Ribosomal Protein L23 Negatively Regulates Cellular Apoptosis via the RPL23/Miz-1/C-Myc Circuit in Higher-Risk Myelodysplastic Syndrome

Abstract

Ribosomal protein (RP) L23 is a negative regulator of cellular apoptosis, and RPL23 overexpression is associated with abnormal apoptotic resistance in CD34+ cells derived from patients with higher-risk myelodysplastic syndrome (MDS). However, the mechanism underlying RPL23-induced apoptotic resistance in higher-risk MDS patients is poorly understood. In this study, we showed that reduced RPL23 expression led to suppressed cellular viability, increased apoptosis and G1-S cell cycle arrest. Gene microarray analysis comparing RPL23-knockdown and control cells identified an array of differentially expressed genes, of which, Miz-1, was upregulated with transactivation of the cell cycle inhibitors p15Ink4b and p21 Myelodysplastic syndrome (MDS) is a clonal haematopoietic stem cell disorder that is characterized by peripheral cytopenia, hypercellular bone marrow (BM), and increased mortality due to a substantial risk of progression to acute myeloid leukaemia (AML)1 . Lower-risk (IPSS score ≤1.0) MDS patients primarily suffer from BM hypercellularity and peripheral cytopenia resulting from a significantly increased rate of apoptosis among BM haematopoietic cells. In contrast, BM haematopoietic cells in higher-risk (IPSS score >1.0) MDS patients demonstrate resistance to apoptosis, and some of these patients subsequently develop AML 2-4 . These opposing biological characteristics of BM haematopoietic cells lead to differences in disease progression and prognosis. However, the molecular pathogenesis underlying the apoptotic resistance observed in the BM haematopoietic cells of higher-risk MDS patients is poorly understood.Ribosomal proteins (RPs), which are components of ribosomal subunits, are ubiquitous RNA-binding proteins that carry out multiple auxiliary extraribosomal functions and are moderately related to tumourigenesis [5][6][7] . Ribosomal protein L23 (RPL23) is a protein component of the 60S large ribosomal subunit. As previously reported, RPL23 acts as a negative regulator of apoptosis by suppressing Miz-1-induced transcriptional activation of the cell cycle inhibitors p15Ink4b 8 and p21 Cip1 9, 10 . Myc-associated zinc-finger protein (Miz-1) is a ubiquitous transcription factor that has been functionally characterized as a transcriptional inducer of p15Ink4b and p21 Cip1. However, in the presence of its repressor, c-Myc, the function of Miz-1 shifts from activation to transcriptional repression. c-Myc and Miz-1 form a co-repressor complex that silences Miz-1's target genes, including the cell-cycle inhibitors p15Ink4b and p21 Cip1. Furthermore, Miz-1 quantitatively competes against and overcomes the inhibitory effects of c-Myc by competitively binding to promoters of its target genes 11,12 . A microarray analysis comprising 40,000 cDNA gene chip arrays conducted by Sridhar et al. 13 identified 11 differentially expressed genes between CD34+ BM cells from tMDS (patients whose MDS transformed to AML) and sMDS (patients who remained stable). Overexpression of genes encoding five ribosomal proteins and