Autophagy mediates survival of pancreatic tumour‐initiating cells in a hypoxic microenvironment

Abstract

Involvement of dysregulated autophagy in cancer growth and progression has been shown in different tumour entities, including pancreatic ductal adenocarcinoma (PDA). PDA is an extremely aggressive tumour characterized by a small population of highly therapy-resistant cancer stem cells (CSCs) capable of self-renewal and migration. We examined whether autophagy might be involved in the survival of CSCs despite nutrition and oxygen deprivation typical for the hypoxic tumour microenvironment of PDA. Immunohistochemistry revealed that markers for hypoxia, CSCs and autophagy are co-expressed in patient-derived tissue of PDA. Hypoxia starvation (H/S) enhanced clonogenic survival and migration of established pancreatic cancer cells with stem-like properties (CSC(high)), while pancreatic tumour cells with fewer stem cell markers (CSC(low)) did not survive these conditions. Electron microscopy revealed more advanced autophagic vesicles in CSC(high) cells, which exhibited higher expression of autophagy-related genes under normoxic conditions and relative to CSC(low) cells, as found by RT-PCR and western blot analysis. LC3 was already fully converted to the active LC3-II form in both cell lines, as evaluated by western blot and detection of accumulated GFP-LC3 protein by fluorescence microscopy. H/S increased formation of autophagic and acid vesicles, as well as expression of autophagy-related genes, to a higher extent in CSC(high) cells. Modulation of autophagy by inhibitors and activators resensitized CSC(high) to apoptosis and diminished clonogenicity, spheroid formation, expression of CSC-related genes, migratory activity and tumourigenicity in mice. Our data suggest that enhanced autophagy levels may enable survival of CSC(high) cells under H/S. Interference with autophagy-activating or -inhibiting drugs disturbs the fine-tuned physiological balance of enhanced autophagy in CSC and switches survival signalling to suicide.

Role of autophagy in the maintenance and function of cancer stem cells

“…In addition, antiangiogenic agents increased the population of CSCs on hypoxia and this phenomenon could limit their anticancer activity (Conley et al, 2012). In line with the hypothesis of a role of autophagy in the adaptation of CSCs to environmental perturbations, interfering with autophagic flux decreased the survival of either pancreatic cancer cells with stem-like properties under hypoxic conditions (Rausch et al, 2012) or CD133 + liver CSCs on oxygen-and nutrient-deprivation (Song et al, 2013). Importantly, a co-expression between hypoxia, stemness and autophagy markers was found in immunohistochemical analyses performed on tissue samples from PDAC patients (Rausch et al, 2012).…”

Synectin promotes fibrogenesis by regulating PDGFR isoforms through distinct mechanisms

“…It is hypothesized that the increased autophagy within HSC is necessary for lipid breakdown to provide the necessary energy source to undergo activation through the process of lipophagy (30). Such dichotomous findings are noted in cancer literature, where autophagy has been reported as both promotional and inhibitory for tumor growth (31)(32)(33)(34)(35). These divergent findings likely reflect the complexity of the role of autophagy in vivo.…”

The acidic microenvironment as a possible niche of dormant tumor cells

“…Low levels of autophagic activity are observed under physiological conditions, while in tumor cells, autophagy promotes cell survival during the frequent nutrient restriction and hypoxia [49]. Autophagy was found to sustain the maintenance of cancer stem cell phenotype, and cooperates to chemo-and radio-resistance [50][51][52][53]. Inhibition of autophagy, on the contrary, increases chemo-and radio-sensitivity [54].…”