PMC:5833766 / 18877-24924 JSONTXT

{"project":"2_test","denotations":[{"id":"29358623-12202786-79067453","span":{"begin":439,"end":441},"obj":"12202786"},{"id":"29358623-24651011-79067454","span":{"begin":613,"end":614},"obj":"24651011"},{"id":"29358623-23940223-79067455","span":{"begin":856,"end":858},"obj":"23940223"},{"id":"29358623-24202392-79067456","span":{"begin":1007,"end":1009},"obj":"24202392"},{"id":"29358623-18455122-79067457","span":{"begin":1100,"end":1101},"obj":"18455122"},{"id":"29358623-23765757-79067458","span":{"begin":1102,"end":1104},"obj":"23765757"},{"id":"29358623-26378037-79067459","span":{"begin":1105,"end":1107},"obj":"26378037"},{"id":"29358623-25483192-79067459","span":{"begin":1105,"end":1107},"obj":"25483192"},{"id":"29358623-21656832-79067459","span":{"begin":1105,"end":1107},"obj":"21656832"},{"id":"29358623-25594178-79067460","span":{"begin":1189,"end":1190},"obj":"25594178"},{"id":"29358623-26980765-79067461","span":{"begin":1191,"end":1192},"obj":"26980765"},{"id":"29358623-26921108-79067462","span":{"begin":1965,"end":1967},"obj":"26921108"},{"id":"29358623-19401591-79067463","span":{"begin":2062,"end":2064},"obj":"19401591"},{"id":"29358623-15718470-79067464","span":{"begin":2093,"end":2096},"obj":"15718470"},{"id":"29358623-18455122-79067465","span":{"begin":2414,"end":2415},"obj":"18455122"},{"id":"29358623-22500797-79067466","span":{"begin":3145,"end":3147},"obj":"22500797"},{"id":"29358623-24206664-79067467","span":{"begin":3221,"end":3223},"obj":"24206664"},{"id":"29358623-27023784-79067468","span":{"begin":3224,"end":3226},"obj":"27023784"},{"id":"29358623-28292700-79067469","span":{"begin":3523,"end":3525},"obj":"28292700"},{"id":"29358623-21460621-79067469","span":{"begin":3523,"end":3525},"obj":"21460621"},{"id":"29358623-21205641-79067469","span":{"begin":3523,"end":3525},"obj":"21205641"},{"id":"29358623-21258367-79067470","span":{"begin":3683,"end":3685},"obj":"21258367"},{"id":"29358623-25980607-79067471","span":{"begin":3696,"end":3698},"obj":"25980607"},{"id":"29358623-21258367-79067472","span":{"begin":3794,"end":3796},"obj":"21258367"},{"id":"29358623-25892232-79067473","span":{"begin":4219,"end":4221},"obj":"25892232"},{"id":"29358623-24161930-79067474","span":{"begin":4492,"end":4494},"obj":"24161930"},{"id":"29358623-27748764-79067475","span":{"begin":4690,"end":4692},"obj":"27748764"},{"id":"29358623-23765757-79067476","span":{"begin":5451,"end":5454},"obj":"23765757"}],"text":"Discussion\nEGFR is a major therapeutic target for cancer treatment. Two classes of therapeutic agents have been developed, monoclonal antibodies that block the binding of EGFR activating ligands, and small molecules of TKIs that reversibly or irreversibly occupy the ATP binding pocket of EGFR. However, the therapeutic efficacy of these EGFR inhibitors has been disappointing. Most cancers of epithelial origin express or overexpress EGFR21,22. However, only a few types of cancer, such as non-small cell lung cancer (NSCLC) and KRAS wild-type colorectal cancer, exhibit significant, but transient, effectiveness1. Furthermore, for even the most responsive cancers, the NSCLC, only those with gain-of-function mutations in EGFR tyrosine kinase activity respond well to TKIs, whereas cancers that express wild-type EGFR either respond poorly, or not at all23. Finally, without exception, cancers that initially respond to TKIs develop acquired resistance to the drugs, often within a few months of treatment24. We have previously reported that EGFR possesses kinase-independent pro-survival functions7,19,25–27, a hypothesis that is also supported by recent reports from other laboratories8,9. The KID functions of EGFR offer a new window for targeting EGFR expressing cancers.\nThe current study identifies a novel mechanism and signal pathway underlying EGFR’s KID function(s), inhibition of mitophagy via repression of the mTORC2/Akt pathway (summarized in Fig. 7c). The differential responses of PKC, mTORC1, and MAPK of the two types of cancer cells to loss-of-EGFR indicate that the KID functions of regulating these pathways are cell type dependent, and the commonality of mTORC2/Akt in mediating loss-of-EGFR-induced mitophagy among the different cell types argues that this EGFR’s kinase-independent anti-mitophagy pathway is a more fundamental mechanism. Our data are consistent with the observation that Akt activation is also pro-mitophagic in macrophage28. It is intriguing that Akt can be activated by membranous growth factor receptors such as EGFR29 and intracellularly by mTORC218, however these different routes of Akt activation lead to complete different cellular responses, cell growth/survival by the former route and mitophagy/cell death by the later one. The activation of mTORC2 upon loss-of-EGFR protein explains the Akt activation in response to EGFR knockdown that we observed previously7. The mechanism underlying the differential roles of Akt merits further investigation.\nIn this study, for the first time, we have shown that EGFR suppresses mitophagy by a mechanism that is independent of its tyrosine kinase activity, and activation of mTORC2 induces mitophagy in cancer cells. In addition, we report Herdegradin, a synthetic peptide that is capable of downregulating EGFR, activating mTORC2, and inducing mitophagic cell death in a manner that is similar to the mitophagic cell death caused by EGFR knockdown. The mTOR kinase is a component of two distinct protein kinase complexes, mTORC1 and mTORC2. Although much is known about the function of mTORC1, our knowledge of the biological role of mTORC2 is limited15. It is known that mTORC1 stimulates mitochondrial activity and biogenesis30,31, and we show in the current study that activation of mTORC2 promotes mitophagy. The mTORC1 pathway is anti-autophagic and anti-mitophagic via inhibiting some downstream autophagy related proteins, such as the Unc-51-like kinase 1 (ULK1), which are otherwise activated by pro-autophagic mechanisms32–34. Studies have shown that phosphorylation of the ULK1 protein at serine 555 by AMPK is pro-autophagic and pro-mitophagic in response to nutrient starvation35 or hypoxia36, whereas phosphorylation at the serine 757 site by mTOR1 is anti-autophagic and anti-mitophagic35. In the current study, we found that loss-of-EGFR either induced by siRNA or by Herdegradin decreased the levels of S555-phosphorylated ULK1, whereas increased the levels of S757-phosphorylated ULK1 (Fig. S2), which is opposite to the phosphorylation changes of ULK1 responding to starvation or hypoxia-induced autophagy, but it is consistent with the recent finding that the S757 of ULK1 can also be phosphorylated by AKT37 given that AKT is activated by loss-of-EGFR, and suggests that ULK1 might be uniquely involved in the loss-of-EGFR-induced mitophagy, however its role needs to be defined by further studies.\nAlthough several studies have shown that mTORC1 often represses mTORC2 activity38, developing dual-inhibitors for both mTORC1 and mTORC2 has been a major approach of targeting the mTOR pathways for cancer treatment, and these inhibitors have exhibited limited clinical benefits39. Given the opposing roles of mTORC1 and mTORC2 on the fate of mitochondria, it is proposed that concurrent inhibition of mTORC1 and activation of mTORC2 might be a better strategy for cancer treatment, however specific mTORC2 activators are yet to be developed. Our EGFR-downregulating peptide showed potent effects on mTORC1 inhibition and mTORC2 activation, it may serve as a valuable tool for developing specific mTORC2 activators.\nOur previous studies have shown that EGFR can exist in two statuses in cancer cells, a kinase responsive status that governs the classical EGFR’s kinase-dependent functions and a kinase-independent status that maintains cell survival by interacting with crucial pro-survival proteins such the sodium/glucose co-transporter 119. The repression of mTORC2 by KID function of EGFR shown herein adds another functional component to the kinase-independent status of EGFR. The KID functions of EGFR might be a critical survival node for cancers that overexpress wild-type EGFR as these cancers are innately resistant to EGFR TKIs, where the KID functions of EGFR are elevated due to overexpression, and for cancers that have acquired resistance to EGFR TKIs, where EGFR’s function has been shifted to its KID functions by TKIs. Co-targeting EGFR’s KD and KID functions may hold a new promise of treating EGFR-positive cancers."}