Transcriptional and Translational control of the expression of the EMT transcription factor ZEB2 in cancer cells

Epithelial-mesenchymal transition (EMT) is a crucial process involved in the metastasis and invasion of cancer cells. EMT promotes migration, invasion and separation of individual cells from the primary cancer enabling them to access the vascular system and promoting tumour dissemination. In different types of human cancer, invading cells recapitulate elements of embryonic EMT pathways to extravasate into the blood stream and form metastases at distant sites. Several pleiotropically activated transcription factors, categorised as master regulators of EMT (EMT-TFs), acting downstream of EMT pathways have a central role in cancer. EMT-TFs include Zn finger transcription factors of SNAIL (SNAIL1 and SNAIL2) and ZEB (ZEB1 and ZEB2) families, basic helix-loop-helix (bHLH) proteins E47, TWIST1, TWIST2, a forkhead transcription factor FOXC2, and a few other relatively less studied proteins. Though ZEB family members, ZEB1 and ZEB2, are both efficient executers of EMT programs in human cancer and their roles in tumourigensis might be different. In particular, ZEB2, but not ZEB1 exhibited tumour-suppressive features in malignant melanoma and, possibly, hepatocellular carcinoma. Regulation of ZEB proteins expression occurs at different levels. The activity of three putative transcriptional enhancer controlling ZEB2 gene expression were analysed, and found that RE-4 regulatory element known to control ZEB2 expression in human embryonic stem cells (hESC) is also active in malignant melanoma cells. However, in contrast with hESC, in melanoma cells ZEB2 expression is not regulated by core pluripotency factors. Instead, in these cells, the activity of RE-4 element is affected by ZEB1- or ZEB2- induced EMT programs. By studying post-transcriptional level of ZEB regulation, a novel mechanism that limits ZEB2 protein synthesis was described in this study. A protein motif adjacent to the smad-binding domain within ZEB2 protein induces ribosome stalling and compromises translation. The activity of this motif is dependent on triplets of rare codons, Leu(UUA)-Gly(GGU)-Val(GUA). Introducing these stretches in the homologous region of ZEB1 has no effect on protein expression. By using retroviral expression of pBABE-ZEB2-mut, it was shown that these stretches might contribute to EMT. The data suggest that rare codons play a regulatory role in the context of appropriate protein structures and we speculate that pools of tRNA available for protein translation influence configuration of EMT programs in cancer cells.