Inflammation and Cancer: From the Development of Personalized Indicators to Novel Therapeutic Strategies

Abstract

Colorectal (CRC) and hepatocellular carcinoma (HCC) are associated with chronic inflammation, which plays a role in tumor development and malignant progression. An unmet medical need in these settings is the availability of sensitive and specific noninvasive biomarkers. Their use will allow surveillance of high-risk populations, early detection, and monitoring of disease progression. Moreover, the characterization of specific fingerprints of patients with nonalcoholic fatty liver disease (NAFLD) without or with nonalcoholic steatohepatitis (NASH) at the early stages of liver fibrosis is necessary. Some lines of evidence show the contribution of platelets to intestinal and liver inflammation. Thus, low-dose Aspirin, an antiplatelet agent, reduces CRC and liver cancer incidence and mortality. Aspirin also produces antifibrotic effects in NAFLD. Activated platelets can trigger chronic inflammation and tissue fibrosis via the release of soluble mediators, such as thromboxane (TX) A₂ and tumor growth factor (TGF)-β, and vesicles containing genetic material (including microRNA). These platelet-derived products contribute to cyclooxygenase (COX)-2 expression and prostaglandin (PG)E₂ biosynthesis by tumor microenvironment cells, such as immune and endothelial cells and fibroblasts, alongside cancer cells. Enhanced COX-2-dependent PGE₂ plays a crucial role in chronic inflammation and promotes tumor progression, angiogenesis, and metastasis. Antiplatelet agents can indirectly prevent the induction of COX-2 in target cells by inhibiting platelet activation. Differently, selective COX-2 inhibitors (coxibs) block the activity of COX-2 expressed in the tumor microenvironment and cancer cells. However, coxib chemopreventive effects are hampered by the interference with cardiovascular homeostasis via the coincident inhibition of vascular COX-2-dependent prostacyclin biosynthesis, resulting in enhanced risk of atherothrombosis. A strategy to improve anti-inflammatory agents' use in cancer prevention could be to develop tissue-specific drug delivery systems. Platelet ability to interact with tumor cells and transfer their molecular cargo can be employed to design platelet-mediated drug delivery systems to enhance the efficacy and reduce toxicity associated with anti-inflammatory agents in these settings. Another peculiarity of platelets is their capability to uptake proteins and transcripts from the circulation. Thus, cancer patient platelets show specific proteomic and transcriptomic expression profiles that could be used as biomarkers for early cancer detection and disease monitoring.

Keywords: COX-2; HCC; NSAIDs; aspirin; colorectal cancer; drug delivery; inflammation; platelets.

FIGURE 1

Aspirin (acetylsalicylic acid, ASA) acts by irreversibly acetylating cyclooxygenase (COX)-1 and COX-2. Prostaglandin H synthase (PGHS)-1 and -2 (also known as COX-1 and COX-2) are microsomal homodimeric heme glycoproteins that catalyze two key reactions in the biosynthesis of prostanoids: the bis-dioxygenation of arachidonic acid to form PGG₂ (by the cyclooxygenase activity) and the reduction of PGG₂ to PGH₂ (by the peroxidase activity). In addition to PGH₂, the monohydroxy acids 11(R)-hydroxyeicosatetraenoic acid 11(R)-hydroxyeicosatetraenoic acid 11R-(HETE), 15S-HETE, and 15R-HETE are generated as minor products of the cyclooxygenase reaction. COX-isozymes function as conformational heterodimers comprised of a regulatory allosteric monomer and a catalytic monomer. The catalytic monomer has a bound heme, whereas the allosteric monomer does not. The cyclooxygenase active site is a hydrophobic channel (reported in grey), and Arg-120 is located near its mouth. Arg-120 forms an ionic bond with the carboxylate group of arachidonate, and this interaction is an important contributor to the overall strength of arachidonate binding to COX-1 more than COX-2. Arg-120 is also the binding site for Aspirin. At the top of the channel is located Tyr-385, which is the critical catalytic amino acid for the cyclooxygenase reaction. Ser-529 and Ser-516 are the aminoacids acetylated by Aspirin. The acetylation of COX-isozymes is associated with the formation of salicylic acid, a weak COX inhibitor. The acetylation of only one monomer of COX dimer is sufficient for causing profound inhibition of cyclooxygenase activity. The acetylation of COX-1 at Ser-516 inhibits the catalytic activity of cyclooxygenase and prevents the generation of PGG₂. In contrast, acetylated COX-2 at Ser-516 gains a novel catalytic activity and forms 15 R-HETE. 15R-HETE could be transformed to 15(R)epi-lipoxin (LX)A₄ and 15epi-LXB₄ in cells expressing the 5-lipoxygenase (5-LOX), the enzyme also responsible for leukotriene biosynthesis.

FIGURE 2

Clinical implications of eicosanoid inhibition in intestinal tumorigenesis. Activated platelets release soluble mediators, including thromboxane A₂ (TXA₂), prostaglandin E₂ (PGE₂), 12S-hydroxyeicosatetraenoic acid (12S-HETE), growth factors, cytokines, and platelet-derived medium-sized extracellular vesicles (mEVs) containing microRNAs, which contribute to the phenotypic changes of the cells in the stromal microenvironment. Interactions between epithelial cells and stromal cells undergo an alteration that induces the development of intestinal neoplasia. A crucial event is the enhanced biosynthesis of PGE₂ in the intestinal mucosa, occurring in the early stages of tumor development via the cyclooxygenase (COX)-1 activity, in association with the suppression of the prostaglandin-degrading enzyme 15-prostaglandin dehydrogenase (15-PGDH). Later, the overexpression of COX-2 contributes to the generation of aberrant levels of PGE₂ in the stromal compartment and epithelial cells. PGE₂ plays multifaceted roles in cancer promotion, including proliferation, migration, epithelial-mesenchymal transition (EMT), and immune escape. Tumor cells that undergo this phenomenon acquire a migratory capacity that facilitates metastatic colonization. By inhibiting platelet activation, antiplatelet agents can indirectly restrain the induction of COX-2 in target cells. Differently, selective COX-2 inhibitors (coxibs) affect COX-2 activity already expressed in the tumor microenvironment and cancer cells. 12S-HETE generated by 12-lipoxygenase (LOX) may have protumorigenic effects, and selective 12-LOX inhibitors are in clinical development.

FIGURE 3

Platelet activation in response to intestinal damage is crucial in chronic inflammation/fibrosis. Platelet activation in response to intestinal epithelial damage contributes to acute inflammation by promoting leukocyte recruitment to restore normal tissue function. However, exaggerated platelet activation is associated with an elevated release of thromboxane (TX) A₂ and prostaglandin (PG)E₂, growth factors, angiogenic factors, cytokines, and chemokines, as well as medium-sized extracellular vesicles (mEVs) rich in genetic material (mRNAs and microRNAs). These factors activate stromal cells (such as myofibroblasts and immune cells), thus increasing the production and release of growth factors and inflammatory mediators, including PGE₂, due to cyclooxygenase (COX)-2 induction. Platelet-derived TXA₂ induces phenotypic and functional changes in myofibroblasts, such as the reduction of α-Alpha Smooth Muscle Actin (SMA) and the increase of vimentin fibronectin RhoA expression; these events lead to an enhanced capacity to proliferate and migrate, thus, contributing to chronic intestinal inflammation and fibrosis. The inhibition of platelet COX-1 activity (by low-dose Aspirin) or blocking the TXA₂ receptor (TP) can mitigate chronic intestinal inflammation and fibrosis.

FIGURE 4

Extrinsic and intrinsic pathways promote chronic inflammation involved in developing hepatocellular carcinoma. Two distinct pathways are involved in chronic inflammation during hepatocarcinogenesis. The extrinsic pathway is triggered by exogenous factors, such as pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) from dying cells. These, in turn, are recognized by specific receptors expressed in inflammatory cells and platelets. PAMPs and DAMPs activate platelets, which release medium-sized extracellular vesicles (mEVs) able to exit the vasculature, reach tissues, and sustain inflammation, thus increasing cancer risk. The intrinsic pathway, induced by alteration in cancer-associated genetic factors, activates the expression of inflammation-related programs, thus contributing to the generation and perpetuation of an inflammatory milieu. All these events further contribute to the overexpression of COX-2 and enhanced PGE₂ biosynthesis, promoting the transformation of normal epithelial cells to cancer. Antiplatelet drugs, such as low-dose Aspirin, can affect the early step of tumor development by inhibiting platelet function. In contrast, selective COX-2 inhibitors (coxibs) may exert an antitumor effect by inhibiting the synthesis of PGE₂ in leukocytes and tumor cells.

FIGURE 5

Platelet’s contribution in drug delivery systems. Different approaches can be used to deliver drugs to the tumor microenvironment by platelets: (1) platelets can be loaded with the anticancer drug via the open canalicular system; (2) genetically modified megakaryocytes (MKs) can give rise to mature platelets expressing programmed death-1 (PD-1) [involved in the immune checkpoint PD-1/programmed death-ligand 1 (PDL-1)] that can also be loaded with the anticancer drug; (3) monoclonal antibodies against PDL-1 (aPDL1) can be covalently bound to resting platelets; (4) melanin nanoparticles (MNPs) can be coated with platelet membranes.