Hypoxia induced CA9 inhibitory targeting by two different sulfonamide derivatives including Acetazolamide in human Glioblastoma

HIF-1α regulated genes are mainly responsible for tumour resistance to radiation- and chemo-therapy. Among these genes, carbonic anhydrase isoform IX (CA9) is highly over expressed in many types of cancer especially in high grade brain cancer like Glioblastoma (GBM). Inhibition of the enzymatic activity by application of specific chemical CA9 inhibitor sulphonamides (CAI) like Acetazolamide (Aza.), the new sulfonamide derivative carbonic anhydrase inhibitor (SU.D2) or indirect inhibitors like the HIF-1α inhibitor Chetomin or molecular inhibitors like CA9-siRNA are leading to an inhibition of the functional role of CA9 during tumorigenesis. Human GBM cells were treated with in vitro hypoxia (1, 6, or 24 h at 0.1%, O₂). Aza. application was at a range between 250 and 8000 nM and the HIF-1α inhibitor Chetomin at a concentration range of 150–500 nM. Cell culture plates were incubated for 24 h under hypoxia (0.1% O₂). Further, CA9-siRNA constructs were transiently transfected into GBM cells exposed to extreme hypoxic aeration conditions. CA9 protein expression level was detectable in a cell-type specific manner under normoxic conditions. Whereas U87-MG exhibited a strong aerobic expression, U251 and U373 displayed moderate and GaMG very weak normoxic CA9 protein bands. Aza. as well as SU.D2 displayed inhibitory characteristics to hypoxia induced CA9 expression in the four GBM cell lines for 24 h of hypoxia (0.1% O₂) at concentrations between 3500 and 8000 nM, on both the protein and mRNA level. Parallel experiments using CA9-siRNA confirmed these results. Application of 150–500 nM of the glycolysis inhibitor Chetomin under similar oxygenation conditions led to a sharply reduced expression of both CA IX protein and CA9 mRNA levels, indicating a clear glucose availability involvement for the hypoxic HIF-1α and CA9 expression in GBM cells. Hypoxia significantly influences the behaviour of human tumour cells by activation of genes involved in the adaptation to hypoxic stress. The main objective in malignant GBM therapy is either to eradicate the tumour or to convert it into a controlled, quiescent chronic disease. Aza., SU.D2, Chetomin or CA9-siRNA possesses functional CA9 inhibitory characteristics when applied against human cancers with hypoxic regions like GBM. They may be used as alternative or in conjunction with other direct inhibitors possessing similar functionality, thereby rendering them as potential optimal tools for the development of an optimized therapy in human brain cancer treatment.