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The Role of Angiopoietin-like 4 in Lipid Homeostasis



The Role of Angiopoietin-like 4 in Lipid Homeostasis


Nora Elizabeth Forbes Gray

Doctor of Philosophy in Molecular and Biochemical Nutrition

University of California, Berkeley

Professor Jen-Chywan Wang, Chair

Alterations in the regulation of lipid homeostasis are major causes of metabolic diseases like obesity, insulin resistance and the metabolic syndrome. These diseases affect millions of people and therefore constitute a pressing public health concern. The mobilization of lipids is a key regulatory step in lipid homeostasis and the proteins that mobilize lipids from adipocytes to other tissues are therefore potential targets for therapeutic interventions. One such protein is angiopoietin-like 4 (Angptl4), a secreted protein induced by fasting and glucocorticoid treatment that inhibits lipoprotein lipase (LPL) and induces intracellular adipocyte lipolysis. These studies seek to characterize the role of Angptl4 in modulating lipid homeostasis.

We found that Angptl4 is involved in the lipolytic response to fasting, glucocorticoids and catecholamines and in each case it modulates intracellular cAMP levels. We further discovered that purified Angptl4 can directly increase cAMP and lipolysis in adipocytes in a dose-dependent manner. We were also able to dissociate ability of Angptl4 to inhibit LPL from its ability to induce lipolysis by determining that just the C-terminal domain, which cannot inhibit LPL, can induce adipocyte lipolysis. Initial attempts to uncover the mechanism by which Angptl4 modulates cAMP showed that soluble adenylate cyclase is important for the lipolytic response to Angptl4. Additionally, Angptl4 treatment increases activation of focal adhesion kinase (FAK) and inhibition of FAK attenuates the lipolytic response to Angptl4, suggesting the potential involvement of integrins in Angptl4 signaling in adipocytes although more research is needed to confirm this possibility.

In addition to its expression in white adipose tissue, Angptl4 levels are also high in the liver where its expression is regulated by glucocorticoids. Because mice lacking Angptl4 are protected from glucocorticoid-induced fatty liver and hyperlipidemia we wanted to investigate the lipogenic role of Angptl4 in the liver. Using stable isotope labeling and gene expression analysis we found that glucocorticoids increase the rate of triglyceride synthesis as well as de novo lipogenesis but this effect is blunted in mice that lack Angptl4. There was also differential hepatic expression of genes involved in lipogenesis in the mice without Angptl4. Treatment of hepatocytes with purified Angptl4 revealed that the increase in triglyceride synthesis is not a direct effect of the protein, nor are the alterations in the expression of most lipogenic genes. Two notable exceptions, however, are Agpat1 and Agpat2, the expression of which was induced by Angptl4 treatment. This is a previously undescribed role of Angptl4 and further research is necessary to understand the mechanism by which it could be modulating transcription.

A third tissue with high expression of Angptl4 is brown adipose tissue. Because of its role in lipolysis and the fact that lipolysis is critical for adaptive thermogenesis, we investigated the how Angptl4 might be involved in thermoregulation. We found that mice lacking Angptl4 maintained a consistently lower body temperature during cold exposure. We further determined that lipolysis in brown adipose tissue was impaired in these mice and the induction of thermogenic genes was compromised relative to wild types. We also found that mice without Angptl4 maintain a lower body temperature during fasting than WT mice. These results indicate a potential role of Angptl4 in thermogenesis, which could have important implications for obesity.

These studies confirm that Angptl4 is important for the regulation of lipid homeostasis. A more complete understanding of its mechanism, including the identification of a receptor that can mediate its intracellular effects, will be crucial for evaluating the potential of Angptl4 as a therapeutic target for diseases of deregulated lipid metabolism.

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