The polyamine/hypusine signaling promotes colorectal cancer cell growth by regulating c-Myc translation

Colorectal cancer is the most frequent malignancy of the gastroenteric system and it is the third most common cancer among males and the second among women. The development of colorectal cancer is determined by progressive genetic and epigenetic alterations which, starting from the formation of adenomatous polyps, lead to the formation of malignant lesions. The mutations include pathways regulatory genes involved in development, proliferation and survival of cancer cells, such as APC/beta catenin, Ras-BRAF/MAPK, p53, pI3K/AKT and SMAD/TGFbeta. In most cases, the aberrant activation of these pathways converges in the overexpression of the oncogene c-Myc. Therefore, inhibition of this oncogene could represent an effective anti-cancer strategy in many types of colorectal cancer. Previous studies have shown that the hypusination, post-translation modification that activates the elongation factor EIF5A, plays an important role in tumour progression. This modification is mediated by two enzymes: DHPS and DOHH that use spermidine as a substrate, an over-expressed polyamine in many solid tumours. The aim of this project was to understand the role of hypusination pathway in CRC and the effect of its pharmacological and genetical targeting. We have shown how inhibition of the EIF5A hypusination, induced by treatment with GC7 or by gene ablation of DHPS in colorectal cancer cells (HCT116, HT29, SW480 and LoVo), has a strong anti-proliferative effect on CRC cells in vitro and in vivo. By analysing the activation status of major pathways involved in tumour onset and progression, we have shown that the DHPS-EIF5A1 axis regulates c-Myc protein levels. The regulation does not alter the mRNA levels or the stability of c-Myc but its translation. Through multi/single site mutagenesis experiments we identified the region of c-Myc involved in EIF5A-mediated regulation. In conclusion, this work shows that the oncogene c-Myc is a translation target of the elongation factor EIF5A; the inhibition of the hypusination causes the blocking of the translation of this oncogene and a decrease in colorectal cancer cells proliferation in vitro and in vivo. Such evidence could be the basis for new therapeutic approaches in colorectal tumours based on the blocking of c-Myc translation by DHPS pharmacological or genetical inhibition.