKT phosphorylation and, in consequence, insulin-induced glucose uptake. Although some prospective cohort studies point to ZAG expression in AT as a useful biomarker to predict insulin sensitivity, our data establish ZAG as a negative modulator of insulin action.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4465909&req=5

pone.0129644.g005: ZAG increases basal glucose uptake but impairs insulin-induced glucose uptake in human subcutaneous adipocytes by acting as a β1/2-AR agonist.ZAG enhanced GLUT4 gene exression and basal glucose uptake via β1-AR. In addition, ZAG might also activate PPA2 via β2-AR, inhibiting insulin-induced AKT phosphorylation and, in consequence, insulin-induced glucose uptake. Although some prospective cohort studies point to ZAG expression in AT as a useful biomarker to predict insulin sensitivity, our data establish ZAG as a negative modulator of insulin action.
Mentions: Although controversial, it has been also suggested that ZAG might inversely reflect the status of insulin sensitivity in obesity. This notion is supported by the positive association found between the expression of ZAG in AT and some key components of the insulin signaling pathway, such as IRS1 [14] and GLUT4 [16]. However, results regarding circulating ZAG levels and insulin resistance indices such as HOMA-IR are conflicting [12, 16, 20]. Moreover, not only obesity but also cachexia and HIV-associated lipodystrophy, where ZAG expression in AT has been inversely associated to fat mass, are related with insulin resistant states [26]. Thus, some caution should be exercised when extrapolating information from observational clinical studies in pathological conditions in an attempt to understand the physiological function of a protein. Indeed, the inverse association between circulating ZAG levels and HOMA in obesity, found by some authors [19], was not observed in a cohort of lean patients classified according to HOMA-IR (this work). Our study clearly shows that treatment of human adipocytes with ZAG increases basal glucose uptake but induces an insulin resistant state, similar to other catabolic adipokines such as TNF-alpha [28]. Analogous to other stress conditions related to insulin resistance [21, 22], ZAG produces paradoxical effects in human subcutaneous adipocytes. Accordingly, ZAG increases glucose uptake, correlating with an increase in GLUT1 and GLUT4 expression, but also activates PP2A, inhibiting insulin-induced AKT phosphorylation and in turn, glucose uptake (Fig 5). This detrimental effect is also observed in other insulin sensitive cells such as myocytes and hepatocytes, suggesting a non-tissue-specific effect of ZAG. Although previous studies by Tisdale et al. have reported that the effects of ZAG on body weight and insulin sensitivity in ob/ob mice might be mediated through β3- or possibly β2-ARs [37], a recent report demonstrates that ZAG does not behave as a typical β3/2-AR agonist [36]. Our own results would suggest that ZAG elicits its inhibitory effects on insulin signaling in human fat cells by acting as a β2-AR agonist, whereas basal glucose responses are controlled by ZAG signaling through β1-AR (Fig 5). These findings are in agreement with the important metabolic differences described for rodent and human AT, where β3-AR would represent a minor player in human white AT [30]. It is well known that β-adrenergic agonists can stimulate glucose uptake in several tissues independently of insulin [38, 39]. Insulin and β2-adrenoreceptor pathways seem not to be additive and utilize similar mechanisms to increase glucose uptake, however, some differences have been revealed in specific insulin-signaling intermediates [39]. In agreement with the paradoxical effects observed with ZAG, other GPCR agonists distinct to those activating β-ARs, such as endothelin-1, stimulates glucose uptake but also desensitizes cells to acute insulin treatment [40]. Our study also provides compelling evidence for the participation of the Ser/Thr phosphatase PP2A as an effector molecule in the ZAG signaling cascade. PP2A has been primarily described as a negative regulator of the insulin signaling pathway since it impairs AKT activation [32]. Nevertheless, certain transduction pathways activated by some insulin-sensitizing adipokines are also elicited by PP2A activation [25, 41], suggesting that PP2A specificity depends on its regulatory subunits.

Bottom Line: ZAG treatment correlated with an increase in PP2A activity.Silencing of the PP2A catalytic subunit abrogated the negative effect of ZAG on insulin-stimulated AKT phosphorylation and glucose uptake but not on GLUT4 expression and basal glucose uptake.ZAG circulating levels were unchanged in a lean patient cohort stratified for HOMA-IR.

View Article: PubMed Central - PubMed

Affiliation: Hospital Universitari de Tarragona Joan XXIII, Institut d´Investigació Sanitària Pere Virgili, Universitat Rovira i Virgili, Tarragona, Spain; Centro de Investigación Biomédica en Red de Diabetes y Enfermedades Metabólicas Asociadas, Instituto de Salud Carlos III, Madrid, Spain.

ABSTRACT

Objective: Evidence from mouse models suggests that zinc-α2-glycoprotein (ZAG) is a novel anti-obesity adipokine. In humans, however, data are controversial and its physiological role in adipose tissue (AT) remains unknown. Here we explored the molecular mechanisms by which ZAG regulates carbohydrate metabolism in human adipocytes.

Methods: ZAG action on glucose uptake and insulin action was analyzed. β1 and β2-adrenoreceptor (AR) antagonists and siRNA targeting PP2A phosphatase were used to examine the mechanisms by which ZAG modulates insulin sensitivity. Plasma levels of ZAG were measured in a lean patient cohort stratified for HOMA-IR.

Results: ZAG treatment increased basal glucose uptake, correlating with an increase in GLUT expression, but induced insulin resistance in adipocytes. Pretreatment of adipocytes with propranolol and a specific β1-AR antagonist demonstrated that ZAG effects on basal glucose uptake and GLUT4 expression are mediated via β1-AR, whereas inhibition of insulin action is dependent on β2-AR activation. ZAG treatment correlated with an increase in PP2A activity. Silencing of the PP2A catalytic subunit abrogated the negative effect of ZAG on insulin-stimulated AKT phosphorylation and glucose uptake but not on GLUT4 expression and basal glucose uptake. ZAG circulating levels were unchanged in a lean patient cohort stratified for HOMA-IR. Neither glucose nor insulin was associated with plasma ZAG.

Conclusions: ZAG inhibits insulin-induced glucose uptake in human adipocytes by impairing insulin signaling at the level of AKT in a β2-AR- and PP2A-dependent manner.

No MeSH data available.


Related in: MedlinePlus