Increased levels of the Akt-specific phosphatase PH domain leucine-rich repeat protein phosphatase (PHLPP)-1 in obese participants are associated with insulin resistance

Abstract

Aims/hypothesis We determined the contribution to insulin resistance of the PH domain leucine-rich repeat protein phosphatase (PHLPP), which dephosphorylates Akt at Ser473, inhibiting its activity. We measured the abundance of PHLPP in fat and skeletal muscle from obese participants. To study the effect of PHLPP on insulin signalling, PHLPP (also known as PHLPP1) was overexpressed in HepG2 and L6 cells. Methods Subcutaneous fat samples were obtained from 82 morbidly obese and ten non-obese participants. Skeletal muscle samples were obtained from 12 obese and eight non-obese participants. Quantification of PHLPP-1 in human tissues was performed by immunoblotting. The functional consequences of recombinant PHLPP1 overexpression in hepatoma HepG2 cells and L6 myoblasts were investigated. Results Of the 82 obese participants, 31 had normal fasting glucose, 33 impaired fasting glucose and 18 type 2 diabetes. PHLPP-1 abundance was twofold higher in the three obese groups than in non-obese participants (p=0.004). No differences were observed between obese participants with normal fasting glucose, impaired fasting glucose or type 2 diabetes. PHLPP-1 abundance was correlated with basal Akt Ser473 phosphorylation (r=−0.48; p=0.001), BMI (r=0.44; p< 0.0001), insulin (r=0.35; p<0.0001) and HOMA (r=0.38; p<0.0001). PHLPP-1 abundance was twofold higher in the skeletal muscle of 12 obese participants than in that of eight non-obese participants (p<0.0001). Insulin treatment of HepG2 cells resulted in a dose-and time-dependent upregulation of PHLPP-1. Overexpression of PHLPP1 in HepG2 cells and L6 myoblasts resulted in impaired insulin signalling involving Akt/glycogen synthase kinase 3, glycogen synthesis and glucose transport. Conclusions/interpretation Increased abundance of PHLPP-1, production of which is regulated by insulin, may represent a new molecular defect in insulin-resistant states such as obesity.

Insulin Receptor Signaling in Normal and Insulin-Resistant States

“…Two novel members of the PP2C family involved in regulation of insulin action are the PH domain leucine-rich repeat protein phosphatases PHLPP-1 and -2, which dephosphorylate both Akt and PKCs (Brognard and Newton 2008). Overexpression of PHLPP1 in cells impairs Akt and glycogen synthase kinase 3 activity, resulting in decreased glycogen synthesis and glucose transport (Andreozzi et al 2011). Elevated levels of PHLPP1 have been found in adipose tissue and skeletal muscle of obese and/or diabetic patients and correlate with decreased Akt2 phosphorylation (Cozzone et al 2008;Andreozzi et al 2011).…”

PHLPP: a putative cellular target during insulin resistance and type 2 diabetes

“…and coworkers (Andreozzi et al 2011), impaired insulin action was associated with higher PHLPP1 expression in the adipose and skeletal muscle tissues of obese patients. Further, liver and skeletal muscle cells transfected with recombinant PHLPP1 demonstrated decreased GSK-3α/β phosphorylation and glycogen synthesis confirming the fact that changes in PHLPP1 abundance contribute to insulin resistance (Aviv & Kirshenbaum 2010).…”

“…Overexpression of PHLPP isoforms is associated with several human diseases, including cancer, obesity, cardiovascular disorders and diabetes (Miyamoto et al 2010, Andreozzi et al 2011. However, limited studies on its role in the diabetic milieu underscore the medical relevance of PHLPPs and make it an attractive drug target for the treatment during insulin-resistant states.…”

Pleckstrin Homology Domain Leucine-rich Repeat Protein Phosphatase (PHLPP): A New Player in Cell Signaling

“…Impaired activation of Akt isozymes, specifically Akt2, reduces glucose transport and ultimately leads to insulin resistance, which is commonly associated with obesity and type 2 diabetes (41,42). Indeed, PHLPP1 protein levels are significantly higher in skeletal muscle and adipose tissue from obese human subjects compared with nonobese subjects, correlating with lower Akt (Ser-473) phosphorylation (43). Consistent with this, PHLPP1 mRNA is enhanced in type 2 diabetic individuals, correlating with decreased hydrophobic motif phosphorylation of Akt2 (43).…”

“…Indeed, PHLPP1 protein levels are significantly higher in skeletal muscle and adipose tissue from obese human subjects compared with nonobese subjects, correlating with lower Akt (Ser-473) phosphorylation (43). Consistent with this, PHLPP1 mRNA is enhanced in type 2 diabetic individuals, correlating with decreased hydrophobic motif phosphorylation of Akt2 (43). Overexpression of PHLPP1 in insulin-responsive cell lines results in a decrease in insulin-induced Akt (Ser-473) phosphorylation, diminished glycogen synthesis, and reduced glucose transport (44).…”