Meta-Biol

• Pathways

RUNX3, like other RUNX family members, is transcribed from two promoters - the proximal P2 promoter and the distal P1 promoter. The P2 promoter is positioned within a large CpG island that is frequently methylated in solid tumors, resulting in epigenetic inactivation of the RUNX3 gene (reviewed by Levanon and Groner 2004). RUNX3 transcription is affected by SMAD4 levels. RUNX3 may directly upregulate its own transcription through a positive feedback loop (Whittle et al. 2015). Under hypoxic conditions, RUNX3 transcription is downregulated. Hypoxic silencing of RUNX3 involves hypoxia-induced upregulation of the histone methyltransferase G9a and histone deacetylase HDAC1, which leads to increased dimethylation of histone H3 at lysine residue K9 (K10 when taking into account the initiator methionine) and reduced acetylation of histone H3 at the RUNX3 promoter (Lee et al. 2009).
RUNX3 protein levels are inversely related to the levels of microRNA miR-130b. Based on in silico analysis, RUNX3 is predicted to be the target of miR-130b, but binding assays and 3'UTR reporter assays have not been done to confirm this (Lai et al. 2010, Paudel et al. 2016).
Similar to RUNX1 and RUNX2, RUNX3 forms a transcriptionally active heterodimer with CBFB (CBF-beta) (Kim et al. 2013). RUNX3 activity can be regulated by changes in RUNX3 localization. SRC protein tyrosine kinase phosphorylates RUNX3 on multiple tyrosine residues, inhibiting its translocation from the cytosol to the nucleus and thus inhibiting RUNX3-mediated transcription (Goh et al. 2010). Subcellular localization of RUNX3 may be affected by PIM1-mediated phosphorylation (Kim et al. 2008).
The P1 and P2 promoters regulate RUNX3 transcription in a cell-type/differentiation dependent manner, giving rise to the p44 and p46 isoforms of RUNX3, respectively. Several splicing isoforms have also been reported. One example is the generation of a 33 kDa protein isoform (p33) by alternative splicing. The RUNX3 p33 isoform lacks the Runt domain and is unable to transactivate the regulatory regions of integrin genes. The p33 isoform is induced during maturation of monocyte-derived dendritic cells (MDDC), leading to reduced expression of genes involved in inflammatory responses, such as IL8 (interleukin-8) (Puig-Kroger et al. 2010).
E3 ubiquitin ligases MDM2 (Chi et al. 2009), SMURF1 and SMURF2 (Jin et al. 2004) are implicated in RUNX3 polyubiquitination and degradation.

RNA polymerase II (Pol II) is the central enzyme that catalyses DNA- directed mRNA synthesis during the transcription of protein-coding genes. Pol II consists of a 10-subunit catalytic core, which alone is capable of elongating the RNA transcript, and a complex of two subunits, Rpb4/7, that is required for transcription initiation.
The transcription cycle is divided in three major phases: initiation, elongation, and termination. Transcription initiation include promoter DNA binding, DNA melting, and initial synthesis of short RNA transcripts. The transition from initiation to elongation, is referred to as promoter escape and leads to a stable elongation complex that is characterized by an open DNA region or transcription bubble. The bubble contains the DNA-RNA hybrid, a heteroduplex of eight to nine base pairs. The growing 3-end of the RNA is engaged with the polymerase complex active site. Ultimately transcription terminates and Pol II dissocitates from the template.

Gene expression encompasses transcription and translation and the regulation of these processes. RNA Polymerase I Transcription produces the large preribosomal RNA transcript (45S pre-rRNA) that is processed to yield 18S rRNA, 28S rRNA, and 5.8S rRNA, accounting for about half the RNA in a cell. RNA Polymerase II transcription produces messenger RNAs (mRNA) as well as a subset of non-coding RNAs including many small nucleolar RNAs (snRNA) and microRNAs (miRNA). RNA Polymerase III Transcription produces transfer RNAs (tRNA), 5S RNA, 7SL RNA, and U6 snRNA. Transcription from mitochondrial promoters is performed by the mitochondrial RNA polymerase, POLRMT, to yield long transcripts from each DNA strand that are processed to yield 12S rRNA, 16S rRNA, tRNAs, and a few RNAs encoding components of the electron transport chain. Regulation of gene expression can be divided into epigenetic regulation, transcriptional regulation, and post-transcription regulation (comprising translational efficiency and RNA stability). Epigenetic regulation of gene expression is the result of heritable chemical modifications to DNA and DNA-binding proteins such as histones. Epigenetic changes result in altered chromatin complexes that influence transcription. Gene Silencing by RNA mostly occurs post-transcriptionally but can also affect transcription. Small RNAs originating from the genome (miRNAs) or from exogenous RNA (siRNAs) are processed and transferred to the RNA-induced silencing complex (RISC), which interacts with complementary RNA to cause cleavage, translational inhibition, or transcriptional inhibition.