ABSTRACT
Bcr-Abl (break-point cluster region-abelson), the oncogenic trigger of chronic myelogenous leukemia (CML), has previously been shown to up-regulate the expression and activity of sphingomyelin synthase 1 (SMS1), which contributes to the proliferation of CML cells; however, the mechanism by which this increased expression of SMS1 is mediated remains unknown. In the current study, we show that Bcr-Abl enhances the expression of SMS1 via a 30-fold up-regulation of its transcription. Of most interest, the Bcr-Abl-regulated transcription of SMS1 is initiated from a novel transcription start site (TSS) that is just upstream of the open reading frame. This shift in TSS utilization generates an SMS1 mRNA with a substantially shorter 5' UTR compared with its canonical mRNA. This shorter 5' UTR imparts a 20-fold greater translational efficiency to SMS1 mRNA, which further contributes to the increase of its expression in CML cells. Therefore, our study demonstrates that Bcr-Abl increases SMS1 protein levels via 2 concerted mechanisms: up-regulation of transcription and enhanced translation as a result of the shift in TSS utilization. Remarkably, this is the first time that an oncogene-Bcr-Abl-has been demonstrated to drive such a mechanism that up-regulates the expression of a functionally important target gene, SMS1.-Moorthi, S., Burns, T. A., Yu, G.-Q., Luberto, C. Bcr-Abl regulation of sphingomyelin synthase 1 reveals a novel oncogenic-driven mechanism of protein up-regulation.
Subject(s)
Carcinogenesis/genetics , Fusion Proteins, bcr-abl/genetics , Membrane Proteins/genetics , Nerve Tissue Proteins/genetics , Transferases (Other Substituted Phosphate Groups)/genetics , Up-Regulation/genetics , 5' Untranslated Regions/genetics , Cell Line, Tumor , HL-60 Cells , HeLa Cells , Humans , K562 Cells , Open Reading Frames/genetics , RNA, Messenger/genetics , Transcription Initiation Site/physiology , Transcription, Genetic/geneticsABSTRACT
Bcr-abl1 oncogene causes a shift in the transcription start site of the SMS1 gene (SGMS1) encoding the sphingomyelin (SM) synthesizing enzyme, sphingomyelin synthase 1 (SMS1). This results in an mRNA with a significantly shorter 5'-UTR, called 7-SGMS1, which is translated more efficiently than another transcript (IIb-SGMS1) with a longer 5'UTR in Bcr-abl1-positive cells. Here, we determine the effects of these alternative 5'UTRs on SMS1 translation and investigate the key features underlying such regulation. First, the presence of the longer IIb 5'UTR is sufficient to greatly impair translation of a reporter gene. Deletion of the upstream open reading frame (-164 nt) or of the predicted stem-loops in the 5'UTR of IIb-SGMS1 has minimal effects on SGMS1 translation. Conversely, deletion of nucleotides -310 to -132 enhanced transcription of IIb-SGMS1 to reach that of 7-SGMS1. We thus suggest that regulatory features within nucleotides -310 and -132 modulate IIb-SGMS1 translation efficiency.