Liu Yen-Chun, Li Feng, Handler Jesse, Huang Cheng Ran Lisa, Xiang Yan, Neretti Nicola, Sedivy John M, Zeller Karen I, Dang Chi V
Program in Pathobiology, Johns Hopkins University School of Medicine, Baltimore, Maryland, United States of America.
PLoS One. 2008 Jul 16;3(7):e2722. doi: 10.1371/journal.pone.0002722.
The c-Myc transcription factor is a master regulator and integrates cell proliferation, cell growth and metabolism through activating thousands of target genes. Our identification of direct c-Myc target genes by chromatin immunoprecipitation (ChIP) coupled with pair-end ditag sequencing analysis (ChIP-PET) revealed that nucleotide metabolic genes are enriched among c-Myc targets, but the role of Myc in regulating nucleotide metabolic genes has not been comprehensively delineated.
METHODOLOGY/PRINCIPAL FINDINGS: Here, we report that the majority of genes in human purine and pyrimidine biosynthesis pathway were induced and directly bound by c-Myc in the P493-6 human Burkitt's lymphoma model cell line. The majority of these genes were also responsive to the ligand-activated Myc-estrogen receptor fusion protein, Myc-ER, in a Myc null rat fibroblast cell line, HO.15 MYC-ER. Furthermore, these targets are also responsive to Myc activation in transgenic mouse livers in vivo. To determine the functional significance of c-Myc regulation of nucleotide metabolism, we sought to determine the effect of loss of function of direct Myc targets inosine monophosphate dehydrogenases (IMPDH1 and IMPDH2) on c-Myc-induced cell growth and proliferation. In this regard, we used a specific IMPDH inhibitor mycophenolic acid (MPA) and found that MPA dramatically inhibits c-Myc-induced P493-6 cell proliferation through S-phase arrest and apoptosis.
CONCLUSIONS/SIGNIFICANCE: Taken together, these results demonstrate the direct induction of nucleotide metabolic genes by c-Myc in multiple systems. Our finding of an S-phase arrest in cells with diminished IMPDH activity suggests that nucleotide pool balance is essential for c-Myc's orchestration of DNA replication, such that uncoupling of these two processes create DNA replication stress and apoptosis.
c-Myc转录因子是一个主要调控因子，通过激活数千个靶基因来整合细胞增殖、细胞生长和代谢。我们通过染色质免疫沉淀（ChIP）结合双末端标签测序分析（ChIP-PET）鉴定直接的c-Myc靶基因，结果显示核苷酸代谢基因在c-Myc靶标中富集，但Myc在调控核苷酸代谢基因中的作用尚未得到全面描述。
方法/主要发现：在此，我们报告在P493-6人伯基特淋巴瘤模型细胞系中，人类嘌呤和嘧啶生物合成途径中的大多数基因被c-Myc诱导并直接结合。在Myc缺失的大鼠成纤维细胞系HO.15 MYC-ER中，这些基因中的大多数也对配体激活的Myc-雌激素受体融合蛋白Myc-ER有反应。此外，这些靶标在体内转基因小鼠肝脏中也对Myc激活有反应。为了确定c-Myc调控核苷酸代谢的功能意义，我们试图确定直接的Myc靶标肌苷单磷酸脱氢酶（IMPDH1和IMPDH2）功能丧失对c-Myc诱导的细胞生长和增殖的影响。在这方面，我们使用了一种特异性的IMPDH抑制剂霉酚酸（MPA），发现MPA通过S期阻滞和凋亡显著抑制c-Myc诱导的P493-6细胞增殖。
结论/意义：综上所述，这些结果证明了c-Myc在多个系统中对核苷酸代谢基因的直接诱导作用。我们发现IMPDH活性降低的细胞出现S期阻滞，这表明核苷酸库平衡对于c-Myc协调DNA复制至关重要，以至于这两个过程的解偶联会产生DNA复制应激和凋亡。
