PKR shRNA downregulates PKR expression in B16-F10 melanoma cells and reduces the metastatic potential of these tumor cells. In the present study, we examined the effect of the intratumoral injection of PKR shRNA-expressing plasmid on the growth of B16-F10 melanoma

The RNA-dependent protein kinase (PKR) is a serine/threonine kinase that is involved in the regulation of important cell processes such as apoptosis, signal transduction, cell proliferation and differentiation. However, the role played by PKR in cancer remains controversial. RNA interference (RNAi) has currently become an important technique in understanding gene function. Previously, we showed that PKR shRNA downregulates PKR expression in B16-F10 melanoma cells and reduces the metastatic potential of these tumor cells. In the present study, we examined the effect of the intratumoral injection of PKR shRNA-expressing plasmid on the growth of B16-F10 melanoma in mice. The results showed that this treatment significantly reduced tumor growth. Thus, these findings suggested that PKR acts as a tumor suppressor, a finding that is consistent with our previous study on the experimental model of metastasis. Moreover, the results suggested that this effect may be mediated by the transcription factor NF-κB. The present study confirmed the hypothesis that the direct administration of RNAi-based therapeutics in the target tumor is a promising approach for overcoming the obstacles of systemic delivery. The results also suggested that the intratumoral injection of PKR shRNA-expressing vector is a novel therapeutic approach for human solid tumors such as cutaneous melanoma and breast cancer, since PKR is overexpressed in these tumors. Introduction The RNA-dependent protein kinase (PKR) is a serine/threonine kinase that was originally identified as a component of the interferon (IFN)-induced anti-viral response (1,2). In response to viral infection, IFN induces the expression of a number of genes, including the human PKR gene. The anti-viral action of PKR is due to its activation by viral double-stranded RNA (dsRNA) which results in the phoshorylation of the α-subunit of eukaryotic translational initiation factor 2 (eIF 2α) with the subsequent shut-down of protein synthesis and inhibition of virus replication. It was demonstrated that PKR is ubiquitously expressed and can also be activated by cellular RNAs (3,4) and proteins (5,6), suggesting that the function of PKR is beyond the anti-viral defense. Several studies have shown that PKR plays a role in the regulation of important cell processes such as apoptosis, signal transduction, cell proliferation and differentiation (1,4,7,8). It has been reported that PKR acts as a tumor suppressor or stimulator protein, depending on the tumor type (1,7,8). The role of PKR in cancer has been extensively studied, however, the results are controversial. In a previous study, we showed that a point mutation in the RNA-binding domain I results in the loss of PKR activity in acute lymphoblastic leukemia (9). By contrast, PKR silencing by RNA interference (RNAi) significantly reduced the number of pulmonary metastatic nodules in the B16-F10 melanoma. In that experimental model of metastasis, our findings suggested that the effect of PKR knockdown is mediated by the transcription factor NF-κB through the reduction of the phosphorylation of its inhibitor IκBβ by PKR (10). RNAi has become a powerful tool for investigating gene function in mammalian cells (10,11). The potential therapeutic of RNAi technology has also been explored in cancer since RNAi is able to selectively knockdown critical genes involved in cell proliferation (12). The RNAi technology holds promise for cancer treatment, however, a number of obstacles remain to be overcome before RNAi can be used in the clinic. Notably, the main challenge is to develop methods to specifically deliver RNAi to tumor cells. In the last few years, several strategies have been used to surmount this barrier (13-15) and Intratumoral injection of PKR shRNA expressing plasmid inhibits B16-F10 melanoma growth NAYARA DELGADO ANDRé1, VIVIANE ALINE OLIVEIRA SILVA2, MARIA ANGELICA EHARA WATANABE3 and FERNANDO LuIz DE LuCCA2 1Federal university of São João del-Rei, 35501296 Divinópolis, MG; 2Department of Biochemistry and Immunology, Ribeirão Preto Medical School, university of São Paulo, 14049-900 Ribeirão Preto, SP; 3Department of Pathological Sciences, Biological Sciences Center, State university of Londrina, 86057-970 Londrina, PR, Brazil Received June 16, 2014; Accepted August 4, 2014 DOI: 10.3892/or.2014.3410 Correspondence to: Professor Fernando Luiz De Lucca, Department of Biochemistry and Immunology, Ribeirão Preto Medical School, university of São Paulo, 14049-900 Ribeirão Preto,