The mitochondrial disruptor devimistat (CPI-613®) synergizes with genotoxic anticancer drugs in colorectal cancer therapy in a Bim-dependent manner

Colorectal cancer (CRC) is one of the most frequent tumor entities, with an increasing incidence and mortality in younger adults in Europe and the US. 5-year survival rates for advanced CRC are still low, highlighting the need for novel targets in CRC therapy. Here, we investigated the therapeutic potential of the compound devimistat (CPI 613®) that targets altered mitochondrial cancer cell metabolism and its synergism with the antineoplastic drugs 5-fluorouracil (5-FU) and irinotecan (IT) in CRC. Devimistat exerted a comparable cytotoxicity in a panel of established CRC cell lines and patient-derived short-term culture independent of their genetic and epigenetic status, whereas human colonic epithelial cells were more resistant indicating tumor selectivity. These findings were corroborated in intestinal organoid and tumoroid models. Mechanistically, devimistat disrupted mitochondrial membrane potential and severely impaired mitochondrial respiration, resulting in CRC cell death induction independent of p53. Combination treatment of devimistat with 5-FU or IT demonstrated synergistic cell killing in CRC cells as shown by Combenefit modelling and Chou-Talalay analysis. Increased cell death induction was revealed as major mechanism involving downregulation of anti-apoptotic genes and accumulation of pro-apoptotic Bim, which was confirmed by its genetic knockdown. In human CRC xenograft mouse models, devimistat showed anti-tumor activity and synergized with IT, resulting in prolonged survival and enhanced therapeutic efficacy. In human tumor xenografts, devimistat prevented IT-triggered p53 stabilization and caused synergistic Bim induction. Taken together, our study revealed devimistat as a promising candidate in CRC therapy by synergizing with established antineoplastic drugs in vitro and in vivo.

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

The protein O6-methylguanine-DNA methyltransferase (MGMT) is able to repair the mutagenic O6-methylguanine adduct back to guanine. In this context, it may protect against colorectal cancer (CRC) formation associated with N-nitroso compounds. Such compounds may be endogenously formed by nitrosylation of amino acids, which can give rise to mutagenic O6-methylguanine (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 (HCEC). 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 CRC.

Synthesis and in vitro characterization of the genotoxic, mutagenic and cell-transforming potential of nitrosylated heme

Data from epidemiological studies suggest that consumption of red and processed meat is a factor contributing to colorectal carcinogenesis. Red meat contains high amounts of heme, which in turn can be converted to its nitrosylated form, NO-heme, when adding nitrite-containing curing salt to meat. NO-heme might contribute to colorectal cancer formation by causing gene mutations and could thereby be responsible for the association of (processed) red meat consumption with intestinal cancer. Up to now, neither in vitro nor in vivo studies characterizing the mutagenic and cell transforming potential of NO-heme have been published due to the fact that the pure compound is not readily available. Therefore, in the present study, an already existing synthesis protocol was modified to yield, for the first time, purified NO-heme. Thereafter, newly synthesized NO-heme was chemically characterized and used in various in vitro approaches at dietary concentrations to determine whether it can lead to DNA damage and malignant cell transformation. While NO-heme led to a significant dose-dependent increase in the number of DNA strand breaks in the comet assay and was mutagenic in the HPRT assay, this compound tested negative in the Ames test and failed to induce malignant cell transformation in the BALB/c 3T3 cell transformation assay. Interestingly, the non-nitrosylated heme control showed similar effects, but was additionally able to induce malignant transformation in BALB/c 3T3 murine fibroblasts. Taken together, these results suggest that it is the heme molecule rather than the NO moiety which is involved in driving red meat-associated carcinogenesis.

Curcumin Administered as Micellar Solution Suppresses Intestinal Inflammation and Colorectal Carcinogenesis

Colorectal cancer (CRC) is one of the most common cancers and preventive strategies based on natural compounds are highly desirable. Curcumin, the principal bioactive compound in Curcuma longa, was described to have multiple beneficial health effects. A drawback, however, is the low bioavailability due to its insolubility in water. Here, we studied whether nanoscaled micellar curcumin with improved bioavailability administered in drinking water reduces inflammation and CRC formation in a mouse model. C57BL6 wild-type (WT) mice and a strain defective in the DNA repair enzyme O⁶-methylguanine-DNA methyltransferase (MGMT) were used, in which tumors were induced by azoxymethane (AOM) followed by dextran sodium sulfate (DSS). Inflammation and tumor formation were determined by mini-colonoscopy. Micellar curcumin (mCur) administered in drinking water significantly reduced AOM/DSS-induced colorectal inflammation in both WT and MGMT-deficient mice as compared to animals receiving drinking water with micelles not containing curcumin. In line with this, the tumor yield and tumor score were significantly lower in mCur-treated mice compared to the control group. No adverse effects were observed in animals receiving mCur daily for at least three months. Overall, our data show that chronic oral administered micellar curcumin is well tolerated and reduces chemical-induced gut inflammation and CRC formation in mice.Impact: The study shows that micellar curcumin with high bioavailability chronically administered at low and physiologically relevant concentration suppresses inflammation and carcinogenesis in a mouse colorectal tumor model.

Mechanism of colorectal carcinogenesis triggered by heme iron from red meat

Colorectal cancer (CRC) is one of the major tumor entities worldwide, with an increasing incidence in younger people. CRC formation is causally linked to various genetic, life-style and dietary risk factors. Among the ladder, the consumption of red meat has emerged as important risk factor contributing to CRC. A large body of evidence shows that heme iron is the critical component of red meat, which promotes colorectal carcinogenesis. In this review, we describe the uptake and cellular fate of both heme and inorganic iron in intestinal epithelial cells. Next, an overview on the DNA damaging properties of heme iron is provided, highlighting the DNA adducts relevant for CRC etiology. Moreover, heme triggered mechanisms leading to colonic hyperproliferation are presented, which are intimately linked to changes in the intestinal microbiota induced by heme. A special focus was set on the impact of heme iron on innate and adaptive immune cells, which could be relevant in the context of CRC. Finally, we recapitulate in vivo studies providing evidence for the tumor-promoting potential of dietary heme iron. Altogether, heme iron affects numerous key pathways involved in the pathogenesis of CRC.

Divergent roles of the Drosophila melanogaster globins

In contrast to long-held assumptions, the gene repertoire of most insects includes hemoglobins. Analyses of the genome of the fruitfly Drosophila melanogaster identified three distinct hemoglobin genes (glob1, glob2, and glob3). While glob1 is predominantly associated with the tracheal system and fat body, glob2 and glob3 are almost exclusively expressed in the testis. The physiological role of globins in Drosophila is uncertain. Here, we studied the functions of the three globins in a cell culture system. Drosophila Schneider 2 (S2) cells were stably transfected with each of the three globins and the empty vector as control. Under hypoxia (1% atmospheric O₂), only glob1 overexpression enhanced the activity of mitochondrial oxidases and the ATP content. However, the positive effect of glob1 expression disappeared after 24h hypoxia, suggesting metabolic adaptations of the S2 cells. glob2 and glob3 had no positive effect on hypoxia-survival. After application of oxidative stress by H₂O₂, glob2 dramatically enhanced the viability of S2 cells. Evaluation of the intracellular localization of the globins using specific antibodies and green fluorescent protein-fusion constructs suggested that glob1 and glob2 most likely reside in the cytoplasm, while glob3 is associated with structures that may represent parts of the intracellular transport machinery. In silico analyses of public RNA-Seq data from different developmental stages provided that glob1 is co-expressed with genes of the aerobic energy metabolism, while glob2 and glob3 expression can be related to spermatogenesis and reproduction. Together, the results indicate divergent functions of the Drosophila globins: glob1 may play a role in the O₂-dependent metabolism while glob2 may protect spermatogenesis from reactive oxygen species.

DNA damage response curtails detrimental replication stress and chromosomal instability induced by the dietary carcinogen PhIP

PhIP is an abundant heterocyclic aromatic amine (HCA) and important dietary carcinogen. Following metabolic activation, PhIP causes bulky DNA lesions at the C8-position of guanine. Although C8-PhIP-dG adducts are mutagenic, their interference with the DNA replication machinery and the elicited DNA damage response (DDR) have not yet been studied. Here, we analyzed PhIP-triggered replicative stress and elucidated the role of the apical DDR kinases ATR, ATM and DNA-PK_cs in the cellular defense response. First, we demonstrate that PhIP induced C8-PhIP-dG adducts and DNA strand breaks. This stimulated ATR-CHK1 signaling, phosphorylation of histone 2AX and the formation of RPA foci. In proliferating cells, PhIP treatment increased the frequency of stalled replication forks and reduced fork speed. Inhibition of ATR in the presence of PhIP-induced DNA damage strongly promoted the formation of DNA double-strand breaks, activation of the ATM-CHK2 pathway and hyperphosphorylation of RPA. The abrogation of ATR signaling potentiated the cell death response and enhanced chromosomal aberrations after PhIP treatment, while ATM and DNA-PK inhibition had only marginal effects. These results strongly support the notion that ATR plays a key role in the defense against cancer formation induced by PhIP and related HCAs.

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

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.