Abstract
The human kallikrein 8 (KLK8) gene, a member of the human tissue kallikrein gene family, encodes a serine protease. The KLK8 protein (hK8) is known to be a favorable prognostic marker in ovarian cancer, but the biological basis of this is not understood. We found that overexpressing the KLK8 gene in highly invasive lung cancer cell lines suppresses their invasiveness. This role in invasiveness was further confirmed by the fact that inhibition of endogenous KLK8 expression with a specific short hairpin RNA reduced cancer cell invasiveness. In situ degradation and cell adhesion assays showed that proteins produced from KLK8 splice variants modify the extracellular microenvironment by cleaving fibronectin. DNA microarray experiments and staining of cells for actin filaments revealed that the degradation of fibronectin by hK8 suppresses integrin signaling and retards cancer cell motility by inhibiting actin polymerization. In addition, studies in a mouse model coupled with the detection of circulating tumor cells by quantitative PCR for the human Alu sequence showed that KLK8 suppresses tumor growth and invasion in vivo. Finally, studies of clinical specimens from patients with non-small cell lung cancer showed that the time to postoperative recurrence was longer for early-stage patients (stages I and II) with high KLK8 expression (mean, 49.9 months) than for patients with low KLK8 expression (mean, 22.9 months). Collectively, these findings show that KLK8 expression confers a favorable clinical outcome in non-small cell lung cancer by suppressing tumor cell invasiveness.
Highlights
Human tissue kallikreins are encoded by a family of 15 structurally homologous genes (KLK) clustered together on chromosome 19q13.4
We compared the gene expression profiles of a CL1-5/KLK8 splice variant and CL1-5/Vector cells using DNA microarray analysis to further investigate the pathways that are affected by hK8-mediated degradation of fibronectin, and which lead to the suppression of cell invasion
To verify whether hK8 is involved in modulating integrin signaling, we examined the phosphorylation of Src, a downstream signaling target in the fibronectin-integrin pathway
Summary
Human tissue kallikreins (hK) are encoded by a family of 15 structurally homologous genes (KLK) clustered together on chromosome 19q13.4. Aberrant amounts of KLK transcripts and/ or hK proteins have been found in several hormonal malignancies, Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/)
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