Aberrant DNA methylation-mediated NF-icB/fatty acid-binding protein 5 (FABP5) feed-forward loop promotes malignancy of colorectal cancer cells

Fatty acid-binding proteins (FABPs) are intracellular lipid-binding proteins that play roles in fatty acid transport and the regulation of gene expression. Dysregulated FABP expression and/or activity have been associated with cancer pathogenesis; in particular, epidermal-type FABP (FABP5) is upregulated in many types of cancer. However, the mechanisms regulating FABP5 expression and its involvement in cancer remain largely unknown. Here, we examined the regulation of FABP5 gene expression in non-metastatic and metastatic human colorectal cancer (CRC) cells. We found that FABP5 expression was upregulated in metastatic compared with nonmetastatic CRC cells as well as in human CRC tissues compared with adjacent normal tissue. Analysis of the DNA methylation status of the FABP5 promoter showed that hypomethylation correlated with the malignant potential of the CRC cell lines. Moreover, FABP5 promoter hypomethylation also correlated with the expression pattern of splice variants of the DNA methyltransferase DNMT3B. ChIP assays and luciferase reporter assays demonstrated that the transcription factor nuclear factor-kappa B (NF-icB) was involved in regulating FABP5 expression. FABP5 expression could be upregulated in metastatic CRC cells by sequential promotion of DNA demethylation followed by activation of NF-& kappa;B. We also found that upregulated FABP5 in turn controlled NF-icB activity through IL-8 production. Collectively, these findings suggest the existence of a DNA methylationdependent NF-icB /FABP5 positive feed-forward loop that may lead to constitutive activation of NF-icB signaling pathway and play a crucial role in CRC progression.