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AutophAgic punctum

www.landesbioscience.com Autophagy 1

Autophagy 10:10, 1–2; october 2014; © 2014 Landes Bioscience

AutophAgic punctum

The transcription factor TWIST1 is a basic helix-loop-helix protein that

regulates epithelial-mesenchymal transi-tion (EMT) in early embryonic morpho-genesis, cancer development, and cancer metastasis. The regulation of TWIST1 remains poorly understood. Recently, we found that autophagy deficiency stabilizes TWIST1 protein through SQSTM1/p62 accumulation. SQSTM1 binds with TWIST1 to inhibit TWIST1 degradation in both autophagosomes and proteasomes. SQSTM1-mediated TWIST1 stabilization promotes EMT in vitro, and tumor growth and metastasis in mice. We propose autophagy as a new mechanism to control the TWIST1 pro-tein levels and activity in cancer develop-ment and progression.

Epithelial-mesenchymal transition is a critical process for early embryonic mor-phogenesis, cancer development, and can-cer metastasis. One of the essential EMT regulators is the basic helix-loop-helix protein and transcription factor TWIST1. TWIST1 induces loss of CDH1/E-cadherin-mediated cell-cell adhesion and thus facilitates EMT. The protein level of TWIST1 is tightly controlled by protea-somal degradation.

Macroautophagy (hereafter autophagy) is a catabolic process for proteolytic degra-dation of cellular proteins, cytoplasm, and organelles in lysosomes. Autophagy dys-function contributes to the pathogenesis of multiple human diseases, such as neurode-generation, microbial infection, metabolic diseases, cardiovascular diseases, aging, and cancer. The protein SQSTM1 serves as both a selective autophagy substrate and

an autophagy receptor protein. SQSTM1 is increased in several human cancers and induced by the oncogene RRAS, suggest-ing that SQSTM1 is oncogenic.

As a homeostatic process, autophagy can serve both oncogenic and tumor-sup-pressive roles depending on the tumori-genic context. The ability of autophagy to restrict inflammation and DNA damage may limit tumorigenesis and tumor pro-gression. SQSTM1 accumulation result-ing from autophagy defects is sufficient to activate NFKB signaling and promote tumorigenesis. The defective clearance of SQSTM1 in autophagy deficiency leads to the constitutive activation of the redox-regulatory transcription factor NFE2L2/NRF2 and contributes to development of liver cancer. Conversely, autophagy may promote tumor formation and metastasis at later stages by protecting tumor cells from environmental stresses. Our recent studies demonstrate that autophagy defi-ciency promotes cell proliferation and migration through SQSTM1-dependent stabilization of TWIST1 (Fig. 1), but not other EMT factors such as SNAI1/Snail, SNAI2/Slug, ZEB1, or TWIST2.

In addition to the autophagosome-lysosome pathway, the proteasomal pro-tein degradation system plays important roles in regulating stability of many short-lived proteins. Although these 2 systems often respond to different physiological or pathological cues, they do interact to regulate protein stabilities. Indeed, dur-ing autophagy stimulation SQSTM1 and TWIST1 colocalize in autophagosomes. However, in autophagy-deficient cells, SQSTM1 binds to TWIST1 and inhibits degradation of TWIST1. Our findings

Autophagy deficiency stabilizes TWIST1 to promote epithelial-mesenchymal transition

Lei Qiang and Yu-Ying He*Section of Dermatology; Department of Medicine; University of Chicago; Chicago, IL USA

Keywords: autophagosome, autophagy, EMT, melanoma, metastasis, p62, proteasome, skin cancer, tumor growth, TWIST1

Submitted: 07/18/2014

Revised: 07/23/2014

Accepted: 07/28/2014

Published Online: 08/13/2014

http://dx.doi.org/10.4161/auto.32171

*Correspondence to: Yu-Ying He; Email: yyhe@medicine.bsd.uchicago.edu

Punctum to: Qiang L, Zhao B, Ming M, Wang N, He TC, Hwang S, Thorburn A, He YY. Regula-tion of cell proliferation and migration by p62 through stabilization of Twist1. Proc Natl Acad Sci U S A 2014; 111:9241–6; PMID:24927592; http://dx.doi.org/10.1073/pnas.1322913111

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2 Autophagy Volume 10 issue 10

SQSTM1 plays a critical role in TGFB-EGF-induced EMT. Indeed, we found that TGFB-EGF induces SQSTM1 expression. Such SQSTM1 induction is crucial for TGFB-EGF-induced EMT and TWIST1 upregulation at the protein levels, while it does not affect TWIST1 mRNA levels. These findings indicate that TWIST1 regulation by SQSTM1 is critical for EMT in a physiological context.

Given that TWIST1 is vital for cancer development, progression, and metastasis and that the incidence of several cancers including both human skin squamous cell carcinoma and melanoma contin-ues to rise at an alarming rate every year, our findings not only add new molecular insights into the pathogenesis of cancer formation and metastasis, but also sug-gest that targeting SQSTM1-mediated TWIST1 stabilization is a promising pre-ventive and therapeutic strategy to prevent and treat cancer.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

This work was supported by the National Institutes of Health (NIH)/National Institute on Environmental Health Sciences Grant ES016936 (to YYH), the American Cancer Society Grant RSG-13-078-01 (to YYH), the University of Chicago Cancer Research Center (P30 CA014599), the CTSA (UL1 TR000430), and the University of Chicago Friends of Dermatology Endowment Fund.

indicate that SQSTM1 inhibits TWIST1 protein degradation in both auto-phagosomes and proteasomes. Autophagy suppresses TWIST1 and EMT by con-trolling the protein levels of SQSTM1 (Fig. 1).

This regulation is relevant to human cancers, since we found that SQSTM1 is upregulated in human skin squamous cell carcinoma. SQSTM1 is also reported to be upregulated in human melanoma and several other human cancers. In addition, we found that SQSTM1 upregulation promotes tumor growth and metastasis of human squamous cell carcinoma cells in mice in a TWIST1-dependent man-ner. We have also shown that autophagy suppression increases while SQSTM1

knockdown decreases tumor growth of human melanoma cells in mice.

At the molecular level, we found that the ubiquitin-associated domain of SQSTM1 is required for SQSTM1 binding to TWIST1 (Fig. 1). Lysine 175 (K175) is critical for TWIST1 binding to SQSTM1 (Fig. 1). SQSTM1 inter-acts with polyubiquitinated TWIST1 at K175 and inhibits TWIST1 degrada-tion through the autophagy and protea-some pathways. Thus, either autophagy suppression or SQSTM1 upregulation increases the TWIST1 protein level and activity in EMT.

To determine the physiological rel-evance of the SQSTM1-TWIST1 interac-tion in EMT, we tested the hypothesis that

Figure 1. Schematic for the regulation of tWiSt1 by SQStm1 through autophagy and the pro-teasome. this figure has been modified from Qiang L, Zhao BZ, ming m, Wang n, he tc, hwang S, thorburn A, he YY. Regulation of cell proliferation and migration by p62 through stabilization of twist1. proc natl Acad Sci uSA 2014; 111:9241–6; pmiD:24927592; http://dx.doi.org/10.1073/pnas.1322913111 by permission of the publisher.