Acidic tissue microenvironment contributes to tumor progression via multiple effects including the activation of angiogenic factors and proteases, reduced cell-cell adhesion, increased migration and invasion, etc. In addition, intratumoral acidosis can influence the uptake of anticancer drugs and modulate the response of tumors to conventional therapy. Acidification of the tumor microenvironment often develops due to hypoxia-triggered oncogenic metabolism, which leads to the extensive production of lactate, protons, and carbon dioxide. In order to avoid intracellular accumulation of the acidic metabolic products, which is incompatible with the survival and proliferation, tumor cells activate molecular machinery that regulates pH by driving transmembrane inside-out and outside-in ion fluxes. Carbonic anhydrase IX (CA IX) is a hypoxia-induced catalytic component of the bicarbonate import arm of this machinery. Through its catalytic activity, CA IX directly participates in many acidosis-induced features of tumor phenotype as demonstrated by manipulating its expression and/or by in vitro mutagenesis. CA IX can function as a survival factor protecting tumor cells from hypoxia and acidosis, as a pro-migratory factor facilitating cell movement and invasion, as a signaling molecule transducing extracellular signals to intracellular pathways (including major signaling and metabolic cascades) and converting intracellular signals to extracellular effects on adhesion, proteolysis, and other processes. These functional implications of CA IX in cancer are supported by numerous clinical studies demonstrating the association of CA IX with various clinical correlates and markers of aggressive tumor behavior. Although our understanding of the many faces of CA IX is still incomplete, existing knowledge supports the view that CA IX is a biologically and clinically relevant molecule, exploitable in anticancer strategies aimed at targeting adaptive responses to hypoxia and/or acidosis.

Abstract. Most solid tumors display extracellular acidosis, which only partially overlaps with hypoxia and induces distinct adaptive changes leading to aggressive phenotype. Although acidosis is mainly attributable to excessive production of lactic acid, it also involves carbonic anhydrase (CA) IXmediated conversion of CO 2 to an extracellular proton and a bicarbonate ion transported to cytoplasm. CA IX is predominantly expressed in tumors with poor prognosis and its transcription and activity are induced by hypoxia. Here we investigated whether low extracellular pH in absence of hypoxia can influence CA IX expression in cell lines derived from glioblastoma, a tumor type particularly linked with acidosis. Our data show that extracellular acidosis increased the level of CA IX protein, mRNA and the activity of minimal CA9 promoter that contains binding sites for HIF-1 and SP-1 transcription factors. Mutation within each of these two biding sites reduced the promoter activity, but did not eliminate the increase by acidosis. Transfection of HIF-1· cDNA produced additive inducing effect with acidosis. Normoxic acidosis was accompanied by HIF-1· protein accumulation and transiently increased phosphorylation of ERK1/2. Expression of a dominant-negative mutant of ERK2 reduced the CA9 promoter activity in both standard and acidic conditions. Similar result was obtained by inhibitors of MAPK and PI3K pathways, whose combination completely suppressed CA IX expression and abolished induction by acidosis. Altogether, our results suggest that acidosis increases the CA IX expression via a hypoxia-independent mechanism that operates through modulation of the basic CA9 transcriptional machinery.

CA IX is a hypoxia-induced, cancer-associated carbonic anhydrase isoform with functional involvement in pH control and cell adhesion. Here we describe an alternative splicing variant of the CA9 mRNA, which does not contain exons 8 -9 and is expressed in tumour cells independently of hypoxia. It is also detectable in normal tissues in the absence of the full-length transcript and can therefore produce false-positive data in prognostic studies based on the detection of the hypoxia-and cancer-related CA9 expression. The splicing variant encodes a truncated CA IX protein lacking the C-terminal part of the catalytic domain. It shows diminished catalytic activity and is intracellular or secreted. When overexpressed, it reduces the capacity of the full-length CA IX protein to acidify extracellular pH of hypoxic cells and to bind carbonic anhydrase inhibitor. HeLa cells transfected with the splicing variant cDNA generate spheroids that do not form compact cores, suggesting that they fail to adapt to hypoxic stress. Our data indicate that the splicing variant can functionally interfere with the full-length CA IX. This might be relevant particularly under conditions of mild hypoxia, when the cells do not suffer from severe acidosis and do not need excessive pH control. British Journal of Cancer (2008) Carbonic anhydrase IX (CA IX) is one of 12 enzymatically active carbonic anhydrase isoforms expressed in the human body. These zinc metalloenzymes catalyse a reversible conversion of carbon dioxide to bicarbonate and proton, and thereby contribute to modulation of ion transport and maintenance of acid-base balance. The CA isoforms show remarkable diversity in their enzyme activity, kinetic properties, distribution in tissues, localisation in subcellular compartments and sensitivity to inhibitors. This diversity enables them to fulfill specific roles in metabolically active organs and in various physiological situations .In contrast to the majority of CA isoforms that are mostly present in differentiated cells of the normal tissues, CA IX expression is predominantly associated with a broad range of tumours derived from cells, which contain no or low level of CA IX (Potter and Harris, 2003). The only normal tissues that show moderate-to-abundant expression of CA IX belong to gastrointestinal tract and comprise epithelia of the glandular stomach, small intestine, and gallbladder (Pastorekova et al, 1997).The tumour-related expression pattern of CA IX is principally determined by a strong activation of CA9 gene transcription via a hypoxia-inducible factor (HIF), which binds to hypoxia responsive element (HRE) localised in the minimal CA9 promoter proximal to transcription start site at À10/À3 position (Wykoff et al, 2000). The HIF transcription factor significantly changes the expression profile of weakly oxygenated tumour cells by the activation of genes that either support their survival and adaptation to hypoxic stress or lead to their death. As a result, hypoxia selects more aggressive tumour cells with increased capability...

Carbonic anhydrase IX (CA IX) is a well-recognized hypoxia marker with promising diagnostic and therapeutic value. CA IX regulates the pH in hypoxic tumor cells and, thereby, contributes to microenvironmental acidosis and cell migration. To gain a better insight into the molecular processes driven by CA IX, we performed gene expression profiling of HT-1080 fibrosarcoma cells subjected to CA IX depletion by shRNA silencing. We identified the focal adhesion pathway as being significantly inhibited in the absence of CA IX and confirmed this finding by functional assays. Thus, we obtained the first direct evidence for the role of CA IX in focal adhesion.

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Abstract. Carbonic anhydrase IX (CA IX) is regarded as one of the most prominent markers of tumor hypoxia with potential to serve as a diagnostic biomarker, prognostic indicator as well as tumor therapeutic target. The aim of the present study was to perform an in-depth analysis of CA IX expression in blood and tissue samples and to evaluate the significance of CA IX status for different renal cell carcinomas (RCCs). The expression of CA IX was determined in blood and tissue samples from 74 kidney cancer patients using reverse transcription polymerase chain reaction (RT-PCR), enzyme-linked immunosorbent assay (ELISA), Western blotting (WB) and immunohistochemistry (IHC). The CA IX status was correlated with RCC type and tumor stage. IHC and WB provided evidence for a significantly higher expression of CA IX in clear cell RCC (CCRCC) specimens compared to other RCCs. RT-PCR assay revealed that 32.42% of all RCC patients possess CA9-positive cells in peripheral blood and three-quarters of CA9-positive patients were diagnosed with CCRCC. When the patients were subdivided according to tumor stage, decreased positivity was observed with higher tumor stage (50% in T1 vs. 17% in T3). Serum CA IX levels determined by ELISA were significantly higher in CCRCC patients than in non-CCRCC. A significant association between s-CA IX and CCRCC tumor stage was also determined (T1-87.51 vs. T3-341.98 pg/ml, p=0.046). We demonstrated that the CA IX expression profiles in blood and tissue samples from 74 kidney cancer patients are closely correlated with their histological subtypes. This is the first study reporting CA IX expression in blood and tissue samples from kidney cancer patients determined by four different methods.

Pheochromocytomas (PHEOs) and paragangliomas (PGLs) related to mutations in the mitochondrial succinate dehydrogenase (SDH) subunits A, B, C, and D, SDH complex assembly factor 2, and the von Hippel-Lindau (VHL) genes share a pseudohypoxic expression profile. However, genotype-specific differences in expression have been emerging. Development of effective new therapies for distinctive manifestations, e.g., a high rate of malignancy in SDHB- or predisposition to multifocal PGLs in SDHD patients, mandates improved stratification. To identify mutation/location-related characteristics among pseudohypoxic PHEOs/PGLs, we used comprehensive microarray profiling (SDHB: n = 18, SDHD-abdominal/thoracic (AT): n = 6, SDHD-head/neck (HN): n = 8, VHL: n = 13). To avoid location-specific bias, typical adrenal medulla genes were derived from matched normal medullas and cortices (n = 8) for data normalization. Unsupervised analysis identified two dominant clusters, separating SDHB and SDHD-AT PHEOs/PGLs (cluster A) from VHL PHEOs and SDHD-HN PGLs (cluster B). Supervised analysis yielded 6937 highly predictive genes (misclassification error rate of 0.175). Enrichment analysis revealed that energy metabolism and inflammation/fibrosis-related genes were most pronouncedly changed in clusters A and B, respectively. A minimum subset of 40 classifiers was validated by quantitative real-time polymerase chain reaction (quantitative real-time polymerase chain reaction vs. microarray: r = 0.87). Expression of several individual classifiers was identified as characteristic for VHL and SDHD-HN PHEOs and PGLs. In the present study, we show for the first time that SDHD-HN PGLs share more features with VHL PHEOs than with SDHD-AT PGLs. The presented data suggest novel subclassification of pseudohypoxic PHEOs/PGLs and implies cluster-specific pathogenic mechanisms and treatment strategies.

Clear cell renal cell carcinoma (ccRCC) is the most frequent type of kidney cancer. In order to better understand the biology of ccRCC, we accomplished the gene profiling of fresh tissue specimens from 11 patients with the renal tumors (9 ccRCCs, 1 oncocytoma and 1 renal B-lymphoma), in which the tumor-related data were compared to the paired healthy kidney tissues from the same patients. All ccRCCs exhibited a considerably elevated transcription of the gene coding for carbonic anhydrase IX (CAIX). Moreover, the ccRCC tumors consistently displayed increased expression of genes encoding the glycolytic pathway enzymes, e.g. hexokinase II (HK2) and lactate dehydrogenase A (LDHA) and a decreased expression of genes for the mitochondrial electron transport chain components, indicating an overall reprogramming of the energetic metabolism in this tumor type. This appears to be accompanied by altered expression of the genes of the pH regulating machinery, including ion and lactate transporters. Immunohistochemical staining of tumor tissue sections confirmed the increased expression of CAIX, HK2 and LDHA in ccRCC, validating the microarray data and supporting their potential as the energetic metabolism-related biomarkers of the ccRCC.