Distinct roles of MK2 and MK5 in cAMP/PKA- and stress/p38MAPK-induced heat shock protein 27 phosphorylation

Using HEK293 cells, we show that MK2, MK3, and MK5 are expressed in these cells, but MK3 protein levels are very moderate. Stress- and cAMP-elevating stimuli, as well as ectopic expression of active MKK6 plus p38MAPK or the catalytic subunit of PKA trigger HSP27 phosphorylation, and specific inhibitors of p38MAPK and PKA prevent this phosphorylation. Depletion of MK2, but not MK3 and MK5 diminished stress-induced HSP27 phosphorylation, while only knockdown of MK5 reduced PKA-induced phosphoHSP27 levels. Stimulation of the p38MAPK, but not the PKA pathway, caused activation of MK2.Our results suggest that in HEK293 cells MK2 is the HSP27 kinase engaged in stress-induced, but not cAMP-induced phosphorylation of HSP27, while MK5 seems to be the sole MK to mediate HSP27 phosphorylation in response to stimulation of the PKA pathway. Thus, despite the same substrate specificity towards HSP27, MK2 and MK5 are implicated in different signaling pathways causing actin reorganization.The mitogen-activated protein kinase (MAPK) pathways control crucial cellular processes such as proliferation, differentiation, cell survival, apoptosis, gene regulation, and motility [1,2]. The typical mammalian MAPK pathways include the well-characterized MEK1-2/ERK1-2, JNK1-3, MEK5/ERK5, and p38MAPK pathways, while the atypical MAPK pathways include the less studied ERK3, ERK4, and ERK7 [2-6]. The classical MAPK pathways consist of a three partite module in which a MAPK kinase kinase phosphorylates and activates downstream MAPK kinases, which in turn phosphorylate and activate MAPK. MAPK can then phosphorylate non-protein kinase substrates or yet other protein kinases. The latter are referred to as mitogen-activated protein kinase-activating protein kinases (MAPKAPK) [1,2]. Based on structural and functional homology, the MAPKAPKs are divided in four subfamilies: ribosomal S6 kinase (RSK) with the members RSK1-4; mitogen- and stress-activated kinase (MSK) comprising MSK1 and MSK2; MAPK-interact