Autophagy is a major intracellular pathway for protein degradation, recycling proteins and eliminating aged or damaged organelles. The process begins with the sequestration of cellular organelles and cytoplasm in a double-membrane autophagosome. Autophagosomes then fuse with lysosomes, and the materials inside are degraded to amino acids and fatty acids. Autophagy is rapidly induced when the cells need to eliminate damaged cytoplasmic components or organelles as well as during oxidative stress, infection, ischemia-reperfusion or mitochondrial dysfunction. Some drugs, such as rapamycin, induce autophagy. Recently, we reported that the tumor suppressor ARHI regulates autophagy and tumor dormancy in ovarian cancer cells. Re-expression of ARHI in multiple human ovarian cancer cell lines induces autophagy by blocking PI3K signaling, inhibiting mammalian target of rapamycin (mTOR), and upregulating ATG4. ARHI also colocalizes with MAP-LC3 in autophagosomes. Furthermore, ARHI is required for spontaneous and rapamycin-induced autophagy in normal and malignant cells [
12]. The present study further confirms that the re-expression of ARHI induces autophagy in breast cancer cells and enhances the inhibitory effects of paclitaxel. ARHI activation could be used as a new therapeutic strategy in breast cancer.
Although apoptotic cell death in cancer has been intensively studied over the last decade, the role of autophagy in cell death has received less attention. Autophagy has recently emerged as an important mediator of the programmed cell death (PCD) pathway. Under certain circumstances, the inhibition of autophagy can trigger apoptosis, and the upregulation of autophagy protects against the onset of apoptosis [
15]. By contrast, autophagy can function as a second mode of PCD (Type II PCD) that is distinct from apoptosis [
16]. Some reports suggest that autophagy protects cells from nutrient depletion stress, but paradoxically, excessive autophagy results in cell death. These results also suggest that the induction of autophagy depends on orchestrated interactions between cancer cells and other cells in the tumor microenvironment during the neoplastic process [
17]. Moreover, tumorigenesis is associated with the downregulation of autophagy, and genes that mediate autophagy have been shown to be tumor suppressors [
18]. In this study, we addressed the question of what happens when autophagy meets apoptosis by combining agents that induce autophagy and apoptosis and observing their interaction [
19]. Our studies revealed that TSA and DAC could activate ARHI expression and induces autophagy, but not apoptosis; it is true that this treatment modulated more genes, but we further confirmed that ARHI is required during autophagy induction. When TSA and DAC were combined with paclitaxel, a chemotherapy agent that induces apoptosis, we observed additive inhibitory effects on breast cancer cell growth. Paclitaxel alone did not induce autophagy in these breast cancer cell lines, but an increased fraction of autophagic cells was found when paclitaxel was combined with TSA and DAC. Conversely, apoptosis was not induced when ARHI was activated by TSA and DAC, but the fraction of apoptotic cells was increased when paclitaxel was added.
Whether the induction of autophagy contributes to cell survival or death during apoptosis may relate to the level and duration of autophagy. Low levels of autophagy could promote survival by eliminating damaged intracellular proteins and organelles. This process could increase cell survival by preventing apoptosis. Conversely, excessive autophagy, such as that observed during the induction of ARHI re-expression by TSA and DAC, eventually led to the destruction of essential proteins and organelles beyond a certain threshold, resulting in cell death. In examining various signaling pathways, we found ARHI re-expression mainly inhibited the autophagy-related pathways, such as pAKT and pmTOR (Figure
2), while paclitaxel mainly inhibited the apoptosis-related pathway. When these signaling changes were combined, cancer cell death was hastened. Our studies may thus shed light on a new approach for combining ARHI gene therapy with chemotherapy.