Acidic tumor microenvironment downregulates hMLH1 but does not diminish 5-fluorouracil chemosensitivity

Abstract

Human DNA mismatch repair (MMR) recognizes and binds 5-fluorouracil (5FU) incorporated into DNA and triggers a MMR-dependent cell death. Absence of MMR in a patient's colorectal tumor abrogates 5FU's beneficial effects on survival. Changes in the tumor microenvironment like low extracellular pH (pHe) may diminish DNA repair, increasing genomic instability. Here, we explored if low pHe modifies MMR recognition of 5FU, as 5FU can exist in ionized and non-ionized forms depending on pH. We demonstrate that MMR-proficient cells at low pHe show downregulation of hMLH1, whereas expression of TDG and MBD4, known DNA glycosylases for base excision repair (BER) that can remove 5FU from DNA, were unchanged. We show in vitro that 5FU within DNA pairs with adenine (A) at high and low pH (in absence of MMR and BER). Surprisingly, 5FdU:G was repaired to C:G in hMLH1 -deficient cells cultured at both low and normal pHe, similar to MMR-proficient cells. Moreover, both hMSH6 and hMSH3, components of hMutSα and hMutSβ, respectively, bound 5FU within DNA(hMSH6 > hMSH3) but is influenced by hMLH1. We conclude that an acidic tumor microenvironment triggers downregulation of hMLH1, potentially removing the execution component of MMR for 5FU cytotoxicity, whereas other mechanisms remain stable to implement overall 5FU sensitivity.

Explosive mutation accumulation triggered by heterozygous human Pol ε proofreading-deficiency is driven by suppression of mismatch repair

“…In this scenario, mutations introduced by the mutant Pol ε could accumulate slowly even in the presence of genotypically and epigenetically wild type mismatch repair. A number of conditions have been shown to transiently and reversibly lower mismatch repair protein levels and inhibit mismatch repair activity, including hypoxia, oxidative damage, inflammation, reduced pH, exposure to adriamycin or cadmium and treatment with mutagenic dNTP analogs ( Banerjee and Flores-Rozas, 2005 ; Francia et al, 2005 ; Larson and Drummond, 2001 ; Mihaylova et al, 2003 ; Chang et al, 2002 ; Hile et al, 2013 ; Iwaizumi et al, 2013 ; Lu et al, 2014 ; Negishi et al, 2002 ). The variable nature and duration of such a suppression event would be expected to result in a complex effect on microsatellite instability.…”

EMAST is a Form of Microsatellite Instability That is Initiated by Inflammation and Modulates Colorectal Cancer Progression

“…Hypoxia and cellular pH changes can also alter DNA mismatch repair function. In particular, hypoxia and low extracellular pH reduces MLH1 expression [ 37 , 38 , 39 , 40 ], apparently via hypoxia-induced transcription repressors and decreased histone methylation at the MLH1 promoter [ 37 , 40 , 41 ]. Because PMS2 stability is dependent on association with its heterodimer partner MLH1, loss of MLH1 protein destabilizes PMS2 [ 11 , 39 ].…”

Trispecific natural killer cell nanoengagers for targeted chemoimmunotherapy

“…S5). The encapsulated EPI underwent pH-dependent controlled release at physiological conditions, with approximately half of the encapsulated EPI released at the weakly acidic (pH 6.0) extracellular tumor microenvironment (27) and early endosomal conditions (28) in the first 24 hours ( Fig. 1D and fig.…”