Cytokine gene expression and ERK activation is modulated by PP2A in THP1 cells exposed to asbestos

Cytokines released by alveolar macrophages play a role in the development of pulmonary fibrosis in patients with asbestosis. Our prior studies show that ERK is a negative regulator of TNF‐α production in monocytes, and that an ERK‐specific phosphatase (MKP‐3) dephosphorylates ERK and increases TNF‐α gene expression in monocytes stimulated with asbestos. Thus, we hypothesized that protein phosphatase 2A (PP2A) may also dephosphorylate ERK in monocytic (THP1) cells. We found that asbestos exposure enhances phosphatase activity in lysates but this activity was reduced near control levels by pretreating cells with okadaic acid or by RNA knockdown of PP2Ac (catalytic subunit). Using an in vitro assay, lysates obtained from cells exposed to asbestos dephosphorylated active ERK, but ERK dephosphorylation was inhibited by PP2Ac siRNA. Asbestos exposure also reduced phosphorylated ERK (pERK) in THP1 cells; however, pERK levels can be increased by okadiac acid or by expression of a dominant negative PP2A, or by RNA knockdown of PP2Ac. Last, affinity purification of an expressed ERK‐V5‐6XHis construct revealed direct binding of PP2Ac to ERK in THP1 cells exposed to asbestos. These finding suggest that PP2A may contribute to ERK inhibition and reduced TNF‐α production in monocytic cells exposed to asbestos. This work supported by NIH grant ES‐015981 and ES‐014871.