Kim Dong Uk, Nam Jehyun, Cha Matthew D, Kim Sang-Woo
Department of Biological Sciences, Pusan National University, Busan 46241, Republic of Korea.
Department of Biological Sciences, The University of Texas at Dallas, Richardson, TX 75080, USA.
Oncol Lett. 2019 Mar;17(3):3589-3598. doi: 10.3892/ol.2019.9996. Epub 2019 Jan 31.
Colorectal cancer (CRC) is a complex disease involving numerous genetic abnormalities. One of the major characteristics of CRC is enhanced Wnt signaling caused by loss-of-function mutations in the adenomatous polyposis coli (APC) gene. Previously, it has been demonstrated that the majority of malignant phenotypes following APC deletion in adult murine small intestines could be rescued when Myc, a downstream target of the Wnt pathway, was deleted. This indicated that Myc is a critical regulator of CRC development following APC loss. Previous studies reported that cyclic adenosine 3',5'-monophosphate (cAMP) can influence the AKT/mammalian target of rapamycin (mTOR) survival pathway in cancer and Myc is a critical downstream molecule of AKT/mTOR signaling. Phosphodiesterase 4D (PDE4D), a member of the cAMP-specific PDE4 family, has been associated with drug resistance in CRC. However, the association between PDE4D and Myc remains unclear. To investigate the potential role of PDE4D in Myc regulation in CRC, the present study evaluated the expression levels of PDE4 subtypes in DLD-1 CRC cells. Additionally, the effects of PDE4 inhibitors on Myc expression and oncogenic properties were analyzed by western blot analysis, reverse transcription-quantitative polymerase chain reaction, colony formation and soft agar assays. It was demonstrated that cAMP/PDE4D signals serve a critical role in regulating Myc expression in DLD-1 CRC cells. Furthermore, PDE4D was identified to be a main hydrolyzer of cAMP and suppression of PDE4D using selective inhibitors of PDE4 increased intracellular cAMP levels, which resulted in a marked decrease in the oncogenic properties of DLD-1 cells, including colony formation, cell proliferation and anchorage-independent growth. Notably, the current data imply that cAMP represses Myc expression via the downregulation of AKT/mTOR signaling, which was abolished by high PDE4D activities in DLD-1 cells. Additionally, a natural polyphenol resveratrol in combination with forskolin elevated the concentration of cAMP and enhanced the expression of Myc and the malignant phenotype of DLD-1 cells, reproducing the effect of known chemical inhibitors of PDE4. In conclusion, the present study identified that cAMP/PDE4D signaling is a critical regulator of Myc expression in DLD-1 and possibly other CRC cells.
结直肠癌（CRC）是一种涉及众多基因异常的复杂疾病。CRC的主要特征之一是由腺瘤性息肉病 coli（APC）基因功能丧失突变导致的Wnt信号增强。此前已证明，当Wnt通路的下游靶点Myc被敲除时，成年小鼠小肠中APC缺失后的大多数恶性表型能够得到挽救。这表明Myc是APC缺失后CRC发展的关键调节因子。先前的研究报道，环磷酸腺苷（cAMP）可影响癌症中的AKT/雷帕霉素哺乳动物靶点（mTOR）生存通路，且Myc是AKT/mTOR信号的关键下游分子。磷酸二酯酶4D（PDE4D）是cAMP特异性PDE4家族的成员，与CRC中的耐药性有关。然而，PDE4D与Myc之间的关联仍不清楚。为了研究PDE4D在CRC中对Myc调控的潜在作用，本研究评估了DLD-1 CRC细胞中PDE4亚型的表达水平。此外，通过蛋白质免疫印迹分析、逆转录-定量聚合酶链反应、集落形成和软琼脂试验分析了PDE4抑制剂对Myc表达和致癌特性的影响。结果表明，cAMP/PDE4D信号在调节DLD-1 CRC细胞中Myc表达方面起关键作用。此外，PDE4D被确定为cAMP的主要水解酶，使用PDE4选择性抑制剂抑制PDE4D可提高细胞内cAMP水平，这导致DLD-1细胞的致癌特性显著降低，包括集落形成、细胞增殖和非锚定依赖性生长。值得注意的是，目前的数据表明，cAMP通过下调AKT/mTOR信号来抑制Myc表达，而DLD-1细胞中高PDE4D活性可消除这种抑制作用。此外，天然多酚白藜芦醇与福司可林联合使用可提高cAMP浓度，并增强Myc的表达以及DLD-1细胞的恶性表型，重现了已知PDE4化学抑制剂的作用效果。总之，本研究确定cAMP/PDE4D信号是DLD-1以及可能其他CRC细胞中Myc表达的关键调节因子。
