Diacylglycerol kinase θ counteracts protein kinase C-mediated inactivation of the EGF receptor

Genome-wide meta-analysis reveals shared new loci in systemic seropositive rheumatic diseases

OBJECTIVE

Immune-mediated inflammatory diseases (IMIDs) are heterogeneous and complex conditions with overlapping clinical symptoms and elevated familial aggregation, which suggests the existence of a shared genetic component. In order to identify this genetic background in a systematic fashion, we performed the first cross-disease genome-wide meta-analysis in systemic seropositive rheumatic diseases, namely, systemic sclerosis, systemic lupus erythematosus, rheumatoid arthritis and idiopathic inflammatory myopathies.

METHODS

We meta-analysed ~6.5 million single nucleotide polymorphisms in 11 678 cases and 19 704 non-affected controls of European descent populations. The functional roles of the associated variants were interrogated using publicly available databases.

RESULTS

Our analysis revealed five shared genome-wide significant independent loci that had not been previously associated with these diseases: NAB1, KPNA4-ARL14, DGQK, LIMK1 and PRR12. All of these loci are related with immune processes such as interferon and epidermal growth factor signalling, response to methotrexate, cytoskeleton dynamics and coagulation cascade. Remarkably, several of the associated loci are known key players in autoimmunity, which supports the validity of our results. All the associated variants showed significant functional enrichment in DNase hypersensitivity sites, chromatin states and histone marks in relevant immune cells, including shared expression quantitative trait loci. Additionally, our results were significantly enriched in drugs that are being tested for the treatment of the diseases under study.

CONCLUSIONS

We have identified shared new risk loci with functional value across diseases and pinpoint new potential candidate loci that could be further investigated. Our results highlight the potential of drug repositioning among related systemic seropositive rheumatic IMIDs.

Overexpression of diacylglycerol kinase η enhances Gαq-coupled G protein-coupled receptor signaling

Multiple genome-wide association studies have linked diacylglycerol kinase η (DGKη) to bipolar disorder (BPD). Moreover, DGKη expression is increased in tissue from patients with BPD. How increased levels of this lipid kinase might affect cellular functions is currently unclear. Here, we overexpressed mouse DGKη in human embryonic kidney 293 cells to examine substrate specificity and signaling downstream of endogenous G protein-coupled receptors (GPCRs). We found that DGKη can phosphorylate diacylglycerol (DAG) with different acyl side chains (8:0, 12:0, 18:1). In addition, overexpression of DGKη enhanced calcium mobilization after stimulating muscarinic receptors with carbachol and after stimulating purinergic receptors with ATP. This effect required DGKη catalytic activity, as assessed using a kinase-dead (G389D) mutant and multiple truncation constructs. DGKη was localized throughout the cytosol and did not translocate to the plasma membrane after stimulation with carbachol. Since protein kinase C (PKC) can be activated by DAG and promotes receptor desensitization, we also examined functional interactions between PKC and DGKη. We found that acute activation of PKC with phorbol 12-myristate 13-acetate shortened carbachol-evoked calcium responses and occluded the effect of overexpressed DGKη. Moreover, inhibition of PKC activity with bisindolylmaleimide I (BIM I) produced the same enhancing effect on carbachol-evoked calcium mobilization as overexpressed DGKη, and overexpression of DGKη produced no additional effect on calcium mobilization in the presence of BIM I. Taken together, our data suggest that DGKη enhances GPCR signaling by reducing PKC activation.

Diacylglycerol kinase θ couples farnesoid X receptor-dependent bile acid signalling to Akt activation and glucose homoeostasis in hepatocytes

DGKs (diacylglycerol kinases) catalyse the conversion of diacylglycerol into PA (phosphatidic acid), a positive modulator of mTOR (mammalian target of rapamycin). We have found that chenodeoxycholic acid and the synthetic FXR (farnesoid X receptor) ligand GW4064 induce the mRNA and protein expression of DGKθ in the HepG2 cell line and in primary human hepatocytes. Reporter gene studies using 1.5 kB of the DGKθ promoter fused to the luciferase gene revealed that bile acids increase DGKθ transcriptional activity. Mutation of putative FXR-binding sites attenuated the ability of GW4046 to increase DGKθ luciferase activity. Consistent with this finding, ChIP (chromatin immunoprecipitation) assays demonstrated that bile acid signalling increased the recruitment of FXR to the DGKθ promoter. Furthermore, GW4064 evoked a time-dependent increase in the cellular concentration of PA. We also found that GW4064 and PA promote the phosphorylation of mTOR, Akt and FoxO1 (forkhead box O1), and that silencing DGKθ expression significantly abrogated the ability of GW4046 to promote the phosphorylation of these PA-regulated targets. DGKθ was also required for bile-acid-dependent decreased glucose production. Taken together, our results establish DGKθ as a key mediator of bile-acid-stimulated modulation of mTORC2 (mTOR complex 2), the Akt pathway and glucose homoeostasis.

The expression of diacylglycerol kinase theta during the organogenesis of mouse embryos

Background

Diacylglycerol kinase (DGK) is a key enzyme that regulates diacylglycerol (DG) turnover and is involved in a variety of physiological functions. The isoform DGKθ has a unique domain structure and is the sole member of type V DGK. To reveal the spatial and temporal expression of DGKθ we performed immunohistochemical staining on paraffin sections of mouse embryos.

Results

At an early stage of development (E10.5 and 11.5), the expression of DGKθ was prominently detected in the brain, spinal cord, dorsal root ganglion, and limb bud, and was also moderately detected in the bulbus cordis and the primordium of the liver and gut. At later stages (E12.5 and 14.5), DGKθ expression persisted or increased in the neocortex, epithalamus, hypothalamus, medulla oblongata, and pons. DGKθ was also evident in the epidermis, and nearly all epithelia of the oropharyngeal membrane, digestive tract, and bronchea. At prenatal developmental stages (E16.5 and E18.5), the expression pattern of DGKθ was maintained in the central nervous system, intestine, and kidney, but was attenuated in the differentiated epidermis.

Conclusion

These results suggest that DGKθ may play important physiological roles not only in the brain, but also in diverse organs and tissues during the embryonic stages.

Cellular functions regulated by phosphorylation of EGFR on Tyr845

The Src gene product (Src) and the epidermal growth factor receptor (EGFR) are prototypes of oncogene products and function primarily as a cytoplasmic non-receptor tyrosine kinase and a transmembrane receptor tyrosine kinase, respectively. The identification of Src and EGFR, and the subsequent extensive investigations of these proteins have long provided cutting edge research in cancer and other molecular and cellular biological studies. In 1995, we reported that the human epidermoid carcinoma cells, A431, contain a small fraction of Src and EGFR in which these two kinase were in physical association with each other, and that Src phosphorylates EGFR on tyrosine 845 (Y845) in the Src-EGFR complex. Y845 of EGFR is located in the activation segment of the kinase domain, where many protein kinases contain kinase-activating autophosphorylation sites (e.g., cAMP-dependent protein kinase, Src family kinases, transmembrane receptor type tyrosine kinases) or trans-phosphorylation sites (e.g., cyclin-dependent protein kinase, mitogen-activated protein kinase, Akt protein kinase). A number of studies have demonstrated that Y845 phosphorylation serves an important role in cancer as well as normal cells. Here we compile the experimental facts involving Src phosphorylation of EGFR on Y845, by which cell proliferation, cell cycle control, mitochondrial regulation of cell metabolism, gamete activation and other cellular functions are regulated. We also discuss the physiological relevance, as well as structural insights of the Y845 phosphorylation.