title: Lipoic acid inhibits the DNA repair protein O 6-methylguanine-DNA methyltransferase (MGMT) and triggers its depletion in colorectal cancer cells with concomitant autophagy induction.
abstract: Alkylating agents are present in food and tobacco smoke, but are also used in cancer chemotherapy, inducing the DNA lesion O (6)-methylguanine. This critical adduct is repaired by O (6)-methylguanine-DNA methyltransferase (MGMT), resulting in MGMT inactivation and degradation. In the present study, we analyzed the effects of the natural disulfide compound lipoic acid (LA) on MGMT in vitro and in colorectal cancer cells. We show that LA, but not its reduced form dihydrolipoic acid, potently inhibits the activity of recombinant MGMT by interfering with its catalytic Cys-145 residue, which was partially reversible by N-acetyl cysteine. Incubation of HCT116 colorectal cancer cells with LA altered their glutathione pool and caused a decline in MGMT activity. This was mirrored by LA-induced depletion of MGMT protein, which was not attributable to changes in MGMT messenger RNA levels. Loss of MGMT protein coincided with LA-induced autophagy, a process resulting in lysosomal degradation of proteins, including presumably MGMT. LA-stimulated autophagy in a p53-independent manner as revealed by the response of isogenic HCT116 cell lines. Knockdown of the crucial autophagy component beclin-1 and chemical inhibitors blocked LA-induced autophagy, but did not abrogate LA-triggered MGMT degradation. Concomitant with MGMT depletion, LA pretreatment resulted in enhanced O (6)-methylguanine levels in DNA. It also increased the cytotoxicity of the alkylating anticancer drug temozolomide in temozolomide-resistant colorectal cancer cells. Taken together, our study showed that the natural compound LA inhibits MGMT and induces autophagy. Furthermore, LA enhanced the cytotoxic effects of temozolomide, which makes it a candidate for a supplement in cancer therapy.
authors.affiliation: Department of Toxicology, University Medical Center Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany.
authors.forename: Anja
authors.initials: A
authors.lastname: Göder
authors.affiliation: Department of Toxicology, University Medical Center Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany.
authors.forename: Georg
authors.initials: G
authors.lastname: Nagel
authors.affiliation: Department of Toxicology, University Medical Center Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Kraus
authors.affiliation: Department of Toxicology, University Medical Center Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany.
authors.forename: Bastian
authors.initials: B
authors.lastname: Dörsam
authors.affiliation: Department of Toxicology, University Medical Center Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany.
authors.forename: Nina
authors.initials: N
authors.lastname: Seiwert
authors.affiliation: Department of Toxicology, University Medical Center Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany.
authors.forename: Bernd
authors.initials: B
authors.lastname: Kaina
authors.affiliation: Department of Toxicology, University Medical Center Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany fahrer@uni-mainz.de.
authors.forename: Jörg
authors.initials: J
authors.lastname: Fahrer
mesh_terms: D000818:Animals; D018906:Antineoplastic Agents, Alkylating / Q000494:pharmacology; D001343:Autophagy / Q000187:drug effects*; D015179:Colorectal Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D003545:Cysteine / Q000378:metabolism; D015254:DNA Modification Methylases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D004260:DNA Repair / Q000187:drug effects; D045643:DNA Repair Enzymes / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D003606:Dacarbazine / Q000031:analogs & derivatives / Q000494:pharmacology; D019008:Drug Resistance, Neoplasm / Q000187:drug effects; D005260:Female; D005978:Glutathione / Q000378:metabolism; D006147:Guanine / Q000031:analogs & derivatives / Q000378:metabolism; D045325:HCT116 Cells / Q000187:drug effects; D006801:Humans; D008297:Male; D008807:Mice, Inbred BALB C; D058990:Molecular Targeted Therapy; D000077204:Temozolomide; D008063:Thioctic Acid / Q000031:analogs & derivatives / Q000494:pharmacology*; D025521:Tumor Suppressor Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D023041:Xenograft Model Antitumor Assays
chemical_list: D018906:Antineoplastic Agents, Alkylating; D025521:Tumor Suppressor Proteins; D006147:Guanine; D008063:Thioctic Acid; D003606:Dacarbazine; C015144:dihydrolipoic acid; C008449:O-(6)-methylguanine; D015254:DNA Modification Methylases; C515491:MGMT protein, human; D045643:DNA Repair Enzymes; D005978:Glutathione; D003545:Cysteine; D000077204:Temozolomide