Abstract 1875: Polo-like kinase 1 positively correlates with N-cadherin and promotes epithelial-mesenchymal transition in melanoma
Authors: Chhabra Gagan , Ndiaye Mary A. , Su Shengqin , Singh Chandra K. , Ahmad Nihal 
Published: August 2019

Polo-like kinase 1 (PLK1), a serine/threonine kinase and important cell cycle regulator, is overexpressed in melanoma and has been linked with enhanced cell proliferation. Recent studies have shown involvement of PLK1 in epithelial-mesenchymal transition (EMT) in certain cancer types. However, the role of PLK1 in EMT in melanoma is not well understood. EMT has been shown to promote metastasis, and is associated with an upregulation of the mesenchymal marker N-cadherin (CDH2), downregulation of the epithelial marker E-cadherin (CDH1), and increased cell migration and invasion ability. Earlier, we demonstrated that PLK1 is involved in EMT and its kinase activity is important for EMT-related modulations in A375 melanoma cells (Cancer Res 2018; 78 (13 Suppl): Abstract #2016). In this study, we employed a human tissue microarray (TMA) co-immunostained for PLK1, N-cadherin, E-cadherin and the melanoma biomarker S100 to determine the association between PLK1 and EMT effectors. This TMA, coupled with high-throughput, multispectral Vectra scanning and inForm analysis, allowed us to objectively analyze and quantify protein levels in 57 clinical tissue specimens of nevus (n=15), primary (n=22) and metastatic melanoma (n=20). We observed an increase in PLK1 expression in primary and metastatic melanoma as compared to benign nevi, with a significant increase in metastatic tumors compared to primary tumors (p=0.004). Similarly, N-cadherin expression was markedly higher in metastatic melanoma (fold-change=2.31; p=0.001) and primary melanoma (fold-change=1.8; p=0.03) when compared to benign nevi. Further, we observed a significant decrease in E-cadherin expression in metastatic tumors compared to primary melanoma (fold-change=2.53; p=0.02). Moreover, using single linear regression analyses between expression of two proteins, we found a significant strong positive correlation between PLK1 and mesenchymal marker N-cadherin (correlation co-efficient R=0.75; p&lt;0.01). We also found a weak but negative correlation between PLK1 and epithelial marker E-cadherin (correlation co-efficient R=-0.25). To further confirm the role of PLK1 in EMT, using shRNA-mediated PLK1 knockdown in SK-MEL-2 cells (a human melanoma line that possesses high metastatic potential), we performed cell migration and invasion assays, as well as immunofluorescence staining for expression of N-cadherin and E-cadherin. Interestingly, we found a decrease in expression of mesenchymal marker N-cadherin and an increase in expression of epithelial marker E-cadherin after PLK1 knockdown. These were accompanied by decreased cell migration and invasion, suggesting that inhibition of PLK1 in these human melanoma cells leads to inhibition of EMT. Overall, these results suggest that PLK1 is an important regulator of EMT in melanoma, and therefore can be exploited as a therapeutic target to inhibit melanoma metastasis.

Citation Format: Gagan Chhabra, Mary A. Ndiaye, Shengqin Su, Chandra K. Singh, Nihal Ahmad. Polo-like kinase 1 positively correlates with N-cadherin and promotes epithelial-mesenchymal transition in melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1875.