Cadherins are transmembrane proteins traditionally known to have a structural role in cellular adhesion. These proteins display tissue-specific changes in expression that have been correlated with poor prognosis in a variety of cancers. N-cadherin expression specifically has been associated with poor oral cancer prognosis. An estimated 80% of oral cancers exhibit aberrant N-cadherin expression. We have previously shown that cadherins can alter growth factor receptor signaling cascades, which have been shown in other studies to direct metabolic changes essential to oral cancer metastasis. The goal of the current study was to characterize the mechanism(s) by which abnormal expression of N-cadherin may alter metabolic activity in oral epithelia. We have utilized a dysplastic oral keratinocyte (DOK) line to develop retrovirally-transduced cells which express control levels or reduced levels of N-cadherin protein. Using these lines, we evaluated the effect of N-cadherin expression on EGF-dependent EGFR phosphorylation as well as downstream effectors within the mammalian target of rapamycin (mTOR) signaling pathway. Western blot analysis of our EGF-treated DOK lines suggested that N-cadherin expression suppressed phosphorylation of EGFR at Tyr 1068, which plays a role in EGFR degradation, and Tyr 1045, which can mediate cell survival through activation of signal transducer and activator of transcription 3 (STAT3). We also found minimal changes in phosphorylation of Tyr 992, a phosphosite associated with mitogenic signaling. EGFR activation has been shown to direct metabolism through phosphatidyl inositol-3 kinase (PI3K)/Akt/mTOR signaling. Our data indicated that N-cadherin expression is necessary for maximal EGF-directed phosphorylation of both Akt at Ser 473 and of mTORC1 at Ser 2448. mTOR has been shown to increase synthesis of the transcription factor hypoxia inducible factor-1 alpha (HIF-1 alpha) to redirect glycolytic intermediates from oxidative metabolism to anaerobic glycolysis by way of lactate dehydrogenase (LDH), a hallmark of the Warburg effect. We found that N-cadherin expression suppressed HIF-1 alpha synthesis but did not appear to alter lactate dehydrogenase expression. It is important to note that reduced expression of N-cadherin did not alter levels of either E- or P-cadherin expression. Taken together, these findings suggest a role for N-cadherin in the regulation of mTOR-related metabolic signaling, and in modulation of the Warburg effect in oral precancer. Given the high incidence of N-cadherin expression in oral cancer as well as its correlation with poor prognosis, elucidating the mechanisms by which N-cadherin regulates metabolism could reveal novel and promising targets for future therapeutic approaches.
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Funding Provided By Midwestern University College of Graduate Studies