-To elucidate the molecular mechanism involved in methylmercury-induced cerebellar disorder, we performed DNA microarray analysis of the cerebellum of methylmercury-treated mice. The expression levels of 21 genes were elevated 2-fold or higher in response to methylmercury, including many genes encoding proteins involved in inflammatory reactions associated with chemokines. The expression levels of 11 genes were reduced by half or more in response to methylmercury.

-Methylmercury is a hazardous heavy metal compound that can damage the central nervous system. The mechanisms of methylmercury toxicity and the biological defense mechanisms against it remain unclear. We employed gene knockdown using siRNA to search for transcription factors involved in the manifestation of methylmercury toxicity. We found that the inhibition of expression of the gene for heat shock transcription factor 1 (HSF1) sensitized HEK293 cells to methylmercury.

-Methylmercury is an environmental pollutant that causes severe central nervous system disorders. We searched for transcription factors involved in the development of methylmercury toxicity and found that the decreased expression of a homeobox protein, HOXB13, in HEK293 cells conferred strong resistance to methylmercury. Key words: Methylmercury, Homeobox protein, HOXB13, Transcription factorCorrespondence: Akira Naganuma (E-mail: naganuma@mail.pharm.tohoku.ac.jp) Toxicogenomics/proteomics ReportThe Journal of Toxicological Sciences (J. Toxicol. Sci.) Vol.35, No.6, 941-944, 2010 Vol. 35 No. 6 941 cells/well in 6-well plates) were rinsed with 1 × PBS. Then, the total RNA was isolated using a FastPure ® RNA kit (Takara, Shiga, Japan), and first-strand cDNA synthesis was performed using the PrimeScrip ® RT reagent kit (Takara) in accordance with the manufacturer's protocol (Ikawa et al., 2008;Chen et al., 2009;Kawakami et al., 2009). We performed real-time PCR reactions (Thermal Cycler Dice ® , Takara) with the following primers: HOXB13-F, 5'-CTGGGTAGGTGGACAATTG-TA-3', and HOXB13-R, 5'-TACGCTGATGCCTGCT-GTCAA-3', for the HOXB13 gene; and GAPDH-F, 5'-GCACCGTCAAGGCTGAGAAC-3', and GAPDH-R, 5'-TGGTGAAGACGCCAGTGGA-3', for the GAP-DH gene. The fold decreases in HOXB13 mRNA levels were determined from standard curves after calibration of the assay. RESULTS AND DISCUSSIONWe selected about 1,000 proteins that are expected to function as transcription factors from DBD, a transcription factor prediction database (http://www.transcriptionfactor.org). Two double-stranded siRNAs targeting protein-coding genes were designed and introduced into HEK293 cells. The cells with introduced siRNAs were cultured for 48 hr in the presence of methylmercuric chloride (4 μM), which suppresses the proliferation of normal cells by about 60%. HOXB13 was identified as a factor whose reduction conferred strong methylmercury resistance to the cells. We introduced double-stranded siRNAs (siRNA Nos. 1 and 2) targeting HOXB13 into HEK293 cells. Cells harboring HOXB13 siRNA No. 1 exhibited slightly stronger methylmercury resistance than control cells. Cells harboring HOXB13 siRNA No. 2 exhibited marked methylmercury resistance (Fig. 1A). Cells harboring both siRNAs exhibited stronger methylmercury resistance than those harboring HOXB13 siRNA No. 2 alone (Fig. 1A) and the lowest HOXB13 mRNA level (about 16% of the level in control cells; Fig. 1B). HOXB13 mRNA levels in the cells harboring HOXB13 siRNA No. 1 or 2 were about 64% and 25% of the levels in the control cells, respectively (Fig. 1B). These results suggest that HOXB13 increases methylmercury toxicity. Decreased HOXB13 expression had little effect on the susceptibility of the cells to three metallic compounds other than methylmercury (Fig. 2). This suggests that HOXB13 may specifically increase methylmercury toxicity. We introduced HOXB13 with a V5 tag fused at the C terminus (HOXB13-V5) into HEK293 cells to investigate the intracellular distribution of HOXB13 and found HOXB13 predomi...