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The role of integrin-linked kinase in tumour angiogenesis and inflammation Tan, Clara Chia-Hua

Abstract

Integrins act as transducers of extracellular matrix-mediated cell signaling [i.e. cell signalling]. A critical signaling component downstream of integrin engagement is integrin-linked kinase (ILK), a serine/threonine protein kinase and adaptor protein, that interacts with the cytoplasmic domains of β1 and β3 integrins. ILK couples integrins and receptor tyrosine kinases to the cytoskeleton, thus mediating multiple downstream signaling events that regulate cell adhesion, survival, proliferation, migration and differentiation. Inappropriate ILK activity as a result of deregulated upstream mechanisms or altered transcription, results in epithelial cell transformation that recapitulates malignant cancer cell behaviour. I provide novel data demonstrating that ILK plays an indispensable role in regulating downstream targets that induce nitric oxide synthase (iNOS) expression and nitric oxide (NO) production in murine macrophages, and cyclooxygenase-2 (COX-2), a pro-inflammatory macromolecule, expression in both murine and human macrophages. Cells treated with a highly specific small molecule ILK inhibitor or transfected with a dominant-negative mutant of ILK showed decreased ILK activity resulting in inhibition of lipopolysaccacharide (LPS) stimulated iNOS and NO production and reduction in COX-2 expression in a protein kinase B (Akt/PKB)-dependent manner. I also propose a novel model for induction of NF-κB transcription, where phosphorylation of IκB at the serine 32 position directly or indirectly by ILK or Akt/PKB targets IκB for degradation; consequently NF-κB translocates to the nucleus to initiate transcription of pro-inflammatory molecules. In addition, ILK and Akt/PKB are stimulated in a PI3-kinase dependent manner. I provide evidence for a new paradigm involving ILK in the improper production of vascular endothelial growth factor (VEGF) in normoxic conditions, shedding light on the role of ILK in tumor angiogenesis [i.e. tumour angiogenesis]. Mechanistically, I show that activated ILK stimulates the expression of hypoxia-inducible factor-1α (HIF-1α) transcription factor, the major inducer of VEGF expression, through the phosphorylation of mammalian target of rapamycin in an Akt/PKB dependent manner in prostate cancer cells. In a positive feedback process, exposure of endothelial cells to VEGF stimulates a transient increase in ILK kinase activity in a PI-3 kinase-dependent manner. Inhibition of ILK activity using small interfering RNA or exposure to the ILK inhibitor uncouples endothelial cell migration and proliferation in response to VEGF.

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