Disruption of peroxisome function leads to metabolic stress, mTOR inhibition, and lethality in liver cancer cells

Highlights

• Peroxisome function is essential in liver cancer cells.

• Normal peroxisome function maintains the activity of mTORC1.

• Autophagy in liver cancer cells is induced by dysfunctional peroxisomes.

Abstract

Peroxisome houses a large number of enzymes involved in lipid and phytochemical oxidation as well as synthesis of bile acid and other specialized lipids. Peroxisome resident enzymes are imported into the organelle via a conserved cargo transport system composed of many peroxins, protein factors essential for the biogenesis of peroxisome. Among the peroxins, PEX5 plays a transporter role, and PEX2, 10, and 12 are thought to form a complex that functions as an E3 ubiquitin ligase to help recycle PEX5 in an ubiquitin modification-dependent process. Previous studies have demonstrated the importance of peroxins in postnatal development especially the development of nerve systems. These studies also show that peroxins or the function of peroxisomes is dispensable for cellular viability. In contrast, however, we report here that PEX2 and other peroxins are essential for the viability of liver cancer cells, probably through altering metabolism and signaling pathways. Our results suggest that peroxins may be potential targets of therapeutics against liver cancer.

Introduction

Peroxisome is a cellular organelle that performs diverse metabolic functions in virtually all eukaryotic cells, including the α- and β-oxidation of fatty acids, cholesterol metabolism, the biosynthesis of bile acids and ether phospholipids, and metabolism of reactive oxygen species (ROS) [1,2]. The biogenesis of peroxisome occurs either by de novo production through budding off the endoreticulum or by the growth and fission of existing organelle [3]. The importation of matrix proteins into peroxisome is carried out by a group of proteins named peroxins (PEXs). PEX5 and PEX7, are carriers which recognize peroxisome targeting signal (PTS1 and PTS2, respectively) in the cargo proteins and transport cargos from cytoplasm onto the surface of peroxisome where they interact with other peroxins to release the cargo into a putative tunnel formed by PEX14 and PEX13, and leave the organelle for another round of importation [[4], [5], [6]]. Although the mechanisms of both cargo and PEX5 (and PEX7) release are still sketchy, it is clear that ubiquitin modification plays an essential role in the process [7,8]. PEX2, 10, and 12 form a ubiquitin E3 ligase complex that modifies PEX5 and promotes PEX5 release from peroxisome into cytoplasm [9,10]. The majority of peroxisome matrix proteins carries PTS1 and therefore are PEX5-dependent for their import into peroxisome.

Intact peroxisomal function is critical for human health and normal development, as defects in peroxisome cause a group of heterogeneous and devastating genetic disorders [9]. There are two categories of peroxisomal disorders: single peroxisomal enzyme deficiency and peroxisome biogenesis disorder (PBD). PBD includes two distinct subtypes: rhizomelic chondrodysplasia punctate type 1 and the Zellweger spectrum disorder [11,12], both of which are linked to mutations in peroxin genes [13]. The Zellweger spectrum disorder is a group of disease conditions, including (from severe to mild) Zellweger syndrome (ZS), neonatal adrenoleukodystrophy (NALD), and infantile Refsum disease. PEX2 was the first peroxin found mutated in Zellweger syndrome [14,15]. Most patients with ZS are characterized by brain morphology changes, including cortical gyral abnormalities, focal or generalized leukoencephalopathy, and brain atrophy [12]. These conditions can all be attributed to defects in the myelination of nerves, although how exactly peroxisomes function in myelin sheeth has not been fully characterized [16,17]. Despite the devastating consequences of peroxisome disorders, peroxisome is apparently dispensable for cellular viability as evidenced by the normal growth of peroxisome-free yeasts [18] and cells derived from patients with Zellweger syndrome [13] or mice with peroxin knockouts [19].

Cancer cells often alter metabolism to support their malignant phenotype. The most noticeably changes in cancer cell metabolism is the Warburg effect [20]. Given the important roles played by peroxisome in metabolism, we sought to determine if the function of this organelle is crucial for cancer cells. Here we report that disruption of peroxisome results in profound metabolic alterations in liver cancer cells that eventually lead to their cessation of proliferation.