STAT3 Is a Potential Therapeutic Target for Histone Deacetylase Inhibitor-Resistant Cutaneous T-Cell Lymphoma

Abstract

Background: Histone deacetylase inhibitors (HDACis), such as vorinostat and romidepsin, are effective treatment agents for various T-cell lymphomas, including cutaneous T-cell lymphoma (CTCL), peripheral T-cell lymphoma, and adult T-cell lymphoma/leukemia. Despite the promising anti-lymphoma activity of HDACis, drug resistance remains a significant clinical problem. Therefore, novel therapeutic strategies are required to overcome HDACi resistance. In this study, we generated HDACi-resistant CTCL cell lines and investigated the mechanisms of HDACi resistance. Aims: This study aimed to identify the mechanism of resistance to HDACis in CTCLs and to explore new therapeutic targets. Methods: The CTCL cell lines MyLa, MJ, and Hut78 were used in this study. Vorinostat-resistant cell lines, namely MyLa resistant, HUT78 resistant, and MJ resistant, were prepared by repeatedly exposing cells to increasing concentrations of vorinostat. To maintain drug resistance, the cells were continuously cultured in a medium containing 2.5 μM vorinostat. Following this, we conducted a comprehensive gene expression analysis using a microarray platform to investigate the deregulated genes in the resistant cell lines. Results: CTCL cell lines resistant to vorinostat were also resistant to romidepsin, suggesting that vorinostat-resistant cell lines showed cross-resistance to other HDACis. Next, we selected genes whose expression was upregulated more than 1.5 times in the three resistant cell lines and performed pathway analysis. Pathway analysis revealed that the upregulated genes were significantly enriched in the cytokine-mediated signaling pathways. Among the upregulated genes, we focused on interleukin-6 signal transducer (IL6ST)/glycoprotein (gp) 130, which forms a heterogeneous complex with IL6 and IL6 receptors, and activates the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway. Furthermore, flow cytometric analysis revealed that surface gp130 expression was significantly upregulated in the resistant cell lines. Moreover, the MyLa-resistant and MJ-resistant cell lines showed increased sensitivity to a gp130 inhibitor (SC144). Gp130 activation could activate the JAK/STAT pathway; therefore, we examined the expression level of the activated forms of STAT3 and p-STAT3 and found that p-STAT3 was significantly upregulated in the resistant cell lines. To investigate whether p-STAT3 upregulation is involved in the development of vorinostat resistance, we performed STAT3 knockdown using siRNA. We confirmed that STAT3 knockdown significantly decreased the survival of the resistant cell lines. Moreover, resistant cell lines showed increased sensitivity to the STAT3-selective inhibitor C188-9. However, compared to the controls, the resistant cell lines showed no difference in their sensitivity towards the JAK1/2 inhibitor ruxolitinib. This suggests that the STAT3 activation occurs not only through the JAK/STAT pathway but also through a pathway that is not mediated by JAK. We also examined the expression level of p-STAT5; however, it was downregulated in the resistant cell lines, which was in contrast to that of p-STAT3. These findings suggest that STAT3 activation, which is mediated by HDAC inhibition, could contribute to the resistance of CTCL to HDACis. Finally, we examined the gp130 and p-STAT3 protein expression in CTCL skin samples, which were obtained from the same patients before and after vorinostat treatment. We found that the gp130 and p-STAT3 expression was significantly increased in the specimens during relapse after vorinostat treatment. Conclusion: Activation of STAT3 in HDACi-resistant CTCL cells was enhanced by gp130, which contributed to their resistance. These results suggest that STAT3 could be the new therapeutic target in HDACi-resistant CTCL and that gp130/STAT3 inhibition could be an important therapeutic strategy. To further elucidate the detailed mechanism of HDACi resistance, analysis of the pathways activating STAT3 without JAK and the target proteins of STAT3 is warranted.