Carbonic anhydrase 9 (CA9) and redox signaling in cancer-associated fibroblasts: Therapeutic implications

Abstract

Recent advances in cancer research have focused on the additional dimension of complexity within solid tumors that constitutes the tumor microenvironment. The tumor microenvironment is comprised of a complex network of cancer cells and secondary stromal cells, which includes endothelial cells, inflammatory cells and cancer-associated fibroblasts (CAFs), and is influenced by environmental factors including nutrient availability and hypoxia. It is becoming increasingly apparent that intratumoral communication between cancer cells and the associated cells of the microenvironment is vital to tumor growth and malignant progression. Furthermore, this cross-talk is not merely mediated by cytokines and growth factors, but may extend to the metabolic reprogramming of both cancer cells and CAFs. In this model, fibroblasts are converted by cancer cells to energy-producing powerhouses, inducing an increased glycolytic rate and autophagy, mediated by enhanced oxidative stress and the redox stabilization of hypoxia-inducible factor-1 (HIF-1).1 This metabolic reprogramming of CAFs not only leads to the production of high-energy metabolites fueling epithelial cancer cells and subsequent tumor growth, but also increases the acidification of the tumor microenvironment via the production and export of acidic metabolites, including lactic and carbonic acids (Fig. 1). In this way, CAFs act to promote a metastatic phenotype in two ways, first via the direct acidification of the microenvironment, accelerating the degradation of the extracellular matrix, promoting invasion, and second by simulating a proinflammatory environment, indirectly promoting the EMT program in epithelial cancer cells. Figure 1. The role of CAFs, CA9, and metabolic symbiosis in promoting the EMT in epithelial cancer cells. For these reasons, this cross-talk between tumor and stromal cells is thought to play an active role in acquiring the capacity for invasion and metastasis. The contribution of pH regulators to the metastatic phenotype has been intensely studied in epithelial cancer cells and has largely focused on the carbonic anhydrase (CA) family of enzymes that catalyze the hydration of carbon dioxide to biocarbonate and protons. Of note is the HIF-1-regulated CAIX, which mediates the acidification of the tumor microenvironment and enhances tumor growth and migration of tumor cells.2 The expression of tumor-associated CAIX in clinical samples is reported to be an independent prognostic factor associated with both poor prognosis and increased incidence of metastasis in many tumor types, and hence may act as a surrogate biomarker of hypoxia.3 Moreover, recent studies have reported an association between the level of soluble plasma CAIX and reduced survival, and radiolabelled antibodies targeting CAIX have been developed as imaging tools for detecting tumor hypoxia in the clinic.4 The recent paper by Fiaschi and colleagues has reported, for the first time, the role of CAIX in a novel mechanism by which CAFs promote metastasis.5 They observed that tumor cells stimulated the de novo expression of CAIX, resulting in extracellular acidification in CAFs in vitro and in CAFs isolated from patient samples of prostate carcinoma, stressing the clinical relevance of this finding. The expression of CAIX in CAFs was associated with ROS-dependent stabilization of HIF-1 in normoxia, and, as such, fits perfectly with previous reports of redox stabilization of HIF-1 in normoxia involved in metabolic rewiring in both prostate and breast cancer.1,6 Functionally, upregulation of CAIX in CAFs resulted in an increase in MMP2 and MMP9 activity, with decreased E-cadherin expression and increased tumor cell invasion, suggestive of activation of EMT in epithelial cancer cells (Fig. 1). Silencing of CAIX in CAFs was sufficient to prevent spontaneous lung metastasis in vivo when co-injected with prostate cancer cells, confirming that CAF-associated CAIX is essential for EMT and metastasis in vivo. The limited tissue distribution of CAIX in normal tissue makes it an especially attractive target for cancer therapy. Several inhibitors and antibodies targeting CAIX are currently in clinical development and have entered clinical trial,7,8 and, taken together, these findings suggest that CAIX may not only be a novel marker of CAFs, but also an important novel therapeutic strategy for targeting both CAF- as well as hypoxia-driven EMT.