Stimulation of tumor necrosis factor α production in human monocytes by inhibitors of protein phosphatase 1 and 2A

Abstract

The protein phosphatase 1 and 2A inhibitor, okadaic acid, has been shown to stimulate many cellular functions by increasing the phosphorylation state of phosphoproteins. In human monocytes, okadaic acid by itself stimulates tumor necrosis factor α (TNF-α) mRNA accumulation and TNF-α synthesis. Calyculin A, a more potent inhibitor of phosphatase 1, has similar effects. TNF-α mRNA accumulation in okadaic acid-treated monocytes is due to increased TNF-α mRNA stability and transcription rate. The increase in TNF-α mRNA stability is more remarkable in okadaic acid-treated monocytes than the mRNA stability of other cytokines, such as interleukin 1α (IL-1α), IL-1β, and IL-6. Gel retardation studies show the stimulation of AP-1, AP-2, and NF-κB binding activities in okadaic acid-stimulated monocytes. This increase may correlate with the increase in TNF-α mRNA transcription rate. In addition to the stimulation of TNF-α secretion by monocytes, okadaic acid appears to modulate TNF-α precursor processing, as indicated by a marked increase in the cell-associated 26-kD precursor. These results suggest that active basal phosphorylation/dephosphorylation occurs in monocytes, and that protein phosphatase 1 or 2A is important in regulating TNF-α gene transcription, translation, and posttranslational modification.