Functional characterization of oncoprotein PLZF and PLZF-RARα, and molecular targets

Abstract

Aberrant transcription repression through chromatin remodeling and histone deacetylation has been postulated as the driving force for tumorigenesis. PLZF (promyelocytic leukemia zinc finger) is initially isolated in fusion protein form with RARα (retinoic acid receptor alpha), which causes in APL (acute promyelocytic leukemia)-type leukemia. IHC(immunohistochemistry) of tumor microtissues arrays and oncomine data indicate that PLZF is aberrantly overexpressed in some human solid tumors, such as kidney renal carcinoma, testis seminoma, brain glioblastoma, and prostate adenocarcinoma. The data suggest that PLZF may be an oncoprotein involved in oncogenesis and cell proliferation. PLZF-RARα is considered as oncoprotein generated by chromosomal translocation between PLZF andRARα genes in the APL-type leukemic patients. Although PLZF-RARα was shown to cause leukemia and the molecular interaction between PLZF-RARα and corepressor-HDAC (Histon deacetylase) complex is important in oncogenesis, the target genes and action mechanism of PLZF-RARα remained largely unknown.I found that PLZF can repress transcription of the p21WAF/CDKN1A gene encoding p21, by binding to the proximal Sp1-binding GC-box 5/6 and the distal p53-responsive elements of the CDKN1A promoter. PLZF can also repress transcription of TP53 and destabilize p53 by deacetylation and ubiquitination, which also decreases transcription of CDKN1A. PLZF interacts with co-repressors, such as mSin3A, NCoR and SMRT, and deacetylateshistones H3 and H4 of the nucleosomes around the p21WAF/CDKN1A promoter. PLZF is an oncoprotein that causes cellular transformation and promotes cell proliferation in the HEK293A, HCT116, and H460 cells. PLZF does not induce apoptosis. PLZF-RARα promotes cell proliferation by repressing the transcription of the CDKN1A gene. Molecular characterization of oncoprotein PLZF-RARα revealed molecular mechanism similar to thePLZF, but PLZF-RARα also showed unique features in transcriptional repression PLZF-RARα is a competitive transcription repressor of p53, RARα and Sp1 and was shown to bind the proximal Sp1-binding GC-boxes 3, 4, 5/6, the RARE and the distal p53-responsive elements of CDKN1A promoter. PLZF-RARα also interacted with co-repressors, such as mSin3A, NCoR and SMRT, thereby deacetylates histones H3 and H4 of the nucleosomes around the CDKN1A promoter. Additionally, I found that PLZFRARα interacts with MBD3-NuRD complex, which leads to epigenetic silencing of the CDKN1A by DNA methylation and heterochromatinformation. PLZF and PLZF-RARα are the oncoproteins that repress transcription of the CDKN1A of the p53 pathway, by direct binding competition to the key regulatory elements such as proximal GC-boxes, distal p53 binding elements and RARE with Sp1, p53, and RARα. PLZF and PLZFRARα also represses transcription of CDKN1A by repressing transcription ofTP53 and by controlling the stability of p53.