Alternatively spliced isoforms of IL-32 differentially influence cell death pathways in cancer cell lines

Abstract

Alternative splicing is a biological mechanism that enables the synthesis of several isoforms with different or even opposite functions. This process must be tightly regulated to prevent unwanted isoform expression favoring pathological processes. Some isoforms of interleukin 32 (IL-32) are reported to be more potent in inducing inflammation, however the role in cell death remains to be investigated. This study demonstrates that IL-32gamma and IL-32beta can induce caspase-8-dependent cell death whereas this was not observed for IL-32alpha. Overexpression of IL-32beta or IL-32gamma but not IL-32alpha, resulted in enhanced expression of the survival cytokine IL-8. Furthermore, restoring the IL-8 signaling pathway by overexpressing CXCR1 in HEK293 cells, rescued IL-32beta but not IL-32gamma-induced cell death. Interestingly, IL-32gamma was able to downregulate CXCR1 and thereby induce cell death. Subsequent studies into the role of IL-32 in thyroid cancer (TC) revealed that several IL-32 isoforms, IL-8, and CXCR1 are expressed in TC cell lines and specimens. Remarkably, TC cell lines were found to produce high concentrations of IL-8, indicating an important role for IL-8 in the survival-signaling pathway in these cells. Intriguingly, a significant correlation between the IL-8 receptor CXCR1 and IL-32gamma was observed in TC specimens, while this was not observed for the other IL-32 splice variants. Blocking IL-32 alternative splicing by Isoginkgetin resulted in predominant expression of IL-32gamma splice variants and cell death in TC cell lines. All together, modulation of IL-32 alternative splicing could represent a novel strategy for the treatment of malignancies, in particular thyroid cancer.