Purpose: Plakophilin 1 (PKP1) is well-known as an important component of the desmosome, a cell structure specialized in spot-like cell-to-cell adhesion. Although desmosomes have generally been associated with tumor suppressor functions, we recently found that PKP1 is recurrently overexpressed in squamous cell lung cancer (SqCLC) to exert an oncogenic role by enhancing the translation of MYC (c-Myc), a major oncogene. In this study, we aim to further characterize the functional relationship between PKP1 and MYC.
Methods: To determine the functional relationship between PKP1 and MYC, we performed correlation analyses between PKP1 and MYC mRNA expression levels, gain/loss of function models, chromatin immunoprecipitation (ChIP) and promoter mutagenesis followed by luciferase assays.
Results: We found a significant correlation between the mRNA levels of MYC and PKP1 in SqCLC primary tumor samples. In addition, we found that MYC is a direct transcription factor of PKP1 and binds to specific sequences within its promoter. In agreement with this, we found that MYC knockdown reduced PKP1 protein expression in different SqCLC models, which may explain the PKP1-MYC correlation that we found. Conversely, we found that PKP1 knockdown reduced MYC protein expression, while PKP1 overexpression enhanced MYC expression in these models.
Conclusions: Based on these results, we propose a feedforward functional relationship in which PKP1 enhances MYC translation in conjunction with the translation initiation complex by binding to the 5'-UTR of MYC mRNA, whereas MYC promotes PKP1 transcription by binding to its promoter. These results suggest that PKP1 may serve as a therapeutic target for SqCLC.
Keywords: Biomarker; Feedforward loop; MYC; Non-small cell lung cancer; Plakophilin; Squamous cell lung cancer.
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