MMP-9 from sublethally irradiated tumor promotes Lewis lung carcinoma cell invasiveness and pulmonary metastasis

Abstract

Matrix metalloproteinases (MMPs) associate with tumor progression and metastasis. We sought to investigate the role of MMP-9 from sublethally irradiated tumor in accelerated pulmonary metastasis of Lewis lung carcinoma (LLC-LM) and the corresponding anti-metastasis strategies in C57BL/6 mice. We used Matrigel-coated Boyden chamber assays and chicken chorioallantoic membrane assays to evaluate the invasion capability of irradiated LLC-LM cells (7.5 Gy), reverse transcription-polymerase chain reaction and the western blot assay to investigate the expression of MMPs by irradiated cells, and small interfering RNA duplexes to inhibit MMP-9 expression. LLC-LM cells differing in MMP-2 or -9 expression were subcutaneously injected into right thighs and the resulting tumors were irradiated (10 Gy × 5) to induce pulmonary metastasis. Radiation significantly enhanced MMP-9 at both the transcriptional and translational levels. MMP-9 siRNA significantly inhibited in vitro radiation-enhanced invasiveness. The number of radiation-accelerated pulmonary metastases was significantly reduced by MMP-9 knockdown and MMP-2/9 knockdown. Reverse transcription-polymerase chain reaction of LLC-LM cells in the blood and lung tissue revealed MMP-9 involvement in radiation-enhanced intravasation. Either higher-dose irradiation (30 Gy × 2) or pretreatment with prototypical MMP-9 inhibitor, zoledronic acid, significantly reduced the number of pulmonary metastases. The viability of irradiated tumor was seen on both positron emission tomography and magnetic resonance imaging, and tumor/serum MMP-9 levels suggested the association of local control of primary tumor and inhibition of time-dependent MMP-9 activities. Our results demonstrate that MMP-9 is crucially involved in radiation-enhanced LLC-LM cell invasiveness in vitro and in pulmonary metastasis from inadequately irradiated primary tumor in vivo.