Mitochondrial Lon protease at the crossroads of oxidative stress, ageing and cancer

Abstract

Lon protease is a nuclear DNA-encoded mitochondrial enzyme highly conserved throughout evolution, involved in the degradation of damaged and oxidized proteins of the mitochondrial matrix, in the correct folding of proteins imported in mitochondria, and in the maintenance of mitochondrial DNA. Lon expression is induced by various stimuli, including hypoxia and reactive oxygen species, and provides protection against cell stress. Lon down-regulation is associated with ageing and with cell senescence, while up-regulation is observed in tumour cells, and is correlated with a more aggressive phenotype of cancer. Lon up-regulation contributes to metabolic reprogramming observed in cancer, favours the switch from a respiratory to a glycolytic metabolism, helping cancer cell survival in the tumour microenvironment, and contributes to epithelial to mesenchymal transition. Silencing of Lon, or pharmacological inhibition of its activity, causes cell death in various cancer cells. Thus, Lon can be included in the growing class of proteins that are not responsible for oncogenic transformation, but that are essential for survival and proliferation of cancer cells, and that can be considered as a new target for development of anticancer drugs.

Keywords: Hypoxia; LONP1; Mitochondria; Pim1; mtDNA.

Fig. 1

Structure of Lon proteases. Lon proteases can be divided in two subfamilies, on the basis or their structure and sequence homology: LonA (its members are present in eubacteria and eukarya, which include human mitochondrial Lon) and LonB (present in archea). The N domain, present only in LonA subfamily, is involved in protein substrate binding and in the oligomerization of the enzyme; the AAA+ domain has an ATPase activity, and the carboxyl-terminal domain (P domain) contains the Ser-Lys dyad at the proteolytic active site. LonB subfamily members can have an additional transmembrane (TM) domain inserted in the AAA+ region, which anchors the protein to the membrane. A mitochondrial-targeting sequence (MTS) is present at the N term of the pre-protein synthesized in eukaryotic cells, which is removed by proteolytic cleavage after importing in mitochondria

Fig. 2

Lon functions in mitochondria of human cells. Lon exerts three main functions in the matrix of human mitochondria: (i) it degrades damaged or oxidized proteins, such as aconitase (Aco2), glutaminase C (GLS-1), steroidogenic acute regulatory protein (StAR), 5-aminolevulinic acid synthase (ALAS-1), Cystathionine β-synthase (CBS); (ii) it regulates mitochondrial metabolism by selective degradation of, and interaction with some subunits of OXPHOS complexes; (iii) it regulates mtDNA replication and maintenance by degrading the phosphorylated form of TFAM

Fig. 3

Effects of changes of Lon expression in cancer cells. Changes in Lon expression have several effects in cancer cells. Lon knockdown leads to: reduction of oxygen consumption rate (OCR), decrease of mitochondrial DNA content (mtDNA), increase of reactive oxygen species (ROS), increased susceptibility to undergo apoptosis and cellular senescence. Lon overexpression affects tumour bioenergetics by inducing OXPHOS remodelling and by shifting energy production to glycolysis. This leads to increased stress adaptations that favours tumour aggressiveness and epithelial–mesenchymal transition (EMT)