Activation of protein kinase CK2 by LPS is mediated by the MAP kinase pathway

Stimulation of macrophages by Gram-negative bacterial lipopolysaccharide (LPS) rapidly leads to the activation of several protein kinases and the subsequent phosphorylation of transcription factors that regulate LPS-inducible genes. Several investigators have shown that the MAP kinases (MAPKs) are rapidly activated following LPS stimulation. We previously reported that LPS stimulation also up-regulates the enzymatic activity of protein kinase CK2, a ubiquitous serine/threonine kinase, although the signaling cascade that leads to CK2 activation in macrophages is unknown. Because MAPKs are known to be rapidly activated by LPS, we performed additional studies in order to determine if CK2 activation was a downstream target of MAPKs. Our studies revealed that CK2 activity was rapidly and transiently up-regulated in LPS-stimulated RAW264.7 murine macrophages. We found that PD98059, an inhibitor of ERK kinase activation by the MAP kinase MEK-1, blocked LPS-induced up-regulation of CK2 activity. In contrast, the p38 kinase inhibitor SB203580 did not block CK2 activation by LPS. This finding suggests that CK2 activation was mediated by the ERK kinase signaling cascade, but not by the p38 kinase cascade. We also found that LPS stimulation resulted in the rapid serine phosphorylation of the catalytic α and α′ subunits of CK2. This contrasts with other studies using growth factor-stimulated fibroblasts in which only phosphorylation of the regulatory β subunit of CK2 was observed. Thus, LPS stimulation leads to rapid activation and cell type-specific phosphorylation of CK2 in macrophages.