CARHSP1 Is Required for Effective Tumor Necrosis Factor Alpha mRNA Stabilization and Localizes to Processing Bodies and Exosomes

Tumor necrosis factor alpha (TNF-alpha) is a critical mediator of inflammation, and its production is tightly regulated, with control points operating at nearly every step of its biosynthesis. We sought to identify uncharacterized TNF-alpha 3' untranslated region (3' UTR)-interacting proteins utilizing a novel screen, termed the RNA capture assay. We identified CARHSP1, a cold-shock domain-containing protein. Knockdown of CARHSP1 inhibits TNF-alpha protein production in lipopolysaccharide (LPS)-stimulated cells and reduces the level of TNF-alpha mRNA in both resting and LPS-stimulated cells. mRNA stability assays demonstrate that CARHSP1 knockdown decreases TNF-alpha mRNA stability from a half-life (t(1/2)) of 49 min to a t(1/2) of 22 min in LPS-stimulated cells and from a t(1/2) of 29 min to a t(1/2) of 24 min in resting cells. Transfecting CARHSP1 into RAW264.7 cells results in an increase in TNF-alpha 3' UTR luciferase expression in resting cells and CARHSP1 knockdown LPS-stimulated cells. We examined the functional effect of inhibiting Akt, calcineurin, and protein phosphatase 2A and established that inhibition of Akt or calcineurin but not PP2A inhibits CARHSP1 function. Subcellular analysis establishes CARHSP1 as a cytoplasmic protein localizing to processing bodies and exosomes but not on translating mRNAs. We conclude CARHSP1 is a TNF-alpha mRNA stability enhancer required for effective TNF-alpha production, demonstrating the importance of both stabilization and destabilization pathways in regulating the TNF-alpha mRNA half-life.