Tumor progression is characterized by a highly proliferative dense tumor core. Neoangiogenesis tries to cater to the increasing needs of the growing tumor, but fails after a point. This leads to nutrient and growth factor deprivation and reduced oxygen supply in the tumor core. Autophagy is perhaps the only process that is promptly induced in response to starvation. Nutrient recycling during starvation is a function of autophagy that is conserved from yeast to mammals [58
]. Thus, autophagy is expected to be an important survival mechanism for tumor cells during starvation. Autophagy is induced by growth factor deprivation and nutrient starvation in breast cancer cell lines and promotes cell survival [60
]. Whether autophagy promotes cell survival under starvation in animal models of breast cancer remains to be studied. Nevertheless, autophagy was shown to be increased in patient-derived mammary tumors and was associated with decreased long-term patient survival [62
In order to combat reduced oxygen availability, tumor cells induce hypoxia-inducible factors (HIFs). HIF-1α is overexpressed in tumors and helps in tumor cell adaptation to hypoxia by altering metabolism, pH, angiogenesis, erythropoiesis, cell migration, invasion, and inflammation [63
]. Recent studies have shown that hypoxia can also lead to the induction of autophagy [65
]. Hypoxia-induced autophagy was mediated via Bnip3 [65
], a Bcl-2-interacting BH3-only protein that was initially shown to function as a cell death effector [66
]. Subsequent studies, however, showed that Bnip3 induces autophagy, as a survival mechanism, by disengaging Bcl-2 from Beclin 1 [65
]. Thus, HIF-1α/Bnip3 represents the primary axis that mediates hypoxia-induced autophagy.
HIF-1α is overexpressed in a subset of breast cancers. Its expression was associated with ER/PR-negative and ErbB2-positive status [68
]. HIF-1α expression is also increased in familial breast cancers in comparison to sporadic tumors [70
]. Moreover, transcriptional activation of HIF-1α target genes was shown to be required for breast cancer progression [71
]. HIF-1α mediates pro-survival autophagy in breast cancer cells in response to PPARγ (peroxisome proliferator-activated receptor γ) activation [72
]. Hypoxia-induced autophagy promotes cell survival in breast cancer cells [73
]. However, HIF-1α is not the only mechanism by which hypoxic cells induce autophagy. Hypoxia can also induce autophagy via activation of AMPK or inhibition of mTOR, independent of HIF-1α [74
Rapidly growing tumor cells also display increased ER stress due to overload on protein synthesis machinery. When the load on protein folding machinery is overwhelming, there can be accumulation of unfolded proteins. As a result, the cell triggers UPR (unfolded protein response). Key enzymes activated during UPR are PERK (PKR-like ER kinase), IRE1 (inositol requiring-1α) and ATF6 (activating transcription factor-6) [76
]. These enzymes attempt to rescue cells by reducing the rate of new protein synthesis and increasing the expression of chaperon proteins. When the ER stress is excessive and unmanageable, UPR triggers ER-associated protein degradation (ERAD), which is responsible for degradation of unfolded proteins. The proteosomal degradation system was the only recycle system known to be triggered in response to ER stress. Recently, autophagy has been shown to work as an alternate pathway to aid protein degradation during ER stress.
ER stress can induce autophagy through several different mechanisms. PERK-mediated phosphorylation of eIF2α and Atg12 was shown to be required for autophagy induction in response to polyglutamine aggregates [77
]. Similarly, PERK-eIF2α mediated radiation-induced autophagy [78
]. On the other hand, IRE1 induced autophagy in response to a variety of ER stressors by activating JNK [79
]. Inhibition of Akt and mTOR has also been implicated in ER stress-induced autophagy [80
]. ER stress and the UPR pathway can also regulate autophagy during starvation and hypoxia [65
]. Such a cooperative regulation of autophagy is expected, as different stresses prevail concomitantly in the tumor core.
A number of studies suggest that inhibition of ER stress-induced autophagy could be exploited for the treatment of breast cancer [81
]. In MCF-7 and MDA-MB-231 breast cancer cell lines, capsaicin treatment induced autophagy, causing resistance to ER stress-mediated apoptosis [81
]. Such protective autophagy was not induced in non-malignant MCF-10A cells [81
]. Moreover, ER stress and autophagy were shown to form the basis of Bortezomib resistance in MCF-7 cells, and inhibition of autophagy increased Bortezomib sensitivity [82
While most studies suggest a protective role for autophagy, some reports show that autophagy may act as a cell death mechanism in response to stress. Autophagy was shown to promote cell death in apoptosis-deficient cells [83
]. Using Bax/Bak double knockout MEFs, Ullman et al.
showed that ER stress induced autophagy in both apoptosis-competent as well as apoptosis-deficient cells [83
]. However, while autophagy was cytoprotective in apoptosis-competent cells, it led to necrotic cell death in apoptosis-deficient cells [83
]. Inhibition of autophagy using 3-methyladenine (3-MA), as well as by genetic knockdown of Atg5, resulted in decreased cell survival in response to ER stress [83
]. Similarly, ER stress-mediated autophagy in response to radiation was cytotoxic in caspase-3/7-deficient cells [78
]. Hypoxia on the other hand was shown to induce autophagic cell death in various cancer cell lines, including apoptosis-competent MDA-MB-231 and ZR-75 breast cancer cell lines [84
]. Thus, the cross-talk between apoptosis and autophagy appears to be stimulus-dependent and warrants further investigation in order to exploit this interaction to achieve maximum killing of cancer cells.