ATF-1, Retinoic Acid Induced 14 (RAI14) and C-Jun form a Novel Regulatory Axis of Invasiveness and Proliferation

Unraveling the molecular mechanisms which transduce microenvironmental cues and integrate such signals with intrinsically aberrant regulation of proliferation and invasiveness of cancer cells, and metastatic cells in particular, should further the development of effective therapeutic strategies. Over-activation of Jun N-terminal Kinase (JNK)/c-Jun signaling was shown to mediate tumorigenic attributes in different tumor models. However, the molecular basis of such aberrant activation and its integration into the tumor cell context merit further study. Proteins which dynamically localize both to the cytoskeleton/membrane interface and the cell nucleus potentially integrate excessive signaling input at the cell surface and modified transcriptional control of gene expression. We identify retinoic acid induced 14 (RAI14) as a candidate for such regulatory function. We opted for glioblastoma cells as a model of invasive and aggressive tumor. The RAI14 message is overexpressed in glioblastoma clinical samples, and this overexpression correlates with poor survival. Glioblastoma cell lines expressed the RAI14 protein, which localized to plasma membrane, actin cytoskeleton and cell nucleus. Antibody-mediated pull-down of RAI14 identified its interaction with multiple cellular components, including the phosphorylated form of the transcription factor ATF-1. Knockdown of RAI14 or ATF-1 reduced proliferation and invasiveness of glioblastoma cells. Moreover, siRNA-mediated knockdown studies also revealed the regulation of c-Jun and phospho-c-Jun expression by both ATF-1 and RAI14, and the regulation of RAI14 expression by ATF-1. Inhibition of JNK activity reduced the expression of phospho- and total c-Jun, and the proliferation and invasiveness of U87-MG cells; while siRNA-mediated knockdown of c-Jun attenuated their proliferation. Together, our data identify an ATF-1-RAI14-c-Jun regulatory circuit in glioblastoma cells, and suggest that such circuit contributes to the invasive and proliferative character of this malignancy.