2015 Volume 38 Issue 6 Pages 795-808
Onset of cancer and neurodegenerative disease occurs by abnormal cell growth and neuronal cell death, respectively, and the number of patients with both diseases has been increasing in parallel with an increase in mean lifetime, especially in developed countries. Although both diseases are sporadic, about 10% of the diseases are genetically inherited, and analyses of such familial forms of gene products have contributed to an understanding of the molecular mechanisms underlying the onset and pathogenesis of these diseases. I have been working on c-myc, a protooncogene, for a long time and identified various c-Myc-binding proteins that play roles in c-Myc-derived tumorigenesis. Among these proteins, some proteins have been found to be also responsible for the onset of neurodegenerative diseases, including Parkinson’s disease, retinitis pigmentosa and cerebellar atrophy. In this review, I summarize our findings indicating the common mechanisms of onset between cancer and neurodegenerative diseases, with a focus on genes such as DJ-1 and Myc-Modulator 1 (MM-1) and signaling pathways that contribute to the onset and pathogenesis of cancer and neurodegenerative diseases.
Products of oncogenes and tumor suppressor genes play pivotal roles in regulation of cell growth, differentiation and death, and aberrant activation of oncogene products and inactivation of tumor suppressor proteins lead to abnormal cell growth, resulting in the onset of cancer. While mutation in both alleles of an oncogene is necessary for tumorigenesis, mutation in either allele of a tumor suppressor gene is sufficient. Onset and pathogenesis of neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease (PD) and Huntington disease, are affected by environmental and genetic factors that lead to abrogation of synaptic function, culminating in neuronal cell death. Although exits of cancer and neurodegenerative diseases are completely opposite, there are some common points of view between both diseases, including growth and death signaling pathways. Diseases are categorized by two types, non-inherited (sporadic) and inherited (familial) cases, and investigation of products of causative genes for familial cases has contributed to the unraveling of molecular mechanisms underlying the onsets of diseases. Furthermore, recent findings, including ours, have shown that some genes are responsible for both cancer and neurodegenerative diseases, thereby advocating the scenario of “common mechanisms of onsets of cancer and neurodegenerative diseases.”
I have been working on oncogene c-myc for a long time and identified various c-Myc-binding proteins. Of the proteins identified, some proteins have been found to directly affect the onset and pathogenesis of both cancer and neurodegenerative diseases. In this review, I show such cases, focusing on DJ-1 and Myc-Modulator 1 (MM-1), and discuss the common mechanism between cancer and neurodegenerative diseases.
The c-myc gene is a protooncogene, and its gene product c-Myc is comprised of 439 amino acids and harbors two domains, Myc-box I/II and basic-helix-loop-helix-leucine zipper (bHLHzip) at the N-terminus and C-terminus, respectively (Fig. 1). c-Myc is a transcription factor and binds to the E-box sequence, CAC GTG, through its basic-helix-loop-helix region after dimerization with another bHLHzip protein, Max, via the leucine zipper structure.1) In addition to the bHLHzip region, Myc-boxes I and II are also essential for transcription activity of c-Myc through association with the co-activator complex.2–4) c-Myc is known to regulate transcription of more than 50% of genes present in cells, including genes related to metabolism, ribosomal synthesis and progression of the cell cycle, thereby leading to increase of cell mass.3–5)
c-Myc comprises two functional domains at the N-terminus and C-terminus. Max binds to the leucine zipper structure to bind to the E-box sequence in the target genes. MM-1 binds to Myc-box II to repress transcriptional activity of c-Myc after recruiting the co-repressor complex containing HDAC. Repressive activity of MM-1 toward c-Myc activity is lost by the A157R mutant, and c-Myc is aberrantly activated, leading to the onset of cancer. MBI: Myc-box I; MBII: Myc-box II; b: basic region; HLH: helix-loop-helix; zip: leucine zipper.
We have found for the first time that c-Myc is a factor that regulates initiation of DNA replication6) and that c-Myc binds to one of the replication origins located at the region upstream of the c-myc gene, which also acts as a transcriptional enhancer, in association with origin recognition complex (ORC) and sequence-specific DNA-binding protein Myc Single-Strand binding Proteins (MSSP), one of the c-Myc-binding proteins we identified.7–11) This case is also true for the replication origin/enhancer in the heat shock protein 70 gene to which c-Myc/NF-Y binds,12) giving rise to the idea of a concerted mechanism of DNA replication and transcription.13,14) Although DNA replication function of c-Myc had been debated for more than 10 years after our proposal,15) many reports supporting DNA replication function of c-Myc have been published and it has finally been corroborated.16)
Amplification and mutation in the c-myc gene have been observed in almost all cancer cells. Translocation of the c-myc gene to other genes has also been observed in Burkitt lymphoma, acute lymphoblastic leukemia and multiple myeloma.17) Of the mutations in c-Myc, threonine 58 (T58) and serine 62 (S62) located in Myc-box 1 have frequently been mutated in cancer cells. c-Myc is an unstable protein and its half-life is only about 20 min.18) Although c-Myc is degraded by the ubiquitin–proteasome system after the cell cycle progresses to the S-phase, mutations of T58 and S62 render c-Myc stable, and the cell cycle-promoting activity of c-Myc therefore continues, resulting in the onset of cancer.19) Although mutation in Myc-box II in cancer cells is rare, Myc-box II is essential for the transforming activity of c-Myc. Transforming activity of c-Myc is lost after mutation in Myc-box II, and c-Myc lacking Myc-box I but containing Myc-box II still possesses transforming activity.20) Thus, both Myc-boxes I and II are necessary for transcriptional activity, but Myc-box II is sufficient for transforming activity of c-Myc. As described above, c-Myc with Max binds to the E-box sequence and positively regulates transcription of E-box-containing genes. In addition, c-Myc indirectly regulates transcription or DNA replication by binding to other DNA-binding factors. There are several cascades that lead to activation or repression of c-Myc activity. To investigate c-Myc function and c-Myc cascades, c-Myc-binding proteins have been identified by using various strategies, including two-hybrid screening and immunoprecipitation followed by mass spectrometry, and more than 30 c-Myc-binding proteins have been identified.21) These proteins are roughly divided into two groups, proteins binding to Myc-box I/II and proteins binding to bHLHzip, and they positively or negatively modulate transcription activity and tumorigenicity of c-Myc.
We have identified approximately 10 c-Myc-binding proteins. Among them, novel proteins are MM-1 and Associate of MYC-1 (AMY-1), which bind to Myc-box II, and MSSP, pTwenty-One and CDK-associated protein-1 (TOK-1) and PAP-1-Associated protein-1 (PAPA-1), which bind to bHLHzip. While MM-1 and TOK-1 negatively regulate c-Myc activity, AMY-1 and MSSP positively regulate c-Myc activity.9,22–25) We have also identified novel c-Myc-modulator proteins, DJ-1 and Pim-1 Associated Protein-1 (PAP-1).26,27) Since MM-1, PAP-1 and DJ-1 were found to be related to onsets of neurodegenerative diseases, I hereafter focus on these three proteins.
MM-1 was isolated by two-hybrid screening using c-Myc as a bait in 1998 and found to bind to Myc-box II.22) MM-1 contains four splicing variants, MM-1α, MM-1β, MM-1γ and MM-1δ from the sequence of chromosome 12, and subcellular localization of MM-1 splicing variants differs in the cytoplasm or nucleus.28) The first isolated MM-1 is MM-1α with additional 13 amino acids from the sequence of chromosome 14 at the N-terminus, but the function of MM-1 has been found to be the same as that of MM-1α. I therefore use MM-1α as MM-1 in this review.
MM-1 negatively regulates transcriptional activity of c-Myc through the TIF1β/mSin3 co-repressor complex,29) and one of the c-Myc/MM-1 target genes is protooncogene c-fms30) (Figs. 1, 2). Missense mutation of amino acid number 157 from alanine to arginine (A157R) was observed in about 50–60% of patients with leukemia and lymphoma and in more than 75% of patients with squamous cell carcinoma of tongue cancer, and A157R lost the activities to repress both the E-box-dependent transcriptional activity of c-Myc and the myc/ras cooperative transforming activity, indicating that MM-1 is a tumor suppressor.31) MM-1 also negatively regulates expression of the wnt4 gene through binding to Egr-1 on the wnt4 gene promoter. The wnt4 gene is located at the top of the Wnt signal that leads to activation of c-myc expression. MM-1 therefore indirectly represses c-myc expression via the Wnt signal.32) Furthermore, MM-1 directly binds to a subunit of the proteasome and to Rabring7, a modulator of ubiquitin ligase, thereby facilitating ubiquitination and subsequent degradation of c-Myc.33,34) Thus, MM-1 represses c-Myc activity in various ways in the nucleus.
Prefoldin is a molecular chaperone comprised of 6 subunits, PFD1–6, and PFD5 has been found to be MM-1.35,36) Prefoldin assists folding of newly synthesized proteins such as actin and tubulin after proteins have been transferred to chaperonin in the cytoplasm. We found that prefoldin inhibited oligomer formation of pathogenic Huntingtin and α-synuclein, causative gene products for Huntington disease and PD, respectively, thereby inhibiting Huntingtin- and α-synuclein-induced neuronal cell death (Fig. 2). Knockdown of MM-1/PFD5 resulted in disruption of the prefoldin complex and loss of protective activity against oligomer formation of pathogenic Huntingtin and α-synuclein.37–39) Prefoldin also modulates aggregation of Amyloid-β (Aβ).40,41) Furthermore, mice possessing a missense mutation in MM-1α at amino acid number 110 from leucine to arginine (L110R) exhibit various symptoms, including degeneration of the cerebellum and retina and male infertility.42) The levels of ubiquitinated proteins and cytotoxicity were higher in L110R MM-1 cells than in wild-type cells under normal conditions and were increased by various stresses. The level of polyubiquitinated protein aggregation was increased in the brains of L110R MM-1α mice.43) These results suggest that MM-1-containing prefoldin plays a role in quality control against protein aggregation and that dysfunction of prefoldin is one of the causes of neurodegenerative diseases.
MM-1 acts as a prefoldin subunit, prefoldin 5, in the cytoplasm. Prefoldin inhibits oligomer formation of pathogenic Huntingtin and α-synuclein, causative gene products of Huntington’s disease and Parkinson’s disease, respectively. MM-1 acts as a tumor suppressor in the nucleus as shown in Fig. 1.
PAP-1 has been isolated by two-hybrid screening using Pim-1 as a bait.27) The Pim-1 gene is an oncogene that cooperates with c-myc in tumorigenesis. Since Pim-1 and c-Myc are usually located in the cytoplasm and nucleus, respectively, and since both proteins are not directly associated, we have screened for Pim-1- and c-Myc-binding proteins and identified two novel proteins, PAP-1 and PAPA-1. Pim-1 directly binds to PAP-1, and PAPA-1 directly binds to PAP-1 and c-Myc, indicating a Pim-1 cascade from Pim-1, PAP-1, PAPA-1 and c-Myc. Missense mutations in the RP9 gene, a causative gene for autonomously dominant inherited retinitis pigmentosa, have been identified, and it has been shown that the PAP-1 gene is identical to the RP9 gene.44) Retinitis pigmentosa is a neurodegenerative disease that occurs in the retina in which retinal apoptosis is elicited. We found that PAP-1 is a transcriptional splicing factor located in the spliceosome and that mutations of PAP-1 observed in RP9 patients disturb correct splicing.45,46) Although the molecular mechanism underlying the onset of retinitis pigmentosa by PAP-1 mutations is still not known, it is thought that PAP-1 regulates splicing of the gene(s) that is expressed in the retina and is responsible for formation or function of the retina and that mutation of PAP-1 compromises proper splicing of the gene, leading to retinal apoptosis.
DJ-1 was isolated in the course of screening for c-Myc-binding proteins in 1997.26) Although DJ-1 does not directly bind to c-Myc, the DJ-1 gene was found to transform NIH3T3 cells in cooperation with the activated ras gene, indicating that the DJ-1 gene is a novel oncogene.26) DJ-1 also accelerates transformation by c-myc through activating the Erk pathway.47) DJ-1 is localized in the cytoplasm, nucleus and mitochondria26,48) and is secreted into culture medium49–51) or serum,52–59) cerebrospinal fluid,53,54,60) saliva61,62) and nipple fluid.63) DJ-1 is translocated from the cytoplasm to nucleus upon addition of a mitogen to the culture medium26) and is translocated to mitochondria after oxidative stress.64)
Increased levels of DJ-1 expression have been observed in various cancer cells and tissues, including breast,51,52) prostate,65,66) lung,67,68) and endometrial,69,70) thyroid,71,72) pancreas,73–76) esophageal,77) ovary,78,79) cervical,80) liver,81) gastric,82,83) supraglottic84) cancers, cholangiocarcinoma,85) glioma/glioblastomas,86,87) laryngeal carcinoma,88) bladder carcinoma89) and melanoma57,90) and leukemia.91,92) Secretion of DJ-1 into the serum or nipple fluid has been found in patients with breast cancer.52,63) Increased levels of DJ-1 expression in cancer cells are parallel to severity of cancer with poor prognosis, including metastasis and invasion.82,83,87,90,93–98)
PD is a progressive neuronal degenerative disease. Since dopaminergic neuron death occurs in the substantia nigra pars compacta, the level of dopamine in the stratus is reduced, leading to movement dysfunction, a phenomenon of PD. Although the molecular mechanism of the onset of PD is still not fully understood, it is thought that oxidative stress and mitochondrial dysfunction are the main causes of PD.99–102) Oxidative stress compromises mitochondrial function, which results in the production of a large amount of reactive oxygen species (ROS), thus triggering activation of cell death pathways and inducing oxidation of proteins. Proteins oxidized by ROS become insoluble. When the activity of protein degradation systems such as the ubiquitin–proteasome system is reduced, insoluble proteins are aggregated, and the aggregations are degraded by chaperone-mediated autophagy. Damaged mitochondria, on the other hand, are degraded by mitochondria-specific autophagy (mitophagy) as described below. When protective systems against oxidative stress and against harmful aggregated proteins are hampered, synaptic dysfunction followed by neuronal cell death is induced. Recent studies have shown that causative gene products for familial PD play critical roles in specific steps during the onset of PD99,103,104) (Fig. 3).
When neuronal cells receive oxidative stress, mitochondria, especially complex I of the respiration complexes, are compromised, facilitating production of large amounts of reactive oxygen species (ROS). ROS enhance oxidative stress to cells, leading to a vicious cycle of oxidative stress. ROS injure and oxidize proteins, making the proteins insoluble. When activity of protein degradation systems such as the ubiquitin–proteasome system is reduced, insoluble proteins are aggregated. Aggregated proteins are degraded by chaperone-mediated autophagy. Damaged mitochondria are degraded by mitophagy. Proteins identified as causative gene products in a familial form of PD play crucial roles in each step during the onset of PD. Reduction and loss of functions of these proteins induce synaptic dysfunction followed by neuronal cell death.
In 2003, mutations, including a large deletion and homozygous missense mutation, were observed in the park7 locus in patients with a familial form of PD, and the DJ-1 gene was identified as park7.105) This is the first case of an oncogene also being a causative gene for a neurodegenerative disease. More than 10 homozygous and heterozygous mutations have been identified in the DJ-1 gene (see the database, http://www.molgen.ua.ac.be/PDmutDB/).106,107) Gene product DJ-1 is comprised of 189 amino acids without a specific domain and it works as a homodimer.108–111) DJ-1 has three cysteine residues at amino acid numbers 46, 53 and 106 (C46, C53 and C106, respectively). DJ-1 is a sensor protein against various stresses, including oxidative stress. When cells receive oxidative stress, C106 of DJ-1 is first oxidized from SH (reduced form) to SOH, SO2H and SO3H forms, and then the other cysteine residues C46 and C53 are oxidized112–114) (Fig. 4). DJ-1 at C106 with SO3H is believed to be an inactive form of DJ-1, and accumulation of excessively oxidized DJ-1 has been observed in brains of patients with sporadic PD, Alzheimer’s disease and Huntington’s disease,115–117) suggesting that DJ-1 participates in the onsets of at least both familial PD and sporadic PD.
DJ-1 has three cysteine residues located at amino acid numbers 46, 54 and 106 (C46, C53 and C106). Of the three cysteine residues, C106 is the most sensitive to oxidative stress and is oxidized from SH to SOH, SO2H and SO3H, and then C46 and C53 are oxidized. This figure was a modified version of Fig. 2 in our previous manuscript.120)
DJ-1 is a multi-functional protein,118–120) having functions in transcriptional regulation,121–135) anti-oxidative stress reaction,64,112,113,136,137) mitochondrial regulation,48,138–146) regulation of signal transduction,96,123,132,147–152) and functions as a protease,58,153–155) a chaperone156,157) and a deglycase158) (Fig. 5). Aberrant activation of and loss of DJ-1 function are thought to lead to onsets of cancer and oxidative stress-related diseases, including PD,105) stroke,159,160) male infertility,161–163) chronic obstructive pulmonary disease (COPD)164) and type 2 diabetes165) (Fig. 5).
DJ-1 is a multifunctional protein, and excess activation and loss of function of DJ-1 activity are related to onset of cancer and various oxidative stress-related diseases, respectively. FAP: familial amyloid polyneuropathy; COPD: chronic obstructive pulmonary disease. This figure was a modified version of Fig. 1 in our previous manuscript.120)
The main function of DJ-1 is thought to be an anti-oxidative stress function. There are several ways for DJ-1 to exert an anti-oxidative stress function: 1) DJ-1 quenches ROS through self-oxidation of its cysteine residues,113,114) 2) DJ-1 activates and stabilizes nuclear factor erythroid-2 related factor 2 (Nrf2), a master transcription factor for redox and detoxification-related genes, through sequestering Keap1, an inhibitor of Nrf2, leading to degradation of Nrf2 by the ubiquitin–proteasome system,126) and 3) DJ-1 upregulates the expression of superoxide dismutase 3 (SOD3) and glutathione ligase, anti-oxidant enzymes, by an unknown mechanism.166,167) Furthermore, DJ-1 directly binds to SOD1 to regulate its activity.168–170) Although DJ-1 is an abundant protein in cells, the contribution level of reduction of oxidative stress by quenching ROS through self-oxidation of DJ-1 is thought to be less than 10% of the total contribution, and aforementioned reactions 2) and 3) contribute to the rest of the anti-oxidative stress function. DJ-1 mutants observed in PD patients have loss of or weaken anti-oxidative stress activity,171) and DJ-1-knockout mice are more vulnerable to neuronal toxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydr (MPTP) than are wild-type mice.172) DJ-1 is expressed in both neuron and glia,115,173,174) and the secretion level of DJ-1 from glia is higher than that from neuron.50) Although it is possible that DJ-1 secreted from glia protects neuron from oxidative stress-induced cell death,160,175,176) there is no direct evidence so far.
DJ-1 binds to various transcription factors to act as a co-activator or co-repressor for regulating their target genes without direct binding to DNA. DJ-1-binding transcription factors or modified proteins identified so far include p53,123,128,132) androgen receptor and its regulatory proteins,121,122,127) polypyrimidine tract-binding protein-associated splicing factor (PSF),125) Keap1, an inhibitor for nuclear factor erythroid-2 related factor 2 (Nrf2),126) sterol regulatory element-binding protein (SREBP),131) Ras-responsive element-binding protein (RREB1)133) and signal transducer and activator of transcription 1 (STAT1)135) to modulate their transcriptional activity, thereby affecting various cell functions. Positive regulation of SREBP1 and RREB1 functions by DJ-1 may participate in the onset and pathogenesis of metabolic syndrome.131,133) Indeed, recent studies have shown relationships of DJ-1 with glucose, hypertension and obesity markers.165,177–183)
As for Parkinson’s disease, dopamine is synthesized from tyrosine by two enzymes, tyrosine hydroxylase (TH) and dopamine decarboxylase (DDC), and packed into vesicles by vesicular monoamine transporter 2 (VMAT2). Expression of the TH gene is negatively regulated by PSF, a repressor of transcription.125) We found that DJ-1 positively regulates TH gene expression by sequestering PSF from the TH gene promoter in human cells but not in mouse cells due to the presence of the PSF recognition sequence in the human TH gene but not in the mouse TH gene.130) Furthermore, we found that DJ-1 directly binds to TH and DDC proteins to activate these enzymatic activities in an oxidative status of C106 of DJ-1-dependent manner184) and that DJ-1 positively regulates VMAT2 by upregulating VMAT2 gene expression and by direct binding to VMAT2 protein.185) These results suggest that DJ-1 directly regulates biosynthesis and secretion of dopamine.
DJ-1 binds to various proteins that play roles in signaling pathways (Fig. 6). DJ-1 binds to p53, a tumor suppressor protein and transcription factor, to modulate p53 activity. p53 is well known to respond to various stresses. When cells are exposed to ultraviolet light (UV), for instance, the expression level of p53 is increased, and p53 is activated by its phosphorylation. Activated p53 attenuates cell growth (G1 arrest) to check the injured level of DNA and facilitates repair of injured DNA. When the injured level of DNA is too severe, p53 up-regulates pro-apoptotic genes such as Bax gene to kill cells containing damaged DNA. DJ-1 modulates p53 activity in two ways: DJ-1 binds to Topors/p53-binding protein3, which is small ubiquitin-related modifier-1 (SUMO-1) ligase. When p53 is sumoylated by Topors, p53 activity is compromised, and Topors-inhibited p53 activity is restored by DJ-1 through the canceling binding of Topors to p53.123) SUMO-1 conjugation modifies the activities of many proteins, including PSF described above,125) and DJ-1 regulates sumoylation levels of proteins by binding to SUMO-1 ligases such as Topors and protein inhibitor of activated STAT (PIAS) family proteins.121,123) DJ-1 itself is sumoylated at lysine 106. Sumoylation of DJ-1 is necessary for full activities of DJ-1, and L166P DJ-1, a pathogenic mutant DJ-1 observed in a PD patient, is excessively sumoylated, resulting in aggregation.186) The other way for DJ-1 to modulate p53 is by directly binding to the DNA-binding region of p53 and inhibiting p53 activity in both oxidative status of C106 of DJ-1- and DNA-binding affinity of p53-dependent manners.132) In this case, DJ-1 inhibits expression of the p53-target gene DUSP1 encoding protein phosphatase, resulting in activation of Erk1/2 and thereby stimulating cell growth.132)
DJ-1 binds to various proteins located in cell growth and cell death pathways and affects positively and negatively cell growth and cell death pathways, respectively.
When cells receive signals from growth factors, their receptors with tyrosine kinase activity are activated by self-phosphorylation. Activated receptors then phosphorylate adaptor proteins to transduce growth signals to Ras or phosphoinositide 3-kinase (PI3K), and Ras or PI3K activates Erk and Akt pathways, respectively, to stimulate cell growth and inhibit apoptosis (Fig. 6). Phosphatase and tensin homolog deleted on chromosome10 (PTEN) is a lipid phosphatase and is a negative regulator of the PI3K/Akt pathway. Under an oxidative stress condition, weakly oxidized DJ-1 inhibits PTEN phosphatase activity by direct interaction with PTEN to activate Akt.147,148) When cells receive a signal from a growth factor such as epidermal growth factor (EGF), DJ-1 activates the Erk pathway by an unknown mechanism.96,149) Thus, DJ-1 facilitates cell growth by activating both Akt and Erk pathways. DJ-1 has been identified as an oncogene in cooperation with activated ras.26) Since Ras activates the Erk pathway, it is thought that the mechanism underlying ras-dependent transforming activity of DJ-1 is activation of the Erk pathway. Death associated protein (Daxx) and homeodomain interacting protein kinase 1 (HIPK1) are activators of apoptosis-activating kinase 1 (ASK1) and stimulate ASK1 in the cytoplasm upon the apoptosis signal.187,188) DJ-1 binds to Daxx in the nucleus to inhibit translocation of Daxx into the cytoplasm.150) DJ-1 also binds to ASK1 and HIPK1.151,152) All of these associations of DJ-1 with Daxx, ASK1 and HIPK1 inhibit the apoptosis-inducing activity of ASK1, leading to cell growth (Fig. 6).
These cell growth and apoptosis signals have been known to be used both in cancer and neurodegenerative diseases. In animal models of and patients with Parkinson’s disease, for instance, highly oxidized DJ-1 is accumulated concomitantly with activation of PTEN, thus reducing Akt and Erk activities and thereby stimulating the apoptosis signaling pathway. In cancer cells, on the other hand, an elevated expression level of DJ-1 is observed with inactivation of PTEN, thus increasing Akt and Erk activities, culminating in the stimulation of cell growth. Also, opposite phenomena of p53 pathways have been reported in cancer and neurodegenerative diseases.
A small amount of DJ-1 is localized in the outer and inner membranes of mitochondria under normal conditions, and DJ-1 is translocated to mitochondria upon oxidative stress.48,64,145) Mitochondria with abnormal morphology such as fragmented mitochondria and with reduced mitochondrial membrane potential are observed in DJ-1-knockout mice.142,143) We have found that DJ-1 binds to subunits of mitochondrial complex I in respiration complexes, enhances its activity, and is co-localized with mitochondrial complex I in the mitochondrial inner membrane.48) Furthermore, the N-terminal 12 amino acids are necessary for DJ-1 to localize to mitochondria, and pathogenic mutants of DJ-1 such as L166P and M26I DJ-1 are localized in mitochondria as monomers.145) Since dimer formation is necessary for DJ-1 to exert its activity, it is reasonable that monomers of L166P and M26I DJ-1 are inactive. DJ-1 also binds to pyrroline-5-carboxylate reductase 1 (PYCR1), a mitochondrial enzyme that catalyzes the last step in proline biosynthesis and responds to oxidative stress, to protect cells from oxidative stress-induced cell death.146) DJ-1 is associated with PGC-1α, a master regulator of mitochondrial gene expression, and enhances the expression of oxidative stress-related genes.189) All of the findings described above suggest a role of DJ-1 in quality control of mitochondria.
Damaged mitochondria are degraded by the mitochondria-specific autophagy system called mitophagy. When mitochondria are injured, mitochondrial membrane potential is reduced. Parkin, a park2 gene product and ubiquitin ligase, is recruited to the outer mitochondrial membrane in damaged mitochondria and is activated through its phosphorylation by Pink1, a park6 gene product and protein kinase. Activated Parkin then ubiquitinates outer mitochondrial membrane-resident proteins, including mitofusion 2 and voltage-dependent anion-selective channel protein 1 (VDAC1), and recruits the macroautophagy system in which damaged mitochondria are engulfed.190) The role of DJ-1 in mitophagy is not clear and is still in debate: some studies have shown that DJ-1 is essential for the basal level of autophagy140) and facilitates Parkin/Pink1-mediated mitophagy.141–143) Further studies are needed to clarify the role of DJ-1 in mitophagy.
Symptomatic treatments have been used for neurodegenerative diseases. Since dopamine level is decreased in the striatum of PD brains and since dopamine does not penetrate the blood brain barrier (BBB), a precursor and releaser of dopamine or an inhibitor of the enzyme that breaks down dopamine have been used for PD therapy. However, neuronal cell death still occurs during these therapies. A drug(s) that inhibits oxidative stress-induced neuronal cell death is therefore needed.
We first examined the effect of recombinant DJ-1 protein on protective activity against toxin 6-hydroxydopamine (6-OHDA)-induced PD in model rats. When 6-OHDA is injected into the substantia nigra, neuronal cell death in both the substantia nigra and striatum occurs, and subsequent locomotion defect is observed. Co-injection of 6-OHDA with recombinant wild-type DJ-1, but not with L166P DJ-1, into the substantia nigra ameliorated such PD symptoms.191) Administration of recombinant DJ-1 also protected against neurodegeneration caused by focal cerebral ischemia and reperfusion in rats, a stroke model of rats.160) These findings suggest that DJ-1 itself is a target for PD and stroke therapies.
As described above, all of the activities of DJ-1 depend on the oxidative level of C106, and excessively oxidized DJ-1, an inactive form of DJ-1, has been observed in brains of patients with PD, Alzheimer’s disease and Huntington’s disease.115–117) Therefore, it is thought that a drug(s) inhibiting excess oxidation of C106 of DJ-1 may serve as a useful drug for PD therapy. We first clarified the X-ray crystal structure of DJ-1 and found that there is a pocket around C106 of DJ-1.108) After in silico screening for DJ-1-binding compounds using a university compound library and Zinc library, various compounds, including compound B/UCP0054278, compound A/UCP0045037 and compound-23, were obtained192–195) (Fig. 7). These compounds inhibit excess oxidation of C106 of DJ-1 and maintain the reduced form DJ-1, thereby inhibiting oxidative stress-induced neuronal cell death and mitigating locomotion defect in mouse and rat PD models concomitantly with increased TH and mitochondrial complex I activities. These compounds are able to penetrate the BBB192,195,196) and are also effective in stroke model rats.197) Protective activities of DJ-1-binding compounds are lost in DJ-1-knockdown or -knockout cells and in DJ-1-knockout mice, indicating the DJ-1-dependency of compounds.192,195,198) These DJ-1-binding compounds are promising for wide ranges of neurodegenerative diseases and need further studies such as a study on their structure–activity relationship to improve their activities.
A. Crystal structure of DJ-1. DJ-1 works as a dimer, and there is a pocket around the C106 region. Compounds that bind to the C106 region have been identified by in silico screening. B. Chemical structures of two DJ-1-binding compounds, compound B and compound-23.
A low level of ROS acts as a second messenger to promote cell growth. A high level of ROS, however, damages DNA, proteins and lipids and compromises mitochondrial function, leading to the onset of various diseases, including cancer and neurodegenerative diseases. Human cells possess defense systems against ROS and ROS-induced cell death, and genes/proteins that have been identified as causes for either cancer or neurodegenerative diseases play crucial roles in these defense systems. In this review, I introduced these players with focus on our findings. In addition, common players in cancer or neurodegenerative diseases that have so far been identified are listed in Table 1. Although the precise mechanisms of commonality and difference between these diseases have just started to be investigated, investigation of these mechanisms is expected to lead to the development of a drug(s) for therapies of mostly inevitable diseases such as cancer and neurodegenerative diseases.
I would like to thank all of the members who have participated in our projects.
The author declares no conflict of interest.