Flavonoids as an effective sensitizer for anti-cancer therapy: insights into multi-faceted mechanisms and applicability towards individualized patient profiles

Abstract

Cost-efficacy of currently applied treatments is an issue in overall cancer management challenging healthcare and causing tremendous economic burden to societies around the world. Consequently, complex treatment models presenting concepts of predictive diagnostics followed by targeted prevention and treatments tailored to the personal patient profiles earn global appreciation as benefiting the patient, healthcare economy, and the society at large. In this context, application of flavonoids as a spectrum of compounds and their nano-technologically created derivatives is extensively under consideration, due to their multi-faceted anti-cancer effects applicable to the overall cost-effective cancer management, primary, secondary, and even tertiary prevention. This article analyzes most recently updated data focused on the potent capacity of flavonoids to promote anti-cancer therapeutic effects and interprets all the collected research achievements in the frame-work of predictive, preventive, and personalized (3P) medicine. Main pillars considered are: - Predictable anti-neoplastic, immune-modulating, drug-sensitizing effects; - Targeted molecular pathways to improve therapeutic outcomes by increasing sensitivity of cancer cells and reversing their resistance towards currently applied therapeutic modalities.

Keywords: Anthocyanidins; Anti-bacterial; Anti-cancer agents; Anti-inflammation; Anti-viral; COVID-19; Chalcones; Chemotherapy; Disease management; Drug-sensitizing effect; Flavanols; Flavanones; Flavones; Flavonoids; Flavonols; Health economy; Health policy; Immunotherapy; Isoflavonoids; Nano-carrier delivery; Phytochemicals; Predictive preventive personalized medicine (3PM/PPPM); Radiotherapy; Signalling pathways; Targeted therapy; Therapy efficacy; Therapy resistance.

Fig. 1

Classification of flavonoids

Fig. 2

Mechanisms of radiotherapy resistance of cancer cells. Explanatory notes: A hypoxic intratumoral microenvironment is a leading cause of radiotherapy failure (decreased ROS formation in irradiated cells under hypoxic conditions is associated with decreased DNA damage and the so-called “oxygen effect”). Indeed, impaired function of mitochondria and glycolytic pathways can be involved in cancer cell radioresistance (anaerobic metabolism and LDH, a marker of resistance, associated with upregulated LDHA under hypoxic conditions). LDH is a marker of perfusion-related hypoxia. Lower oxygen leads to reductions in radiation-induced ROS generation and DNA damage. Upregulation of the oxidative pentose pathway that accompanies glycolysis, activation of LDHA as a result of direct mitochondrial dysfunction or oncogene/HIF-mediated inactivation of mitochondrial function, and inhibition of pyruvate entry into mitochondria by pyruvate-dehydrogenase kinases (regulated by LDHA through HIF) are processes associated with cancer cells radioresistance [13]. Also, aberrantly activated Nrf2 in tumor cells (as a result of Keap1 or Nrf2 somatic mutations or other Keap1/Nrf2-related mechanisms) contributes to high-level resistance of cancer cells [82]. The Keap1 promoter is often hypermethylated in NSCLS and leads to decreased Keap1 mRNA and protein expression; this impairs the Nrf2-Keap1 pathway (resulting in radio- and/or chemo-resistance) [83]. The homologous recombination (HRR) and non-homologous end joining (NHEJ) pathways enhance DNA repair activity and modulate cell sensitivity and resistance to radiotherapy [48]. The repair of DNA damage in dormant cancer stem cells (CSCs) is predominantly performed through NHEJ; consequently, NHEJ inhibition could overcome CSC radioresistance [84]. Indeed, CSCs are considered the primary source of resistance to radiotherapy and chemotherapy while tumor heterogeneity contributes to radiation resistance [11].

Fig. 3

Mechanisms of resistance to chemotherapeutic drugs. Explanatory notes: A) increased drug efflux – proteins in the ATP-binding cassette (ABC) transporter family contain nucleotide-binding domains (NBD) and two transmembrane domains (TMDs); ATP hydrolysis-driven conformational changes of TMD result in unidirectional transport across the lipid bilayer [91]. ABC transporter overexpression is observed in several cancer types and is more predominant in cancer stem cells (CSCs) [92]. ABC transporters, including multidrug resistance protein 1 (MDR-1, ABCB1, P-gp), multidrug resistance-associated protein 1 (MRP1, ABCC1), and breast cancer resistance protein (BCRP, ABCG2), are implicated in drug-resistant cancers [93]. Also, aldehyde dehydrogenase (ALDH) promotes drug resistance. ABC transporters and ALDH are upregulated in normal stem cells, CSCs, and drug-resistant cancer cells [94]. B) Increased DNA repair capacity [95] or tolerance of DNA damage [96] induced by therapeutic agents [95, 96]—base excision repair (BER) involves different proteins (UDG, HAP1, Polβ, XRCC1, and DNA ligase I or III) [96] and nucleotide excision repair (NER) mechanisms involve damage recognition/excision proteins and helicase proteins (DNA damage is recognized by the NER protein XPC-RAD23B, which binds to DNA strand, an oligonucleotide containing the lesion is then excised from the DNA strand, a repair patch is synthesized, and DNA ligases join the patch to the DNA) [96]. NER-induced resistance to platinum-based agents [12, 96] includes the DNA repair endonuclease XPF and the DNA excision repair protein ERCC1. Replication protein A (RPA) is involved in the DNA-damage response (DDR), HR, and NER [12]. Decreased mismatch repair (MMR) promotes damage tolerance and enhanced mutagenicity and chemoresistance in cancer cells (hypermethylation of the hMLH1 gene promoter results in decreased expression of the MLH1 protein involved in the MMR pathway) [12]. C) Genetic and epigenetic factors—TP53 loss results in continued replication and resistance to genotoxic drugs [12]. Abnormal activation of the androgen receptor (AR) signaling pathway (AR over-expression, AR gene amplification, mutations, alterations in coregulators, and continuous androgen release from the tumor tissue or adrenal glands) and abnormal activation of PI3K/Akt or PI3K/Akt/mTOR signaling can lead to the overexpression of ABC transporters and the upregulation of oncogenes and growth factors such as VEGF and c-myc. The acidified tumor micro-environment promotes aerobic glycolysis and MDR (by reducing drug absorption and efficiency). PI3K/Akt regulates aerobic glycolysis to increase energy supply and enhance ABC transporter-mediated drug excretion [97]. The transcription of specific genes essential for resistance is enhanced (e.g. ABCB amplification) [12, 98, 99]. Epigenetic alterations (genome-wide DNA hypomethylation, regional hypermethylation, changes in histone modifications, and alterations in miRNA expression) [12] – carboplatin-induced methylation of the MLH1 CpG island (important for the MMR DNA repair system) is associated with chemoresistance in ovarian cancer; ABCB1 demethylation decreases the accumulation of anti-cancer drugs and promotes the acquisition of the multidrug phenotype [12]. D) Growth factors—cytokine (IL-1, IL-6) production is increased in multidrug cancer cells when compared with drug-sensitive cancer cells [12]. Specific chemotherapeutic agents were ineffective against cancers with increased levels of extracellular fibroblast growth factors (eFGF) [12]. E) Increased metabolism of xenobiotics—altered expression of isoforms of cytochrome (CYPs)—overexpressed CYP1B1, CP4Z1, CYP1B1, and CYP2A7 and phase II enzymes, such as glutathione-S-transferases (GSTs), uridine diphospho-glucuronosyltransferases (UGTs), gamma-glutamyl transferases (γGTs), thiopurine methyltransferases (TPMTs), and dihydropyrimidine dehydrogenases (DPDs) promote the development of multidrug resistance (MDR) [12]. F) CSCs—targeted less by chemotherapeutic drugs (due to slow cell cycle kinetics, high expression of ABC transporters, ALDHs, epithelial-mesenchymal transition, and factors affecting the tumor microenvironment, such as hypoxia, and epigenetic modifications) [100]. F) Other mechanisms include endoplasmatic reticulum (ER) stress—perturbation of ER quality control (ERQC) causes the accumulation of unfolded or misfolded proteins in the ER lumen, resulting in ER stress. The ER stress response (ERSR) is produced to restore homeostasis or activate cell death. ERS is critical for chemo-therapeutic resistance, following the initiation of an ERSR [101]. ROS is increased by the activation of ER stress. Cancer cells induce fluctuations of redox homeostasis through the variation of ROS-regulated machinery, leading to increased tumorigenesis and chemoresistance [102]. The receptor for advanced glycation end products (RAGE) activation leads to drug resistance (pancreatic cancer) [103]. P‐gp overexpression and CSCs are closely associated with the nuclear localization of YB‐1 in cancer cells [50]. NF-kB activation rescues cancer cells from cell death [104]. Galectin-3 is transported from the nucleus to the cytoplasm to stimulate the phosphorylation of Bcl-2 associated death (Bad) protein and the downregulation of Bad; this results in the maintenance of mitochondrial membrane integrity. Consequent effects, including the blockade of cytochrome c release and caspase-3 activation, inhibition of apoptosis, and activation of PARP1, induce chemoresistance through the cytosolic translocation of HMGB1 via PARylation, which is known to induce autophagy by disrupting the interaction between Beclin-1 and Bcl-2 [51]

Fig. 4

Mechanisms of cancer cells resistance to targeted therapy. Explanatory notes: Resistance to targeted therapy often results from reactivation of pathways inhibited by the drug (acquisition of drug-resistant mutations/amplification of the target, re-activation of downstream signalling proteins via activation mechanisms or activating mutations, or activation of compensatory signalling pathways) [123]. Due to commonly observed gene mutations, cancer cells can perform modifications as a response to targeted molecules and thus induce resistance to specific agents [12]. The mutation, amplification, downregulation, and alternative RNA splicing of drug targets all contribute to the resistance of cancer cells to targeted therapy [124]. Moreover, direct restoration of biologic function that was disrupted by a drug [125], activation of compensatory pathways parallel to or downstream of the inhibited pathway (such as pro-angiogenic signalling through PDGFR), activation of pro-survival signalling, and epigenetic alterations (like DNA methylation, histone modifications, and microRNA) also contribute to resistance to targeted treatment [124]