The EGFR, a transmembrane protein receptor and an important member of tyrosine kinase receptors, is commonly elevated in cancers, engaging multiple malignant functions such as aberrant activation of signaling, uncontrolled cell proliferation, vascular invasion and metastasis of the tumors. Accumulating evidence indicates that EGFR expression is significantly correlated with pancreatic cancer, high expression of EGFR frequently suggests a poor prognosis. Although many EGFR antibodies and inhibitors, including cetuximab, afatinib, osimertinib, erlotinib and gefitinib, have been applied to cancer treatment. However, the anticancer efficacy of them have negligible effects on patients with pancreatic cancer, especially in KRAS mutant pancreatic ductal adenocarcinoma. Therefore, it’s urgent to carry out new drugs or novel combination therapy regimens to control pancreatic cancer process.
Recently, chemoprevention substances naturally existing in diets and medicinal plants have attracted widespread attentions. 6-P, a phenolic compound in the rhizome of ginger, was reported that 6-P had potent anti-inflammatory activity, which exerted huge anticancer functions. Previous study suggested 6-P and its derivative 6-G had the ability to reduce the viability of HL-60 cell and induce cell apoptosis. By decreasing STAT3 and inactivating NF-κB signaling, 6-P significantly reduced survival of prostate cancer cells. In addition, 6-P could induce cell apoptosis in oral squamous carcinoma in a dose-dependent manner. In our data, we first proposed that 6-P might be correlated with pancreatic cancer. Subsequently, we constructed tumor proliferation and metastasis model in vitro and in vivo, attempting to uncover the underlying molecular mechanism how 6-P affected pancreatic cancer procession. First, we evaluated the effect of 6-P on proliferation and metastasis of pancreatic cancer cells in different concentrations or in a same concentration for different application time. The results suggested 6-P inhibited pancreatic cancer cell proliferation and metastasis both in a time-dependent manner and a dose-dependent manner. Furthermore, tumor growth was obviously inhibited with 6-P treatment in vivo. Then, we discovered 6-P could reducing the protein expression of EGFR while did not change the mRNA expression of EGFR, suggesting 6-P had less effect at the transcriptional level of EGFR. Therefore, we further explored whether 6-P promoted EGFR degradation via proteasome-dependent degradation. The results suggested 6-P mediated post-translational modifications of EGFR via promoting EGFR ubiquitination, resulting in EGFR degradation. Additionally, we found 6-P reduced the activity of PI3K/AKT signaling via downregulation of EGFR, leading to decreasing abilities of cell proliferation and metastasis.
Ubiquitination plays an important role in protein localization, metabolism, function, regulation and degradation. At the same time, it is also involved in the regulation of cell cycle, proliferation, apoptosis, differentiation, metastasis, gene expression, transcriptional regulation, signal transduction, injury repair, inflammation and immunity, and almost all life activities. Zhang et al found Ginsenoside compound K inhibited the proliferation of liver cancer via promoting the degradation of HIF-1α ubiquitination. Liu et al suggested that Honokiol had an anticancer function via directly interacting with keratin 18 protein in melanoma cells. The interaction between keratin 18 and Honokiol led to the degradation of keratin 18 by ubiquitination. Another compound from ginger, 6-G, was also related with ubiquitination. The study indicated 6-G decreased the expression of USP14, which was a ubiquitin-specific peptidase mainly exerting inhibitory effect on ubiquitnation. Decreasing USP14 elevated the autophagosomes and reduced the survival of lung cancer cell. In our data, 6-P mediated EGFR degradation though enhancing EGFR ubiquitination, resulting in inactivity of PI3K/AKT signaling.
Numerous investigations suggest hyperactivity PI3K/AKT signaling is associated with malignant phenotype of cancer and can accelerate cancer procession. Totiger et al found Urolithin A exerted anticancer effect in pancreatic cancer via downregulating phosphorylation of AKT. Additionally, Amcp, one novel derivative of valepotriate significantly inhibited the PI3K/AKT signaling, suppressing the cell viability and Mcl-1 expression in pancreatic cancer cells. In this study, we confirmed 6-P negatively regulated activity of PI3K/AKT signaling via decreasing EGFR.
In conclusion, our data demonstrate that 6-P exerts anticancer effect though suppressing pancreatic cancer cell growth, viability, invasion and migration. Mechanistically, the inhibitory effect of 6-P mainly based on decreasing the expression of EGFR and inactivity of PI3K/AKT signaling via ubiquitination degradation of EGFR(Fig. 8). Therefore, 6-P might become a supplementary durgs for pancreatic cancer treatment.