SV40-induced expression of calretinin protects mesothelial cells from asbestos cytotoxicity and may be a key factor contributing to mesothelioma pathogenesis.

The calcium-binding protein calretinin has emerged as a useful marker for the identification of mesotheliomas of the epithelioid and mixed types, but its putative role in tumor development has not been addressed previously. Although exposure to asbestos fibers is considered the main cause of mesothelioma, undoubtedly, not all mesothelioma patients have a history of asbestos exposure. The question as to whether the SV40 virus is involved as a possible co-factor is still highly debated. Here we show that increased expression of SV40 early gene products in the mesothelial cell line MeT-5A induces the expression of calretinin and that elevated calretinin levels strongly correlate with increased resistance to asbestos cytotoxicity. Calretinin alone mediates a significant part of this protective effect because cells stably transfected with calretinin cDNA were clearly more resistant to the toxic effects of crocidolite than mock-transfected control cells. Down-regulation of calretinin by antisense methods restored the sensitivity to asbestos toxicity to a large degree. The protective effect observed in clones with higher calretinin expression levels could be eliminated by phosphatidylinositol 3-kinase (PI3K) inhibitors, implying an important role for the PI3K/AKT signaling (survival) pathway in mediating the protective effect. Up-regulation of calretinin, resulting from either asbestos exposure or SV40 oncoproteins, may be a common denominator that leads to increased resistance to asbestos cytotoxicity and thereby contributes to mesothelioma carcinogenesis.

Heterozygosity of SNP513 in intron 9 of the human calretinin gene (CALB2) is a risk factor for colon cancer.

BACKGROUND: The Ca2+-binding protein calretinin (CR), while absent in normal colonocytes, is expressed in the majority of poorly differentiated colon carcinomas, and is hypothesized that mutations in the distal part (from exon 7 to 10) of the CR gene (CALB2) could be responsible for the aberrant CR expression. MATERIALS AND METHODS: Using PCR amplification of genomic DNA and restriction fragment length polymorphism analysis, four single nucleotide polymorphisms (SNPs) were identified in CALB2 intron 9. RESULTS: A significant positive association between the genotype at SNP513 and colon cancer was found: more C/T heterozygotes were found in patients with colon tumors (60%) compared to healthy controls (35%) or patients with lung tumors (38%). Our results indicate that C/T heterozygosity at position 513 is linked to CR expression in colon tumors and colon cancer cell lines, suggesting a link with increased carcinogenesis. CONCLUSION: The SNP513 in human CALB2 may thus be a predictive marker for colon tumors.