The Nematode Caenorhabditis elegans as a Model Organism to Study Metabolic Effects of ω-3 Polyunsaturated Fatty Acids in Obesity

Abstract

Obesity is a complex disease that is influenced by several factors, such as diet, physical activity, developmental stage, age, genes, and their interactions with the environment. Obesity develops as a result of expansion of fat mass when the intake of energy, stored as triglycerides, exceeds its expenditure. Approximately 40% of the US population suffers from obesity, which represents a worldwide public health problem associated with chronic low-grade adipose tissue and systemic inflammation (sterile inflammation), in part due to adipose tissue expansion. In patients with obesity, energy homeostasis is further impaired by inflammation, oxidative stress, dyslipidemia, and metabolic syndrome. These pathologic conditions increase the risk of developing other chronic diseases including diabetes, hypertension, coronary artery disease, and certain forms of cancer. It is well documented that several bioactive compounds such as omega-3 polyunsaturated fatty acids (ω-3 PUFAs) are able to reduce adipose and systemic inflammation and blood triglycerides and, in some cases, improve glucose intolerance and insulin resistance in vertebrate animal models of obesity. A promising model organism that is gaining tremendous interest for studies of lipid and energy metabolism is the nematode Caenorhabditis elegans. This roundworm stores fats as droplets within its hypodermal and intestinal cells. The nematode's transparent skin enables fat droplet visualization and quantification with the use of dyes that have affinity to lipids. This article provides a review of major research over the past several years on the use of C. elegans to study the effects of ω-3 PUFAs on lipid metabolism and energy homeostasis relative to metabolic diseases.

FIGURE 1

Major adipokines and adipose-derived soluble factors in regulating energy homeostasis and immune status. A wide variety of WAT-produced molecules contribute to regulation of lipid and carbohydrate metabolism in health and disease. In obese white adipose tissue (WAT), activation of energy deposition pathways is coupled with elevated proinflammatory signaling, causing obesity-associated chronic inflammation. GLUT4, glucose transporter type 4 protein; MCP-1, monocyte chemoattractant protein-1; PAI-1, plasminogen activator inhibitor-1; IL, interleukin; TNF-α, tumor necrosis factor-α.

FIGURE 2

Metabolism of ω-3 and ω-6 PUFAs in humans. ω-3 Fatty acids are synthesized from the ALA precursor and ω-6 fatty acids are synthesized from the LA precursor. LA is converted to AA. Eicosanoids derived from AA have proinflammatory properties. ALA is subsequently converted to EPA and DHA. The metabolites of EPA and DHA have anti-inflammatory properties. AA, arachidonic acid; ALA, α-linolenic acid; Cox, cyclooxygenase; LA, linoleic acid; Lox, lipoxygenase; LTB4, leukotriene B4; LTB5, leukotriene B5; MaR, maresin; NPD1, neuroprotectin D1; PD1, protectin D1; PGE2, prostaglandin E2; PGE3, prostaglandin E3; RvD, resolvin D; RvE, resolvin E; TXA2, thromboxane A2; TXA3, thromboxane A3.

FIGURE 3

Molecular mediators of the effects of long-chain ω-3 PUFAs. In adipocytes, ω-3 PUFAs modulate gene expression and promote biosynthesis of regulatory proteins, which enhance the utilization of carbohydrates and fats, reduce adipogenesis, increase insulin sensitivity, and ameliorate inflammation. AMPK, 5' AMP-activated protein kinase; CPT-1, carnitine palmitoyl transferase-1; FFAR4, free fatty acid receptor 4; GPR, G-protein–coupled receptor; IRS, insulin substrate receptor; NF-κB, nuclear factor kappa-B; SREBP-1, sterol regulatory element-binding protein-1.

FIGURE 4

Caenorhabditis elegans life cycle at 20°C. The life cycle of this nematode is ∼3.5 d. Under standard laboratory conditions, reproductive adult worms survive for ∼3 wk. The regular ontogenesis includes embryonic stage, 4 larval stages (L1–L4; separated by molts), and adulthood. Under stress conditions (starvation, crowding, high temperature), the roundworm can enter an alternative L3 stage called the Dauer state, which can last for several months. The Dauer larva develops from a pre-Dauer L2 (L2d). Numbers in red underneath the arrows show the time span that the worm stays at the indicated stage.

FIGURE 5

Visualization of intestinal fat droplets in the Caenorhabditis elegans body. (A) Oil Red O staining in N2 wild-type and fat-3 mutant worms. (B) Nile Red staining in N2 wild-type nematode. Because of the transparency of C. elegans’ body, lipid droplets, intestine, and embryos are easily visualized through the use of conventional staining protocols. fat-3 mutants lack delta 6 desaturase.

FIGURE 6

Metabolism of ω-3 and ω-6 PUFAs in Caenorhabditis elegans. Unlike humans, C. elegans is capable of producing ω-3 PUFAs via the enzymatic conversion of various ω-6 PUFAs, biosynthesized from the short-chain stearic and oleic acid precursors. Δ5D, Δ-5 desaturase (encoded by the fat-4 gene); Δ6D, Δ-6 desaturase (encoded by the fat-3 gene); Δ9D, Δ-9 desaturase; Δ12D, Δ-12 desaturase (encoded by the fat-2 gene); ω3D, ω-3 desaturase (encoded by the fat-1 gene). fat encodes fatty acid desaturase genes.