A limit for liver lipid overload
Hepatocytes respond to insulin by accumulating triglycerides and cholesterol. Excessive lipid accumulation in the liver can result in nonalcoholic fatty liver disease (NAFLD), the more severe forms of which are risk factors for the development of liver cirrhosis and cancer. Mayer et al. found that activation of PKD3 by insulin signaling served as a negative feedback mechanism to prevent hepatic lipid accumulation. Mice lacking PKD3 in the liver showed increased insulin signaling, triglyceride and cholesterol synthesis, and steatosis in response to a high-fat diet. In contrast, overexpression of a constitutively active form of PKD3 attenuated insulin signaling in the liver and resulted in insulin resistance. Thus, PKD3 activity curtails insulin signaling and, therefore, lipid synthesis and accumulation in the liver.
Hepatic activation of protein kinase C (PKC) isoforms by diacylglycerol (DAG) promotes insulin resistance and contributes to the development of type 2 diabetes (T2D). The closely related protein kinase D (PKD) isoforms act as effectors for DAG and PKC. Here, we showed that PKD3 was the predominant PKD isoform expressed in hepatocytes and was activated by lipid overload. PKD3 suppressed the activity of downstream insulin effectors including the kinase AKT and mechanistic target of rapamycin complex 1 and 2 (mTORC1 and mTORC2). Hepatic deletion of PKD3 in mice improved insulin-induced glucose tolerance. However, increased insulin signaling in the absence of PKD3 promoted lipogenesis mediated by SREBP (sterol regulatory element-binding protein) and consequently increased triglyceride and cholesterol content in the livers of PKD3-deficient mice fed a high-fat diet. Conversely, hepatic-specific overexpression of a constitutively active PKD3 mutant suppressed insulin-induced signaling and caused insulin resistance. Our results indicate that PKD3 provides feedback on hepatic lipid production and suppresses insulin signaling. Therefore, manipulation of PKD3 activity could be used to decrease hepatic lipid content or improve hepatic insulin sensitivity.
Fig. S1. Stimulation of hepatocytes with DAG suppresses expression of lipogenic genes.
Fig. S2. PKD3 deletion is restricted to the liver.
Fig. S3. Liver-specific PKD3 deletion does not affect metabolism of mice fed an ND.
Fig. S4. PKD3 does not affect proliferation, immune cell infiltration, or apoptosis in the liver.
Fig. S5. TG accumulation in the livers of PKD3liverΔ/Δ mice does not depend on FA oxidation or VLDL secretion.
Fig. S6. The abundance and/or phosphorylation of mTORC1/2 components are not affected by deletion or overexpression of PKD3 in hepatocytes.
Fig. S7. Quantifications of Western blots of control and PKD3-deficient primary hepatocytes.
Fig. S8. Quantifications of Western blots of EGFP- and PKD3ca-transduced primary hepatocytes.
Fig. S9. Liver-specific expression of PKD3ca improves glucose tolerance and insulin sensitivity.
Table S1. List of antibodies used for Western blotting and immunohistochemistry.
Table S2. Sequence of primers used for QPCR and genotyping.
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Volume 12 | Issue 593
Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.
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Received: 31 October 2018
Accepted: 19 July 2019
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We thank O. Sumara for the critical comments to our manuscript, A. Schürmann and C. Baumeister for introducing us to primary hepatocyte isolation, J. Hetzer and D. Heide for performing the immunohistochemical stainings, and K. Aurbach, I. Becker, and D. Cherpokova for experimental assistance. Funding: This project was supported by the EFSD/Janssen Rising Star Fellowship Programme (2013) from the European Foundation for the Study of Diabetes and by the Starting Grant (SicMetabol) from the European Research Council (ERC) and internal funds of the Rudolf Virchow Center for Experimental Biomedicine. R.E.-M. and G.S. were also funded by the Emmy Noether grant from the German Research Foundation (no. Su 820/1-1). M.H. was supported by an ERC Consolidator grant (HepatoMetaboPath). A.S. was supported by a German Research Foundation grant (FOR2314, SCHU2670/1-1). Author contributions: A.E.M. and G.S. conceived the study, designed the experimental procedures, and wrote the manuscript. A.E.M. performed the majority of the experimental work and analyzed the data. M.C.L., A.E.L.V., R.E.-M., J.T.V., and M.E. performed parts of the experiments. W.S. participated in the experimental design and carried out the mass spectrometry. M.L., T.Z., and U.B. provided the PKD3f/f mice. M.H. and A.S. contributed to the experimental design. Competing interests: The authors declare that they have no competing interests. Data and materials availability: The PKD3f/f mice require a material transfer agreement by the PKC Research Consult (Cologne, Germany). All data needed to evaluate the conclusions in the paper are present in the paper or the Supplementary Materials.
H2020 European Research Council: SicMetabol
H2020 European Research Council: HepatoMetaboPath
Deutsche Forschungsgemeinschaft: Su 820/1-1
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