Cell Oncol. (2016) 39:389–396 DOI 10.1007/s13402-016-0290-8
REPORT
Inactivation of the LKB1-AMPK signaling pathway does not contribute to salivary gland tumor development - a short report Natascha Cidlinsky 1 & Giada Dogliotti 1 & Tobias Pukrop 2 & Rudolf Jung 3 & Florian Weber 3 & Michael P. Krahn 1
Accepted: 15 July 2016 / Published online: 1 August 2016 # International Society for Cellular Oncology 2016
Abstract Purpose Activation of AMPK by the tumor suppressor LKB1 represents an essential gatekeeping step for cells under energetic stress to prevent their growth and proliferation by inhibiting mTOR activation, until the energy supply normalizes. The LKB1/AMPK pathway is frequently downregulated in various types of cancer, thereby uncoupling tumor cell growth and proliferation from energy supply. As yet, little information is available on the role of the LKB1/AMPK pathway in tumors derived from salivary gland tissues. Methods We performed LKB1 protein expression and AMPK and mTOR activation analyses in several salivary gland tumor types and their respective healthy control tissues using immunohistochemistry. Results No significant downregulation of LKB1 expression or decreased activation of AMPK or mTOR were observed in any of the salivary gland tumors tested. In contrast, we found that the salivary gland tumors exhibited an increased rather than a decreased AMPK activation. Although the PI3K/Akt pathway was found to be activated in most of the
analyzed tumor samples, the unchanged robust activity of LKB1/AMPK likely prevents (over)activation of mTOR. Conclusion In contrast to many other types of cancer, inactivation or downregulation of the LKB1/AMPK pathway does not substantially contribute to the pathogenesis of salivary gland tumors. Keywords LKB1 . AMPK . mTOR . PI3K-pathway . Salivary gland tumor
Abbreviations ACC adenoid cystic carcinoma AMPK AMP-activated kinase CexPA carcinoma ex pleomorphic adenoma LKB1 liver kinase B1 MEC mucoepidermoid carcinoma mTOR mammalian target of rapamycin PA pleomorphic adenoma PDK1 phosphoinositide-dependent kinase 1 SDC salivary duct carcinoma WT Warthin tumor
Electronic supplementary material The online version of this article (doi:10.1007/s13402-016-0290-8) contains supplementary material, which is available to authorized users. * Michael P. Krahn
[email protected] 1
Molecular and Cellular Anatomy, University of Regensburg, Universitätsstr. 31, 93053 Regensburg, Germany
2
Department of Internal Medicine III, Hematology and Medical Oncology, University Hospital Regensburg, Franz-Josef-Strauss-Allee 11, 93053 Regensburg, Germany
3
Institute for Pathology, University of Regensburg, Franz-Josef-Strauss-Allee 11, 93053 Regensburg, Germany
1 Introduction Liver kinase B1 (LKB1, also known as STK11 in mammals and PAR4 in Caenorhabditis elegans) is a master serinethreonine kinase that can activate various downstream kinases (MARKs, SADs, NUAKs and AMPK) regulating important cellular signaling pathways, including the Wnt, TGFβ and mTOR pathways [1–5]. In addition, it has been reported that LKB1 can mediate the stabilization of p53 and activate the tumor suppressor capacity of PTEN [6, 7].
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The activation of AMPK by LKB1 through phosphorylation of its T-loop under energetic stress, which can be observed in the micro-environments of rapidly growing tumors [8, 9], seems to contribute to the regulation of tumor cell growth and proliferation (depicted in Fig. 1a). In most tumors, deregulation of the PI3K-Akt pathway (by e.g. inactivating mutations in the PTEN gene or activating mutations in genes encoding PI3Ks) results in upregulation of the mTOR pathway, which stimulates cellular proliferation and growth by, besides others, phosphorylating p70, S6 Kinase-1 (S6 K1) and 4E-BP1 (reviewed by [10]). Under low energetic conditions or hypoxia, increased AMP levels can enhance the activation of AMPK by LKB1, resulting in the phosphorylation of TSC2 (an inhibitor of mTOR) and Raptor (the core component of mTOR), both leading to a decreased mTOR activity [11–18]. As a consequence, loss of LKB1 activity (in particular in conjunction with upregulation of PI3K/Akt signaling) results in enhanced mTOR activation, which is associated with increased tumor cell growth and proliferation. Interestingly, it has in recent years become clear that AMPK activators may serve as powerful anti-cancer agents (discussed by [19]). Germline mutations in the LKB1 gene are the main cause for Peutz-Jeghers syndrome (PJS), an autosomal dominant disorder [20, 21]. PJS patients typically suffer from mucocutaneous mispigmentation, hamartomatous polyps in the gastrointestinal tract and an increased risk to develop intestinal and extra-intestinal cancers [22, 23]. In addition to PJS, LKB1 gene mutations have also been identified in sporadic tumors, in particular in small-cell lung cancer, prostate cancer and minimal deviation adenocarcinoma, a subtype of cervix carcinoma (reviewed by [24, 25]). Although biallelic inactivation of LKB1 has been encountered in cervical carcinomas, one LKB1 wild-type allele has been found to be maintained in lung adenocarcinomas [26–30]. Moreover, an inverse correlation between LKB1 protein expression and endometrial cancer grade has been reported [31]. Next to LKB1 gene mutations that affect its kinase activity or destabilize its protein, LKB1 protein expression has also been found to be frequently downregulated in several types of tumors [32]. Head and neck cancer occurs frequently and encompasses both malignant and benign tumors [33–35]. Salivary gland tumors account for ~5 % of all head and neck tumors. Most of these tumors (75 %) arise in the parotid glands, whereas 10 % originate from submandibular glands, 15 % from minor salivary glands of the upper aerodigestive tract and
