Interleukin-32 alpha downregulates the activity of the B-cell CLL/lymphoma 6 protein by inhibiting protein kinase C epsilon-dependent SUMO-2 modification

Abstract

A proinflammatory cytokine IL-32 acts as an intracellular mediator. IL-32 alpha interacts with many intracellular molecules, but there are no reports of interaction with a transcriptional repressor BCL6. In this study, we showed that PMA induces an interaction between IL-32 alpha, PKC epsilon, and BCL6, forming a trimer. To identify the mechanism of the interaction, we treated cells with various inhibitors. In HEK293 and THP-1 cell lines, treatment with a pan-PKC inhibitor, PKC epsilon inhibitor, and PKC delta inhibitor decreased BCL6 and IL-32 alpha protein expression. MAPK inhibitors and classical PKC inhibitor did not decrease PMA-induced BCL6 and IL-32 alpha protein expression. Further, the pan-PKC inhibitor and PKC epsilon inhibitor disrupted PMA-induced interaction between IL-32 alpha and BCL6. These data demonstrate that the intracellular interaction between IL-32 alpha and BCL6 is induced by PMA-activated PKC epsilon. PMA induces post-translational modification of BCL6 by conjugation to SUMO-2, while IL-32 alpha inhibits. PKC epsilon inhibition eliminated PMA-induced SUMOylation of BCL6. Inhibition of BCL6 SUMOylation by IL-32 alpha affected the cellular function and activity of the transcriptional repressor BCL6 in THP-1 cells. Thus, we showed that IL-32 alpha is a negative regulator of the transcriptional repressor BCL6. IL-32 alpha inhibits BCL6 SUMOylation by activating PKC epsilon, resulting in the modulation of BCL6 target genes and cellular functions of BCL6.