Identification and characterization of human MEX-3, a novel family of RNA-binding proteins that regulate cell proliferation

4622 The Cdx2 homeobox gene plays a key role in intestinal morphogenesis and has been proposed to exert a tumor suppressor function in the distal colon. Despite extensive studies on Cdx2, the mechanisms that regulate its expression are poorly understood. We have identified the human homologues of the Caenorhabditis elegans Mex-3 gene. Mex-3 encodes a KH domain protein that specifies the blastomeres identity in the early embryo through the spatial and temporal control of pal-1, the C. elegans ortholog of the Cdx genes. Four distinct human genes called hMex-3A to -3D located respectively on chromosome 1q22-23, 15q24-25, 18q21, 19p13.3 have been characterized. The four related hMex-3 proteins display two RNA-binding KH domains and one RING finger module. We found that the hMex-3A, -3B and -3C are phospho-proteins that shuttle between the nucleus and the cytoplasm owing to the presence of functional NLS and NES sequences. hMex-3 proteins bind in vitro to RNA, a process which requires an intact KH domain. hMex-3A, -3B and -3C promote growth arrest when expressed in HeLa (cervical carcinoma) and G361 (human melanoma) cell lines and this effect was abolished when the KH domain was disrupted. Finally, we obtained evidence that hMex-3A, -3B and -3C proteins negatively regulate Cdx2 expression. Interestingly, immuno-histochemical analyses with an anti-hMex-3B antibody revealed that the protein was expressed in the colonic epithelium, specifically in the perinuclear region of mucin-secreting cells. These findings indicate that the Mex-3-pal1/Cdx2 pathway has been evolutionarily conserved. Combined with the chromosomal localization of hMex-3 genes, which lie in regions frequently subjected to loss of heterozygosity in tumors, these results suggest that hMex-3 are putative tumor suppressor gene candidates.