Abstract

Objective

   Adipose tissue mass is maintained by a balance between lipolysis and
   lipid storage. The contribution of adipose tissue lipogenesis to fat
   mass, especially in the setting of high-fat feeding, is considered
   minor. Here we investigated the effect of adipose-specific inactivation
   of the peroxisomal lipid synthetic protein PexRAP on fatty acid
   synthase (FASN)-mediated lipogenesis and its impact on adiposity and
   metabolic homeostasis.

Methods

   To explore the role of PexRAP in adipose tissue, we metabolically
   phenotyped mice with adipose-specific knockout of PexRAP. Bulk RNA
   sequencing was used to determine transcriptomic responses to PexRAP
   deletion and ^14C-malonyl CoA allowed us to measure de novo lipogenic
   activity in adipose tissue of these mice. In vitro cell culture models
   were used to elucidate the mechanism of cellular responses to PexRAP
   deletion.

Results

   Adipose-specific PexRAP deletion promoted diet-induced obesity and
   insulin resistance through activation of de novo lipogenesis.
   Mechanistically, PexRAP inactivation inhibited the flux of carbons to
   ethanolamine plasmalogens. This increased the nuclear PC/PE ratio and
   promoted cholesterol mislocalization, resulting in activation of liver
   X receptor (LXR), a nuclear receptor known to be activated by increased
   intracellular cholesterol. LXR activation led to increased expression
   of the phospholipid remodeling enzyme LPCAT3 and induced FASN-mediated
   lipogenesis, which promoted diet-induced obesity and insulin
   resistance.

Conclusions

   These studies reveal an unexpected role for peroxisome-derived lipids
   in regulating LXR-dependent lipogenesis and suggest that activation of
   lipogenesis, combined with dietary lipid overload, exacerbates obesity
   and metabolic dysregulation.

   Keywords: Adipose tissue, Cholesterol, De novo lipogenesis, Diabetes,
   Plasmalogen, Liver X receptor

Highlights

     * •
       Adipose-specific knockout of PexRAP promotes diet-induced obesity
       and insulin resistance.
     * •
       PexRAP inactivation decreases ethanolamine plasmalogens,
       peroxisome-derived phospholipids.
     * •
       PexRAP deletion alters adipocyte membrane phospholipid composition,
       causing cholesterol mislocalization and LXR activation.
     * •
       LXR activation induces FASN-mediated lipogenesis, leading to
       diet-induced obesity and metabolic dysregulation.

1. Introduction

   Obesity continues to rise globally, increasing risk and prevalence of
   diabetes, hypertension, and non-alcoholic fatty liver disease [[43]1].
   Unhealthy expansion of adipose tissue in obesity promotes disease
   through complex and multifactorial mechanisms. Adipose tissue mass is
   maintained through a balance between lipolysis and lipid storage.
   Disruption of this balance can lead to storage of lipids in other
   tissues, such as liver and skeletal muscle, impairing their functions
   via lipotoxicity [[44]2]. The modern western diet contains a high
   proportion of calories from energy-rich fats, and in the setting of
   positive energy balance, these lipids are stored in adipose tissue,
   increasing susceptibility to obesity. In such a state of lipid excess,
   the role of lipogenesis, the endogenous synthesis of lipids from simple
   carbon precursors, is not obvious.

   Increasing evidence implicates peroxisomes, organelles specialized for
   lipid metabolism, including lipid synthesis, in metabolic regulation
   [[45]3]. Peroxisomes are versatile metabolic organelles involved in
   ether lipid biosynthesis, fatty acid oxidation, bile acid synthesis,
   and reactive oxygen species production and neutralization [[46]4]. The
   importance of peroxisomes to mammalian physiology is demonstrated by
   the deleterious phenotypes associated with Zellweger spectrum
   peroxisome biogenesis disorders [[47]5]. Ether lipids, such as
   plasmalogens, have recently been shown to regulate ferroptosis
   susceptibility, extend Caenorhabditis elegans lifespan, and function as
   cellular antioxidants [[48][6], [49][7], [50][8]]. Plasmalogens are
   also important for assembly of lipid raft microdomains,
   cholesterol-rich membrane regions involved in cellular signaling
   [[51]9]. Plasmalogen deficiency leads to disruption of lipid rafts and
   cholesterol mislocalization to a perinuclear compartment [[52]10].
   However, the broader physiological roles of these peroxisome-derived
   lipids are unclear. Of note, how peroxisomal lipid synthesis affects
   metabolic homeostasis remains unknown.

   Peroxisomes use the glycolytic intermediate dihydroxyacetone phosphate
   (DHAP) as a substrate for lipid biosynthesis. When conditions favor the
   routing of carbon away from glycolysis and towards peroxisomal ether
   lipid synthesis, peroxisomal membrane protein 2 (encoded by Pxmp2)
   shuttles DHAP into peroxisomes for entry into the acyl DHAP pathway,
   localized in peroxisomes and the endoplasmic reticulum (ER), for
   production of ether lipids and other phospholipid species. In the
   peroxisomal component of this pathway, DHAP is converted into
   acyl-DHAP, which may subsequently have its acyl group exchanged for an
   alkyl group to yield alkyl-DHAP. In the final peroxisome-localized step
   of this pathway, the sn-1 carbonyl of acyl- or alkyl-DHAP is reduced by
   acyl/alkyl DHAP reductase [[53]4]. We previously reported that this
   protein is encoded by Dhrs7b and renamed the protein PexRAP
   (Peroxisomal Reductase Activating PPARγ) due to its role in generating
   partial agonists for the nuclear receptor PPARγ [[54]11].

   PexRAP-catalyzed reduction of acyl- or alkyl-DHAP yields
   lysophosphatidic acid (LPA), a diacyl phospholipid precursor, or
   1-O-Alkyl-G3P (AGP), an ether lipid precursor, respectively. PexRAP is
   localized to both ER and peroxisomes and appears to have distinct,
   location-dependent preferences for acyl-versus alkyl-DHAP in HeLa