PLAC8 Localizes to the Inner Plasma Membrane of Pancreatic Cancer Cells and Regulates Cell Growth and Disease Progression through Critical Cell-Cycle Regulatory Pathways.

Pancreatic ductal adenocarcinoma (PDAC) carries the most dismal prognosis of all solid tumors and is generally strongly resistant to currently available chemo- and/or radiotherapy regimens, including targeted molecular therapies. Therefore, unraveling the molecular mechanisms underlying the aggressive behavior of pancreatic cancer is a necessary prerequisite for the development of novel therapeutic approaches. We previously identified the protein placenta-specific 8 (PLAC8, onzin) in a genome-wide search for target genes associated with pancreatic tumor progression and demonstrated that PLAC8 is strongly ectopically expressed in advanced preneoplastic lesions and invasive human PDAC. However, the molecular function of PLAC8 remained unclear, and accumulating evidence suggested its role is highly dependent on cellular and physiologic context. Here, we demonstrate that in contrast to other cellular systems, PLAC8 protein localizes to the inner face of the plasma membrane in pancreatic cancer cells, where it interacts with specific membranous structures in a temporally and spatially stable manner. Inhibition of PLAC8 expression strongly inhibited pancreatic cancer cell growth by attenuating cell-cycle progression, which was associated with transcriptional and/or posttranslational modification of the central cell-cycle regulators CDKN1A, retinoblastoma protein, and cyclin D1 (CCND1), but did not impact autophagy. Moreover, Plac8 deficiency significantly inhibited tumor formation in genetically engineered mouse models of pancreatic cancer. Together, our findings establish PLAC8 as a central mediator of tumor progression in PDAC and as a promising candidate gene for diagnostic and therapeutic targeting.

A multistep high-content screening approach to identify novel functionally relevant target genes in pancreatic cancer.

In order to foster the systematic identification of novel genes with important functional roles in pancreatic cancer, we have devised a multi-stage screening strategy to provide a rational basis for the selection of highly relevant novel candidate genes based on the results of functional high-content analyses. The workflow comprised three consecutive stages: 1) serial gene expression profiling analyses of primary human pancreatic tissues as well as a number of in vivo and in vitro models of tumor-relevant characteristics in order to identify genes with conspicuous expression patterns; 2) use of 'reverse transfection array' technology for large-scale parallelized functional analyses of potential candidate genes in cell-based assays; and 3) selection of individual candidate genes for further in-depth examination of their cellular roles. A total of 14 genes, among them 8 from "druggable" gene families, were classified as high priority candidates for individual functional characterization. As an example to demonstrate the validity of the approach, comprehensive functional data on candidate gene ADRBK1/GRK2, which has previously not been implicated in pancreatic cancer, is presented.