Derong Hu

Insulin Receptor-mediated p62dok Tyrosine Phosphorylation at Residues 362 and 398 Plays Distinct Roles for Binding GTPase-activating Protein and Nck and Is Essential for Inhibiting Insulin-stimulated Activation of Ras and Akt

A GTPase-activating protein (GAP)-associated 60-kDa protein has been found to undergo rapid tyrosine phosphorylation in response to insulin stimulation. However, whether this protein is a direct in vivo substrate for the insulin receptor (IR) tyrosine kinase and whether the tyrosine phosphorylation plays a role in insulin signaling remain to be established. Here we show that the insulin-stimulated tyrosine phosphorylation of the GAP-associated protein, now identified as p62dok, is inhibited by Grb10, an adaptor protein that binds directly to the kinase domain of the IR, both in vitro and in cells. Replacing Tyr362 and Tyr398 with phenylalanine greatly decreased the IR-catalyzed p62dok tyrosine phosphorylation in vitro, suggesting that these two residues are the major IR-mediated phosphorylation sites. However, mutations at Tyr362 and Tyr398 only partially blocked insulin-stimulated p62dok tyrosine phosphorylation in cells, indicating that p62dok is also a target for other cellular tyrosine kinase(s) in addition to the IR. Replacing Tyr362 with phenylalanine abolished the interaction between p62dok and Nck. Mutations at Tyr362/398 of p62dok disrupted the interaction between p62dokand GAP and decreased the inhibitory effect of p62dok on the insulin-stimulated activation of Ras and Akt, but not mitogen-activated protein kinase. Furthermore, the inhibitory effect of p62dok on Akt phosphorylation could be blocked by coexpression of a constitutively active Ras. Taken together, our findings indicate that p62dok is a direct substrate for the IR tyrosine kinase and that phosphorylation at Tyr362 and Tyr398 plays an essential role for p62dok to interact with its effectors and negatively regulate the insulin signaling pathway.

Grb10 Promotes Lipolysis and Thermogenesis by Phosphorylation-dependent Feedback Inhibition of mTORC1

Identification of key regulators of lipid metabolism and thermogenic functions has important therapeutic implications for the current obesity and diabetes epidemic. Here, we show that Grb10, a direct substrate of mechanistic/mammalian target of rapamycin (mTOR), is expressed highly in brown adipose tissue, and its expression in white adipose tissue is markedly induced by cold exposure. In adipocytes, mTOR-mediated phosphorylation at Ser501/503 switches the binding preference of Grb10 from the insulin receptor to raptor, leading to the dissociation of raptor from mTOR and downregulation of mTOR complex 1 (mTORC1) signaling. Fat-specific disruption of Grb10 increased mTORC1 signaling in adipose tissues, suppressed lipolysis, and reduced thermogenic function. The effects of Grb10 deficiency on lipolysis and thermogenesis were diminished by rapamycin administration in vivo. Our study has uncovered a unique feedback mechanism regulating mTORC1 signaling in adipose tissues and identified Grb10 as a key regulator of adiposity, thermogenesis, and energy expenditure.

Identification of Grb10 as a direct substrate for members of the Src tyrosine kinase family

Treatment of cells with insulin and protein tyrosine phosphatase inhibitors such as vanadate and pervanadate resulted in the tyrosine phosphorylation of Grb10, a Src homology 2 (SH2) and pleckstrin homology domain-containing adaptor protein which binds to a number of receptor tyrosine kinases including the insulin receptor (IR). Although Grb10 binds directly to the kinase domain of the IR, our data show that Grb10 is not a direct substrate for the IR tyrosine kinase. Consistent with this finding, Grb10 tyrosine phosphorylation in cells was inhibited by herbimycin A, a relatively specific inhibitor for members of the Src tyrosine kinase family, and by the expression of dominant negative Src or Fyn. In addition, Grb10 tyrosine phosphorylation was stimulated by expression of constitutively active Src or Fyn in cells and by incubation with purified Src or Fyn in vitro. The insulin stimulated or Src/Fyn-mediated tyrosine phosphorylation in vivo was significantly reduced when Grb10 tyrosine 67 was changed to glycine. This mutant form of Grb10 bound with higher affinity to the IR in cells than that of the wild-type protein, suggesting that tyrosine phosphorylation of Grb10 may normally negatively regulate its binding to the IR. Our data show that Grb10 is a new substrate for members of the Src tyrosine kinase family and that the tyrosine phosphorylation of the protein may play a potential role in cell signaling processes mediated by these kinases.

Inhibition of hGrb10 Binding to the Insulin Receptor by Functional Domain-mediated Oligomerization

hGrb10 is a newly identified Src homology 2 (SH2) and pleckstrin homology (PH) domain-containing protein that binds to autophosphorylated receptor tyrosine kinases, including the insulin and insulin-like growth factor receptors. To identify potential downstream proteins that interact with hGrb10, we screened a yeast two-hybrid cDNA library using the full-length hGrb10γ as bait. A fragment of hGrb10, which included the IPS (insert between the PH and SH2 domain) and the SH2 domains, was found to bind with high affinity to the full-length protein. The interaction between the IPS/SH2 domain and the full-length hGrb10 was further confirmed by in vitroglutathione S-transferase fusion protein binding studies. Gel filtration assays showed that hGrb10 underwent tetramerization in mammalian cells. The interaction involved at least two functional domains, the IPS/SH2 region and the PH domain, both of which interacted with the NH2-terminal amino acid sequence of hGrb10γ (hGrb10γΔC, residues 4–414). Competition studies showed that hGrb10γΔC inhibited the binding of hGrb10 to the tyrosine-phosphorylated insulin receptor, suggesting that this region may play a regulatory role in hGrb10/insulin receptor interaction. We present a model for hGrb10 tetramerization and its potential role in receptor tyrosine kinase signal transduction.

Fine Tuning PDK1 Activity by Phosphorylation at Ser163

3-Phosphoinositide-dependent protein kinase-1 (PDK1) mediates phosphorylation and activation of members of the AGC protein kinase family and plays an essential role in insulin signaling and action. However, whether and how PDK1 activity is regulated in cells remains largely uncharacterized. In the present study, we show that PDK1 undergoes insulin-stimulated and phosphatidylinositol 3-kinase-dependent phosphorylation at Ser244 in the activation loop and at a novel site: Ser163 in the hinge region between the two lobes of the kinase domain. Sequence alignment studies revealed that the residue corresponding to Ser163 of PDK1 in all other AGC kinases is glutamate, suggesting that a negative charge at this site may be important for PDK1 function. Replacing Ser163 with a negatively charged residue, glutamate, led to a 2-fold increase in PDK1 activity. Molecular modeling studies suggested that phosphorylated Ser163 may form additional hydrogen bonds with Tyr149 and Gln223. In support of this, mutation of Tyr149 to Ala is sufficient to reduce PDK1 activity. Taken together, our results suggest that PDK1 phosphorylation of Ser163 may provide a mechanism to fine-tune PDK1 activity and function in cells.

Phosphorylation of adaptor protein containing pleckstrin homology domain, phosphotyrosinebinding domain, and leucine zipper motif 1 (APPL1) at Ser 430 mediates endoplasmic reticulum (ER) stress-induced insulin resistance in hepatocytes

Background: Adaptor protein APPL1 plays a critical role in regulating both adiponectin and insulin signaling pathways. Results: ER stress-induced APPL1 phosphorylation at Ser430 blocks the insulin-sensitizing effect of APPL1 in a PKCα-dependent manner. Conclusion: APPL1 phosphorylation at Ser430 mediates ER stress-induced insulin resistance. Significance: PKCα-mediated APPL1 phosphorylation could be a potential drug target for the treatment of insulin resistance. APPL1 is an adaptor protein that plays a critical role in regulating adiponectin and insulin signaling. However, how APPL1 is regulated under normal and pathological conditions remains largely unknown. In this study, we show that APPL1 undergoes phosphorylation at Ser430 and that this phosphorylation is enhanced in the liver of obese mice displaying insulin resistance. In cultured mouse hepatocytes, APPL1 phosphorylation at Ser430 is stimulated by phorbol 12-myristate 13-acetate, an activator of classic PKC isoforms, and by the endoplasmic reticulum (ER) stress inducer, thapsigargin. Overexpression of wild-type but not dominant negative PKCα increases APPL1 phosphorylation at Ser430 in mouse hepatocytes. In addition, suppressing PKCα expression by shRNA in hepatocytes reduces ER stress-induced APPL1 phosphorylation at Ser430 as well as the inhibitory effect of ER stress on insulin-stimulated Akt phosphorylation. Consistent with a negative regulatory role of APPL1 phosphorylation at Ser430 in insulin signaling, overexpression of APPL1S430D but not APPL1S430A impairs the potentiating effect of APPL1 on insulin-stimulated Akt phosphorylation at Thr308. Taken together, our results identify APPL1 as a novel target in ER stress-induced insulin resistance and PKCα as the kinase mediating ER stress-induced phosphorylation of APPL1 at Ser430.

Endoplasmic Reticulum (ER) Localization Is Critical for DsbA-L Protein to Suppress ER Stress and Adiponectin Down-regulation in Adipocytes

Background: DsbA-L has been shown to promote adiponectin multimerization, but the molecular mechanisms remain unknown. Results: The wild type, but not ER localization-defective mutant of DsbA-L, protects against ER stress and adiponectin down-regulation. Conclusion: ER localization is critical for DsbA-L to suppress ER stress and adiponectin down-regulation in adipocytes. Significance: Increasing DsbA-L expression could protect against obesity-induced ER stress and metabolic disorders. Adiponectin is an adipokine with insulin-sensitizing and anti-inflammatory functions. We previously reported that adiponectin multimerization and stability are promoted by the disulfide bond A oxidoreductase-like protein (DsbA-L) in cells and in vivo. However, the precise mechanism by which DsbA-L regulates adiponectin biosynthesis remains elusive. Here we show that DsbA-L is co-localized with the endoplasmic reticulum (ER) marker protein disulfide isomerase and the mitochondrial marker MitoTracker. In addition, DsbA-L interacts with the ER chaperone protein Ero1-Lα in 3T3-L1 adipocytes. In silico analysis and truncation mapping studies revealed that DsbA-L contains an ER targeting signal at its N terminus. Deletion of the first 6 residues at the N terminus greatly impaired DsbA-L localization in the ER. Overexpression of the wild type but not the ER localization-defective mutant of DsbA-L protects against thapsigargin-induced ER stress and adiponectin down-regulation in 3T3-L1 adipocytes. In addition, overexpression of the wild type but not the ER localization-defective mutant of DsbA-L promotes adiponectin multimerization. Together, our results reveal that DsbA-L is localized in both the mitochondria and the ER in adipocytes and that its ER localization plays a critical role in suppressing ER stress and promoting adiponectin biosynthesis and secretion.

DsbA-L prevents obesity-induced inflammation and insulin resistance by suppressing the mtDNA release-activated cGAS-cGAMP-STING pathway

Significance Activation of the cGAMP-cGAS-STING pathway has recently been shown to mediate virus- or bacteria-induced activation of the innate immune response. Here we report that this pathway also plays an important role in obesity-induced inflammation and metabolic dysfunction, beyond its well-characterized roles in innate immune surveillance. We have also identified adipose disulfide-bond A oxidoreductase-like protein as a key regulator of mitochondrial integrity and function, which protects mice from obesity-induced inflammation and insulin resistance by suppressing mtDNA release-induced activation of the cGAS-cGAMP-STING pathway. Our study suggests that targeting the cGAS-cGAMP-STING pathway in adipose tissue may be an effective approach to ameliorating obesity-induced chronic inflammation and its associated metabolic diseases. Chronic inflammation in adipose tissue plays a key role in obesity-induced insulin resistance. However, the mechanisms underlying obesity-induced inflammation remain elusive. Here we show that obesity promotes mtDNA release into the cytosol, where it triggers inflammatory responses by activating the DNA-sensing cGAS-cGAMP-STING pathway. Fat-specific knockout of disulfide-bond A oxidoreductase-like protein (DsbA-L), a chaperone-like protein originally identified in the mitochondrial matrix, impaired mitochondrial function and promoted mtDNA release, leading to activation of the cGAS-cGAMP-STING pathway and inflammatory responses. Conversely, fat-specific overexpression of DsbA-L protected mice against high-fat diet-induced activation of the cGAS-cGAMP-STING pathway and inflammation. Taken together, we identify DsbA-L as a key molecule that maintains mitochondrial integrity. DsbA-L deficiency promotes inflammation and insulin resistance by activating the cGAS-cGAMP-STING pathway. Our study also reveals that, in addition to its well-characterized roles in innate immune surveillance, the cGAS-cGAMP-STING pathway plays an important role in mediating obesity-induced metabolic dysfunction.