Regulation of WNT signaling molecules by retinoic acid during neuronal differentiation in NT2 cells: Threshold model of WNT action (Review)

NT2/NTera2 cells, derived from human embryonal tumor, differentiate into neuronal cells after treatment with all-trans-retinoic acid (ATRA). We have cloned and characterized 13 out of 19 human WNT genes, and also 9 out of 10 human Frizzled (FZD) genes encoding seven-transmembrane-type WNT receptors, which are potent targets for pharmacogenomics in the post-genomic era, especially in the field of regenerative medicine and clinical oncology. Because WNT signals are implicated in morphogenesis of neural tissues, regulation of 19 WNT genes and 10 FZD genes during the early phase of neuronal differentiation in NT2 cells is reviewed. Multiple WNTs and FZDs are expressed in NT2 cells. WNT2B/WNT13 gene encode 2 isoforms due to alternative splicing of alternative promoter type, and WNT2B isoform 2 (WNT2B2) rather than WNT2B isoform 1 (WNT2B1) is expressed in NT2 cells. WNT3A, WNT8A, WNT8B, WNT10B and WNT11 are down-regulated in NT2 cells after ATRA treatment, while WNT2, WNT7B and WNT14B are up-regulated. FZD4 and FZD10 are up-regulated in NT2 cells after ATRA treatment. Expression of multiple WNT signaling molecules are dramatically changed during the early phase of neuronal differentiation in NT2 cells. Each WNT activates the β-catenin - TCF pathway, the JNK pathway or the Ca2+-releasing pathway in NT2 cells, and summed effects of multiple WNTs might determine the fate of NT2 cells (self-renewal or differentiation) through switching intracellular WNT signaling pathways. The author proposes the threshold model of WNT action.