The Role of Autophagy in Breast Cancer Metastasis

Abstract

Patient morbidity and mortality is significantly increased in metastatic breast cancer. The metastasis process of breast cancer is very complicated and is delicately controlled by various factors. Autophagy is one of the important regulatory factors affecting metastasis in breast cancer by engaging in cell mobility, metabolic adaptation, tumor dormancy, and cancer stem cells. Here, we discuss the effects of autophagy on metastasis in breast cancer and assess the potential use of autophagy modulators for metastasis treatment.

Keywords: autophagy; autophagy modulators; breast cancer; metastasis.

Figure 1

The role of canonical autophagy pathway for migration and invasion in breast cancer metastasis. (A) Epithelial–mesenchymal transition regulator. Autophagy promotes the epithelial–mesenchymal transition (EMT) in breast cancer. Activated autophagy in breast cancer cells activates EMT through increased TFE-2 transcription (and upregulation of NUPR1) and the STAT/MAPK pathway involving IL-6. Autophagy markers related to IL-6 are autophagy markers ATG7 and Beclin1 and TFE-2 related autophagy markers are LC3, Beclin1, and ATG5. During this process, the expression of N-cadherin and vimentin increases, while that of E-cadherin decreases. Cell death caused by anoikis should occur following cancer cell migration after detachment from the extracellular matrix (ECM). Nevertheless, autophagy is involved in inducing anoikis resistance of breast cancer cells through the IKK complex and the PERK-ATF4-CHOP pathway. IKK complex activates LC3, whereas ATG6 and ATG8 are activated by the PERK-ATF4-CHOP pathway. (B) Interaction of breast cancer cells to ECM-related proteins. Paxillin is one of the adhesion proteins that interacts with LC3B. This increases breast cancer cell migration through SRC or Rab7 GTPase. Furthermore, autophagy through LC3 and Beclin1 increases breast cancer cell migration by stabilizing the integrin β1 pathway and inhibiting the SRC pathway and the uPAR-uPA system. However, autophagy through Beclin1 also suggests a mechanism for suppressing migration and invasion of breast cancer cells through Notch1 degradation or CLDN6-related autophagy by ERβ. (C) Crosstalk with CAF. STAT3/AMPK/ULK1 activation occurs in CAFs by cardiotrophin 1 (CTF1/CT-1) secreted from breast cancer cells. This induces α-SMA accumulation, stress fiber formation, and fibroblast activation, which eventually increases breast cancer cell migration and invasion. BC—breast cancer; NUPR1—nuclear protein 1; TFE-2—transcription factor E-2; IL-6—interleukin-6; STAT—signal transducer and activator of transcription; MAPK—mitogen-activated protein kinase; EMT—epithelial–mesenchymal transition; ECM—extracellular matrix; IKK—IκB kinase; PERK—protein kinase R–like endoplasmic reticulum kinase; ATF4—activating transcription factor 4; CHOP—C/EBP homology protein; LC3—light chain 3; SRC—Src family of protein tyrosine kinases; GTP—guanosine tri-phosphate; uPAR—urokinase-type plasminogen activator receptor; uPA—urokinase-type plasminogen activator; Erβ—estrogen receptor beta; CLDN6—claudin 6; AMPK—adenosine monophosphate-activated protein kinase; ULK1—unc-51-like kinase 1; SMA—smooth muscle actin; CAF—cancer-associated fibroblast; ATG—autophagy related.

Figure 2

The role of autophagy in tumor dormancy and cancer stem cell biology in breast cancer metastasis. (A) Tumor dormancy regulator. Activation of autophagy in breast cancer cells promotes tumor dormancy. The mechanisms of autophagy activation include inhibition of the integrin β1 pathway by breast cancer cells detached from the ECM detachment, activation of the PI3K system in breast cancer cells, inhibition of the AKT and mTOR pathways, and DIRAS3/ARHI ectopic expression by chemotherapy. Low-level autophagy in breast cancer cells causes PFKFB3 stabilization, resulting in a switch from dormant to active metastatic tumor cells. (B) Modulator for cancer stem cell maintenance. In breast cancer, autophagy involved in the maintenance of breast cancer stem cells (BCSCs) shows differences according to molecular subtype. LC3B-related autophagy activity was high (and involved BCSC proliferation and pluripotency) in ALDH1-positive stem cells derived from ER-positive breast cancer. Meanwhile, autophagy was involved in CD44high/CD24low BCSC maintenance in HER-enriched breast cancer. Autophagy is involved in BCSC maintenance in TNBC through the IL6/STAT3, EGFR/STAT3, and TGFβ/SMAD pathways. This suggests that autophagy may reduce the number of BCSCs by reducing IL-6 secretion. BC—breast cancer; ECM—extracellular matrix; PI3K—phosphoinositide 3-kinase; mTOR—mammalian target of rapamycin; DIRAS3—DIRAS family GTPase3; ARHI—aplasia Ras homologue member I; PFKFB3—6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3; BCSC—breast cancer stem cells; LC3—light chain 3; ALDH—aldehyde dehydrogenase; IL-6—interleukin-6; ER—estrogen receptor; HER—human epidermal growth factor receptor; TNBC—triple-negative breast cancer; STAT—signal transducer and activator of transcription; EGFR—epidermal growth factor receptor; TGFβ—transforming growth factor-β; SMAD—suppressor of mothers against decapentaplegic homolog.

Figure 3

The role of autophagy in breast cancer brain and bone metastasis. (A) Brain metastasis of breast cancer is promoted by inducing autophagy upregulation through the CXCL12-MIR345-KISS1 axis in astrocytes, a cell present in the brain. TNBC and HER-2-positive breast cancer, which commonly metastases into the brain overexpresses GPR94; GPR94 is a glucose-regulated protein, which induces prosurvival autophagy and allows tumor cells to survive in a low-glucose brain environment. (B) In bone metastasis, autophagy-mediated IL-6 secretion from the osteoblast induces osteoclast differentiation and increases osteolysis and the tumor burden. Bone metastatic breast cancer cells exhibit Runx2 expression, which causes α-tubulin acetylation and increases autophagosome trafficking, thereby increasing cancer-cell survival during bone metastasis. In addition, bone metastatic breast cancer cells show an increase in Rab5a expression that promotes autophagosome formation in the early stage through crosstalk with Beclin1 and contributes to autophagy cargo recruitment by promoting LC3 lipidation in the terminal stage. ULK1 downregulation in breast cancer cells reduces mitophagy and increases reactive oxygen species (ROS) production to induce the inflammasome. This eventually results in cytokine secretion and facilitates bone metastasis through osteoblast recruitment. BC—breast cancer; GPR94—G-protein coupled receptor 94; HER—human epidermal growth factor receptor; TNBC—triple-negative breast cancer; CXCL—chemokine ligand; MIR—microRNA; LC3—light chain 3; ROS—reactive oxygen species; Runx2—Runt-related transcription factor 2; ULK—Unc-51-like kinase.