Abstract
BackgroundAlthough some molecularly targeted drugs for colorectal cancer are used clinically and contribute to a better prognosis, the current median survival of advanced colorectal cancer patients is not sufficient. Autophagy, a basic cell survival mechanism mediated by recycling of cellular amino acids, plays an important role in cancer. Recently, autophagy has been highlighted as a promising new molecular target. The unfolded protein response (UPR) reportedly act in complementary fashion with autophagy in intestinal homeostasis. However, the roles of UPR in colon cancer under autophagic inhibition remain to be elucidated. We aim to clarify the inhibitory effect of autophagy on colon cancer.MethodsWe crossed K19CreERT and Atg5flox/flox mice to generate Atg5flox/flox/K19CreERT mice. Atg5flox/flox/K19CreERT mice were first treated with azoxymethane/dextran sodium sulfate and then injected with tamoxifen to inhibit autophagy in CK19-positive epithelial cells. To examine the anti-cancer mechanisms of autophagic inhibition, we used colon cancer cell lines harboring different p53 gene statuses, as well as small interfering RNAs (siRNAs) targeting Atg5 and immunoglobulin heavy-chain binding protein (BiP), a chaperone to aid folding of unfolded proteins.ResultsColon tumors in Atg5flox/flox/K19CreERT mice showed loss of autophagic activity and decreased tumor size (the total tumor diameter was 28.1 mm in the control and 20.7 mm in Atg5flox/flox/K19CreERT mice, p = 0.036). We found that p53 and UPR/endoplasmic reticulum (ER) stress-related proteins, such as cleaved caspase 3, and CAAT/enhancer-binding protein homologous protein, are up-regulated in colon tumors of Atg5flox/flox/K19CreERT mice. Although Atg5 and BiP silencing, respectively, increased apoptosis in p53 wild type cells, Atg5 silencing alone did not show the same effect on apoptosis in p53 mutant cells. However, co-transfection of Atg5 and BiP siRNAs led to increased apoptosis in p53 mutant cells.ConclusionsBlocking autophagy has potential in the treatment of colon cancer by inducing apoptosis via p53 and ER stress, and suppressing the UPR pathway is a valid strategy to overcome resistance to autophagic inhibition.
Highlights
Some molecularly targeted drugs for colorectal cancer are used clinically and contribute to a better prognosis, the current median survival of advanced colorectal cancer patients is not sufficient
By genetic inhibition of autophagy and CQ treatment, we showed that suppression of autophagy has an anti-colorectal cancer effect via apoptosis induced by p53 activation and endoplasmic reticulum (ER) stress in vivo and in vitro
CK19 and Atg5 expression in normal colon mucosa and AOM/Dextran sodium sulfate (DSS)-derived colon tumors We first examined the distribution of CK19- and Atg5positive epithelial cells in normal colonic mucosa of K19CreERT mice
Summary
Some molecularly targeted drugs for colorectal cancer are used clinically and contribute to a better prognosis, the current median survival of advanced colorectal cancer patients is not sufficient. To examine the anti-cancer mechanisms of autophagic inhibition, we used colon cancer cell lines harboring different p53 gene statuses, as well as small interfering RNAs (siRNAs) targeting Atg and immunoglobulin heavy-chain binding protein (BiP), a chaperone to aid folding of unfolded proteins. Some molecularly targeted drugs, such as anti-vascular endothelial cell growth factor (VEGF) and anti-epidermal growth factor receptor (EGFR) antibodies, are used clinically and contribute to a better prognosis, the current median survival of stage IV colorectal cancer patients receiving chemotherapy is shorter than 3 years [2]. Macroautophagy (hereafter referred to as autophagy), which is a highly conserved cell survival mechanism mediated by the recycling of cellular amino acids [3], has been highlighted as a promising new molecular target [3, 4]. Rosenfeldt et al showed that the effect of autophagic inhibition on pancreatic cancer depends on p53 status [13]
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