Repair of O⁶-carboxymethylguanine adducts by O⁶-methylguanine-DNA methyltransferase in human colon epithelial cells

authored by

Tina Kostka, Michael T Empl, Nina Seiwert, Susanne M Geisen, Pascal Hoffmann, Janine Adam, Bettina Seeger, Jerry W Shay, Markus Christmann, Shana J Sturla, Jörg Fahrer, Pablo Steinberg

Abstract

The protein O6-methylguanine-DNA methyltransferase (MGMT) is able to repair the mutagenic O6-methylguanine (O6-MeG) adduct back to guanine. In this context, it may protect against colorectal cancer formation associated with N-nitroso compounds. Such compounds may be endogenously formed by nitrosylation of amino acids, which can give rise to mutagenic O6-MeG and O6-carboxymethylguanine (O6-CMG) adducts. It is well established that O6-MeG is repaired by MGMT. However, up to now, whether O6-CMG is repaired by this enzyme remains unresolved. Therefore, the aim of the present study was to analyze the fate of both types of O6-guanine adducts in the presence and absence of MGMT activity. To this end, MGMT activity was efficiently blocked by its chemical inhibitor O6-benzylguanine in human colon epithelial cells (HCECs). Exposure of cells to azaserine (AZA) caused significantly higher levels of both O6-MeG and O6-CMG adducts in MGMT-inhibited cells, with O6-CMG as the more abundant DNA lesion. Interestingly, MGMT inhibition did not result in higher levels of AZA-induced DNA strand breaks in spite of elevated DNA adduct levels. In contrast, MGMT inhibition significantly increased DNA strand break formation after exposure to temozolomide (TMZ), a drug that exclusively generates O6-MeG adducts. In line with this finding, the viability of the cells was moderately reduced by TMZ upon MGMT inhibition, whereas no clear effect was observed in cells treated with AZA. In conclusion, our study clearly shows that O6-CMG is repaired by MGMT in HCEC, thereby suggesting that MGMT might play an important role as a tumor suppressor in diet-mediated colorectal cancer.

External Organisation(s)

University of Veterinary Medicine of Hannover, Foundation
University of Kaiserslautern
University of Texas Southwestern Medical Center
TRON - Translational Oncology at the University Medical Center of the Johannes Gutenberg University Mainz
ETH Zurich

Type

Article

Journal

CARCINOGENESIS

Volume

42

Pages

1110-1118

No. of pages

9

ISSN

0143-3334

Publication date

01.08.2021

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Cancer Research

Sustainable Development Goals

SDG 3 - Good Health and Well-being

Electronic version(s)

https://doi.org/10.1093/carcin/bgab049 (Access: Closed)