Overexpression of GRIM-19 in cancer cells suppresses STAT3-mediated signal transduction and cancer growth.

Constitutive activation of signal transducer and activator of transcription 3 (STAT3) is common in many human and murine cancer cells, and its activation leads to cellular transformation. STAT3 pathway inhibitors have been reported to suppress cancer growth. To investigate the antitumor effects of inhibiting the STAT3-mediated signaling cascade in the cancer microenvironment, using a molecular-targeting approach, we focused on the gene associated with retinoid-IFN-induced mortality 19 (GRIM-19). GRIM-19 has been reported to interact physically with STAT3 and inhibit STAT3-dependent signal transduction. We used the nona-arginine (R9)-protein transduction domain (R9-PTD) as a protein carrier to induce high levels of GRIM-19 expression in vitro and in vivo. We generated an R9-PTD-containing GRIM-19 fusion protein (rR9-GRIM19) and successfully induced overexpression in the cytoplasm of cancer cells. Analysis of the expression of downstream molecules of STAT3 confirmed that in vitro rR9-GRIM19 treatment of constitutively activated STAT3 (STAT3c) cancer cells significantly reduced STAT3-dependent transcription. Moreover, intratumoral injections of rR9-GRIM19 in STAT3c cancer-bearing mice significantly suppressed tumor growth. These results suggest that intratumoral injections of rR9-GRIM19 have potential as a novel anticancer therapy in STAT3c cancer due to their ability to inhibit STAT3-mediated signal transduction without major systemic side effects.