Autophagy and Metastasis

By Christina Towers, PhD

The majority of cancer patients die from metastatic disease at secondary sites. The threshold to undergo metastasis is high. Only a minority of cancer cells acquire invasive phenotypes like motility and the ability to survive under detached conditions. Consistently, only few cancer cells acquire stem cell like properties, including a more mesenchymal morphology by way of epithelial to mesenchymal transition (EMT   ), which allow them to survive extravasation and grow at the secondary site. The cellular recycling process, autophagy, can function as a pro-survival mechanism and has been linked to metastatic outgrowth. Pathological studies have suggested a correlative relationship between autophagy and metastasis showing especially high LC3 expression in the metastatic lesions^1-4. Moreover, studies utilizing both genetic and pharmacological inhibition of autophagy have provided evidence for a causal relationship. Loss of autophagy eliminated lung metastasis in a syngeneic breast cancer mouse model after orthotopic injection of the mouse mammary carcinoma 4T1 cell line⁵.

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The connection between Autophagy, Stemness and EMT

Recent studies have identified a variety of mechanisms that mediate the link between autophagy and metastasis. Autophagy can promote a stemness phenotype in cancer stem cells, which maintain a quiescent or dormant state, self-renew, and can regenerate an entire tumor at limiting dilutions^6,7. In chronic myeloid leukemia (CML), the addition of autophagy inhibition to kinase inhibitors eradicated functionally defined CML stem cells⁸. This effect may be linked to autophagy-regulated metabolic changes in the stem cells⁹. Additionally, genetic manipulation of core autophagy proteins in breast cancer models can also diminish the tumor initiating properties of specific cancer stem cell populations in mice¹⁰.

EpCAM/TROP1 expression in human breast carcinoma was detected by immunohistochemical analysis of paraffin embedded human breast carcinoma tissue section with primary antibody to EpCAM/TROP1 Antibody (AUA1) [NB600-1182]. EpCAM expression was seen in association with the membrane and the intercellular spaces.

Autophagy has also been associated with EMT, a hallmark of metastatic dissemination that is linked to increased invasiveness as well as cancer stem cell phenotypes^11-13. Recently, Shinde et al showed that inhibition of autophagy-mediated degradation of RNA processing bodies or p-bodies mitigates the reverse process, mesenchymal-to-epithelial transition (MET), ultimately reducing metastatic outgrowth¹³. Additionally, autophagy regulates focal adhesion dynamics via direct interactions between LC3B and the focal adhesion protein, paxillin, and consequently affects cell migration and metastasis^5,14. Tumor dormancy is described as the long-term survival of quiescent cell populations, often at a secondary site. These cells can eventually initiate growth at a later time resulting in the appearance of metastatic tumors¹⁵. In breast, ovarian, and pancreatic cancer models, autophagy has been shown to promote the survival and outgrowth of dormant cell populations^16-18.

Immunocytochemical analysis of Paxillin in HUVEC-human umbilical vein endothelial cells. HUVEC cells were incubated with 10 µg/mL mouse primary antibody- Anti-Human Paxillin Monoclonal Antibody (MAB4259) for 3 hours at room temperature. A secondary antibody, Anti-Mouse IgG conjugated with NorthernLights^™ 557 (red; NL007) was used for staining. Nuclei were visualized by DAPI (blue) counterstain. Specific Paxillin expression is present in the cytosol and actin filament tips.

Autophagy: Pro-Metastatic or Anti-Metastatic Role

Together, these studies portray a pro-tumorigenic and pro-metastatic role for autophagy in cancer, although there have also been recent studies indicating the opposite. For example, in melanoma the autophagy cargo receptor, p62/SQSTM1– which accumulates in cells after autophagy inhibition, can interact with select RNA-binding proteins to stabilize specific pro-metastatic mRNAs¹⁹. Loss of autophagy causes p62 to interact with and stabilize the pro-EMT protein, Twist-1, resulting in an increase in cell migration^20,21. As autophagy inhibitors move forward in clinical trials, it is important to fully elucidate the pro- and anti-metastatic roles of autophagy in order to generate the most beneficial therapeutics.

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Christina Towers, PhD   
University of Colorado (AMC)
Dr. Towers studies the roles of autophagy, apoptosis and cell death in cancer.

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