Abstract
Double-stranded RNA (dsRNA)-dependent protein kinase (PKR) is an interferon-induced protein kinase that plays a central role in the anti-viral process. Due to its pro-apoptotic and anti-proliferative action, there is an increased interest in PKR modulation as an anti-tumor strategy. PKR is overexpressed in breast cancer cells; however, the role of PKR in breast cancer cells is unclear. The expression/activity of PKR appears inversely related to the aggressiveness of breast cancer cells. The current study investigated the role of PKR in the motility/migration of breast cancer cells. The activation of PKR by a synthesized dsRNA (PIC) significantly decreased the motility of several breast cancer cell lines (BT474, MDA-MB231 and SKBR3). PIC inhibited cell migration and blocked cell membrane ruffling without affecting cell viability. PIC also induced the reorganization of the actin cytoskeleton and impaired the formation of lamellipodia. These effects of PIC were reversed by the pretreatment of a selective PKR inhibitor. PIC also activated p38 mitogen-activated protein kinase (MAPK) and its downstream MAPK-activated protein kinase 2 (MK2). PIC-induced activation of p38 MAPK and MK2 was attenuated by the PKR inhibitor and the PKR siRNA, but a selective p38 MAPK inhibitor (SB203580) or other MAPK inhibitors did not affect PKR activity, indicating that PKR is upstream of p38 MAPK/MK2. Cofilin is an actin severing protein and regulates membrane ruffling, lamellipodia formation and cell migration. PIC inhibited cofilin activity by enhancing its phosphorylation at Ser3. PIC activated LIM kinase 1 (LIMK1), an upstream kinase of cofilin in a p38 MAPK-dependent manner. We concluded that the activation of PKR suppressed cell motility by regulating the p38 MAPK/MK2/LIMK/cofilin pathway.
Highlights
Double-stranded RNA-dependent protein kinase (PKR) is a 551 amino acid serine/threonine protein kinase and is ubiquitously expressed in mammalian cells
We further established the p38 mitogen-activated protein kinase (MAPK)/MAPK-activated protein kinase 2 (MK2)/LIM kinase (LIMK)/cofilin signaling pathway is involved in PKR modulation of cell migration (Fig. 8)
Our study demonstrates that PKR activation inhibits the migration of breast cancer cells independent of cell cycle arrest and the induction of apoptosis, suggesting PKR activity may negatively regulate cell mobility
Summary
Double-stranded RNA (dsRNA)-dependent protein kinase (PKR) is a 551 amino acid serine/threonine protein kinase and is ubiquitously expressed in mammalian cells. The enzyme phosphorylates the a-subunit of protein synthesis initiation factor eIF2, thereby inhibiting translation [1]. PKR mediates the activation of signal transduction pathways by proinflammatory stimuli, including bacterial lipopolysaccharide (LPS), tumor necrosis factor-a (TNF-a) and interleukin 1 (IL-1) [1]. Active PKR regulates diverse downstream substrates and signaling pathways, such as NFkB, p53, protein phosphatase 2A (PP2A), MAPK and STAT1/ STAT3 signaling [3]. PKR has been implicated in the regulation of cell proliferation, apoptosis, differentiation and transformation [5,6]. Activation of PKR results in the inhibition of cell proliferation or the induction of apoptosis and translational suppression; PKR is considered a ‘‘tumor suppressor’’ and considerable attention has been paid to the PKR pathway for its anti-tumor potential [3,7]
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