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.