Abstract

We previously developed a gene-gun-based in vivo screening system and identified shikonin as a potent suppressor of tumor necrosis factor-α (TNF-α) gene expression. Here, we show that shikonin selectively inhibits the expression of TNF-α at the RNA splicing level. Treatment of lipopolysaccharide-stimulated human primary monocytes and THP-1 cells with shikonin resulted in normal transcriptional induction of TNF-α, but unspliced pre-mRNA accumulated at the expense of functional mRNA. This effect occurred with noncytotoxic doses of shikonin and was highly specific, because mRNA production of neither a housekeeping gene nor another inflammatory cytokine gene, interleukin-8 (IL-8), was affected. Moreover, cotreatment with lipopolysaccharide (LPS) and shikonin increased the endpoint protein production of IL-8, accompanied by suppressed activation of the double-stranded RNA-activated protein kinase (PKR) pathway. Because PKR inactivation has been shown to down-regulate the splicing process of TNF-α RNA and interfere with translation, our findings suggest that shikonin may achieve differential modulation of cytokine protein expression through inactivation of the PKR pathway and reveal that regulation of TNF-α pre-mRNA splicing may constitute a promising target for future anti-inflammatory application.