Oncobiotic transformation of the gut microbiome may contribute to the risk of breast cancer. Recent studies have provided evidence that the microbiome secretes cytostatic metabolites that inhibit the proliferation, movement, and metastasis formation of cancer cells. In this study, we show that indolepropionic acid (IPA), a bacterial tryptophan metabolite, has cytostatic properties. IPA selectively targeted breast cancer cells, but it had no effects on non-transformed, primary fibroblasts. In cell-based and animal experiments, we showed that IPA supplementation reduced the proportions of cancer stem cells and the proliferation, movement, and metastasis formation of cancer cells. These were achieved through inhibiting epithelial-to-mesenchymal transition, inducing oxidative and nitrosative stress, and boosting antitumor immune response. Increased oxidative/nitrosative stress was due to the IPA-mediated downregulation of nuclear factor erythroid 2-related factor 2 (NRF2), upregulation of inducible nitric oxide synthase (iNOS), and enhanced mitochondrial reactive species production. Increased oxidative/nitrosative stress led to cytostasis and reductions in cancer cell stem-ness. IPA exerted its effects through aryl hydrocarbon receptor (AHR) and pregnane X receptor (PXR) receptors. A higher expression of PXR and AHR supported better survival in human breast cancer patients, highlighting the importance of IPA-elicited pathways in cytostasis in breast cancer. Furthermore, AHR activation and PXR expression related inversely to cancer cell proliferation level and to the stage and grade of the tumor. The fecal microbiome’s capacity for IPA biosynthesis was suppressed in women newly diagnosed with breast cancer, especially with stage 0. Bacterial indole biosynthesis showed correlation with lymphocyte infiltration to tumors in humans. Taken together, we found that IPA is a cytostatic bacterial metabolite, the production of which is suppressed in human breast cancer. Bacterial metabolites, among them, IPA, have a pivotal role in regulating the progression but not the initiation of the disease.

In breast cancer patients, the diversity of the microbiome decreases, coinciding with decreased production of cytostatic bacterial metabolites like lithocholic acid (LCA). We hypothesized that LCA can modulate oxidative stress to exert cytostatic effects in breast cancer cells. Treatment of breast cancer cells with LCA decreased nuclear factor-2 (NRF2) expression and increased Kelch-like ECH associating protein 1 (KEAP1) expression via activation of Takeda G-protein coupled receptor (TGR5) and constitutive androstane receptor (CAR). Altered NRF2 and KEAP1 expression subsequently led to decreased expression of glutathione peroxidase 3 (GPX3), an antioxidant enzyme, and increased expression of inducible nitric oxide synthase (iNOS). The imbalance between the pro- and antioxidant enzymes increased cytostatic effects via increased levels of lipid and protein oxidation. These effects were reversed by the pharmacological induction of NRF2 with RA839, tBHQ, or by thiol antioxidants. The expression of key components of the LCA-elicited cytostatic pathway (iNOS and 4HNE) gradually decreased as the breast cancer stage advanced. The level of lipid peroxidation in tumors negatively correlated with the mitotic index. The overexpression of iNOS, nNOS, CAR, KEAP1, NOX4, and TGR5 or the downregulation of NRF2 correlated with better survival in breast cancer patients, except for triple negative cases. Taken together, LCA, a metabolite of the gut microbiome, elicits oxidative stress that slows down the proliferation of breast cancer cells. The LCA–oxidative stress protective pathway is lost as breast cancer progresses, and the loss correlates with poor prognosis.

Changes to bacterial metabolite-elicited signaling, in oncobiosis associated with breast cancer, plays a role in facilitating the progression of the disease. We show that indoxyl-sulfate (IS), a tryptophan metabolite, has cytostatic properties in models of breast cancer. IS supplementation, in concentrations corresponding to the human serum reference range, suppressed tumor infiltration to the surrounding tissues and metastasis formation in a murine model of breast cancer. In cellular models, IS suppressed NRF2 and induced iNOS, leading to induction of oxidative and nitrosative stress, and, consequently, reduction of cell proliferation; enhanced oxidative and nitrosative stress are crucial in the subsequent cytostasis. IS also suppressed epithelial-to-mesenchymal transition vital for suppressing cellular movement and diapedesis. Furthermore, IS rendered cells hypometabolic, leading to a reduction in aldehyde-dehydrogenase positive cells. Pharmacological inhibition of the pregnane-X receptor using CH223191 and the aryl-hydrocarbon receptor using ketoconazole diminished the IS-elicited effects, suggesting that these receptors were the major receptors of IS in these models. Finally, we showed that increased expression of the human enzymes that form IS (Cyp2E1, Sult1A1, and Sult1A2) is associated with better survival in breast cancer, an effect that is lost in triple negative cases. Taken together, IS, similar to indolepropionic acid (another tryptophan metabolite), has cytostatic properties and higher expression of the metabolic machinery responsible for the formation of IS supports survival in breast cancer.

Keratinocytes provide the first line of defense of the human body against carcinogenic ultraviolet (UV) radiation. Acute and chronic UVB-mediated cellular responses were widely studied. However, little is known about the role of mitochondrial regulation in UVB-induced DNA damage. Here, we show that poly (ADP-ribose) polymerase 1 (PARP1) and ataxia-telangiectasia-mutated (ATM) kinase, two tumor suppressors, are important regulators in mitochondrial alterations induced by UVB. Our study demonstrates that PARP inhibition by ABT-888 upon UVB treatment exacerbated cyclobutane pyrimidine dimers (CPD) accumulation, cell cycle block and cell death and reduced cell proliferation in premalignant skin keratinocytes. Furthermore, in human keratinocytes UVB enhanced oxidative phosphorylation (OXPHOS) and autophagy which were further induced upon PARP inhibition. Immunoblot analysis showed that these cellular responses to PARP inhibition upon UVB irradiation strongly alter the phosphorylation level of ATM, adenosine monophosphate-activated kinase (AMPK), p53, protein kinase B (AKT), and mammalian target of rapamycin (mTOR) proteins. Furthermore, chemical inhibition of ATM led to significant reduction in AMPK, p53, AKT, and mTOR activation suggesting the central role of ATM in the UVB-mediated mitochondrial changes. Our results suggest a possible link between UVB-induced DNA damage and metabolic adaptations of mitochondria and reveal the OXPHOS-regulating role of autophagy which is dependent on key metabolic and DNA damage regulators downstream of PARP1 and ATM. IntroductionMitochondria regulate their shape, number, distribution, mass, content of mitochondrial DNA (mtDNA), and metabolic capacity in a process called mitochondrial biogenesis, which requires the orchestration of complex transcriptional control of both nuclear and mitochondrial genes [1,2]. The function of mitochondrial biogenesis is to provide quality control of mitochondria by regulating mitochondrial fission, fusion, and mitophagy [3,4] to maximize the energy utilization of mitochondria [5] to meet cellular and environmental demands. Imbalances or perturbations in these processes can lead to mitochondrial dysfunction [3,6].Accumulating evidence suggests that mitochondria also play a central role in skin physiology. Although, involvement of other organ systems predominates in classical mitochondrial disorders, several cutaneous diseases can be linked to mitochondrial dysfunctions [7]. Interestingly, mitochondria lack functional nucleotide excision repair (NER) pathway [8,9] which is responsible for the removal of ultraviolet (UV)-induced DNA lesions including cyclobutane pyrimidine dimers (CPD). Accumulation of these DNA photoproducts in mtDNA leads to mutations and deletions resulting in mitochondrial alterations which have been associated with photoaging [10,11] and are present in melanoma [12], as well as in non-melanoma skin cancers [13,14]. The other types of mitochondrial alterations such as upregulated oxidative phosphorylation (OXPHOS), mitochondrial memb...

There is a growing body of evidence that poly(ADP-ribose) polymerase-2 (PARP2), although originally described as a DNA repair protein, has a widespread role as a metabolic regulator. We show that the ablation of PARP2 induced characteristic changes in the lipidome. The silencing of PARP2 induced the expression of sterol regulatory element-binding protein-1 and -2 and initiated de novo cholesterol biosynthesis in skeletal muscle. Increased muscular cholesterol was shunted to muscular biosynthesis of dihydrotestosterone, an anabolic steroid. Thus, skeletal muscle fibers in PARP2 mice were stronger compared to those of their wild-type littermates. In addition, we detected changes in the dynamics of the cell membrane, suggesting that lipidome changes also affect the biophysical characteristics of the cell membrane. In in silico and wet chemistry studies, we identified lipid species that can decrease the expression of PARP2 and potentially phenocopy the genetic abruption of PARP2, including artificial steroids. In view of these observations, we propose a new role for PARP2 as a lipid-modulated regulator of lipid metabolism.