In normal lung, the predominant cytoplasmic carbonic anhydrase (CA) isozyme (CAII) is highly expressed in amine- and peptide-producing pulmonary neuroendocrine cells where its role involves CO2 sensing. Here, we report robust cytoplasmic expression of CAII by immunohistochemistry in the tumor cells of different native neuroendocrine tumor (NET) types, including typical and atypical carcinoids and small-cell lung carcinomas, and in NET and non-NET tumor cell lines. Because, in both pulmonary neuroendocrine cell and related NETs, the hypercapnia-induced secretion of bioactive serotonin (5-hydroxytryptamine) is mediated by CAII, we investigated the role of CAII in the biological behavior of carcinoid cell line H727 and the type II cell-derived A549 using both in vitro clonogenicity and in vivo xenograft model. We show that short hairpin RNA-mediated down-regulation of CAII resulted in significant reduction in clonogenicity of H727 and A549 cells in vitro, and marked suppression of tumor growth in vivo. CAII-short hairpin RNA cell-derived xenografts showed significantly reduced mitosis (phosphohistone H3 marker) and proliferation associated antigen Ki-67 (Ki67 marker), and significantly increased apoptosis by terminal deoxynucleotidyl transferase dUTP nick end labeling assay. Using an apoptosis gene array, we found no association with caspases 3 and 8, but with a novel association of CAII-mediated apoptosis with specific mitochondrial apoptosis-associated proteins. Furthermore, these xenografts showed a significantly reduced vascularization (CD31 marker). Thus, CAII may play a critical role in NET lung tumor growth, angiogenesis, and survival, possibly via 5-hydroxytryptamine, known to drive autocrine tumor growth. As such, CAII is a potential therapeutic target for the difficult-to-treat lung NETs.
Keywords: apoptosis; carbonic anhydrase II; clonogenicity; tumor; vascularization.