Abstract
Drug resistance has become one of the largest challenges for cancer chemotherapies. Under certain conditions, cancer cells hijack autophagy to cope with therapeutic stress, which largely undermines the chemo-therapeutic efficacy. Currently, biomarkers indicative of autophagy-derived drug resistance remain largely inclusive. Here, we report a novel role of lipid rafts/cholesterol-enriched membrane micro-domains (CEMMs) in autophagosome biogenesis and doxorubicin resistance in breast tumors. We showed that CEMMs are required for the interaction of VAMP3 with syntaxin 6 (STX6, a cholesterol-binding SNARE protein). Upon disruption of CEMM, VAMP3 is released from STX6, resulting in the trafficking of ATG16L1-containing vesicles to recycling endosomes and subsequent autophagosome biogenesis. Furthermore, we found that CEMM marker CAV1 is decreased in breast cancer patients and that the CEMM deficiency-induced autophagy is related to doxorubicin resistance, which is overcome by autophagy inhibition. Taken together, we propose a novel model whereby CEMMs in recycling endosomes support the VAMP3 and STX6 interaction and function as barriers to limit the activity of VAMP3 in autophagic vesicle fusion, thus CEMM deficiency promotes autophagosome biogenesis and doxorubicin resistance in breast tumors.
Highlights
Macroautophagy is an evolutionarily conserved “self-eating” process that results in degradation of longlived proteins and organelles via the lysosomal pathway, which is essential for the maintenance of cellular homeostasis.[1]
Decreased vesicle-associated membrane protein 3 (VAMP3)-STX6 interaction by cholesterol-enriched membrane micro-domains (CEMMs) disruption promotes autophagosome biogenesis To further explore the molecular mechanism underlying the regulatory role of CEMMs in VAMP3 function, we examined the distribution of STX6, a SNARE protein that is known as a cholesterol-binding protein and a binding partner of VAMP3.68,69 Consistent with previous reports, the interaction between STX6 and VAMP3 was confirmed by their colocalization in cells with normal cholesterol and proficient Caveolin 1 (CAV1) levels (Figures 5A and 5B)
CEMM deficiency-induced autophagy is associated with the acquisition of Doxo resistance in breast cancer cells Since we reported the downregulation of CEMMs and enhanced autophagy level were observed in human breast cancer cells and tissues,[32] here we further analyzed the expression of CEMM marker protein CAV1 in both normal and tumor tissues from TCGA database
Summary
Macroautophagy (here referred to as autophagy) is an evolutionarily conserved “self-eating” process that results in degradation of longlived proteins and organelles via the lysosomal pathway, which is essential for the maintenance of cellular homeostasis.[1]. The PtdIns3P effector WD repeat domain, phosphoinositide interacting 2 (WIPI2) further recruits the ATG12-ATG5-ATG16L1 complex, which functions as an E3 to facilitate the lipidation of microtubule associated protein 1 light chain 3 (LC3) and the maturation of autophagosomes.[9,10,11] In addition to the key ATGs mentioned above, autophagosome formation requires membrane fusion driven by SNAREs (SNAP [soluble NSF attachment protein] receptor), a family of proteins known to mediate membrane/vesicle fusion events.[12,13,14,15,16,17] Recent studies suggest that SNAREs are not solely involved in autophagosome-lysosome fusion. Another v-SNARE protein, syntaxin 17 (STX17), which is well known as a key player in autophagosome-lysosome fusion, is reported to promote
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