Bacterial lipopolysaccharide (LPS) exerts profound effects on mammalian hosts in part by inducing macrophages to release tumor necrosis factor-alpha (TNF-alpha); the mechanisms involved are unresolved. The microtubule stabilizer taxol shared two actions of LPS on macrophages: it rapidly decreased TNF-alpha receptors and triggered TNF-alpha release. Both actions of taxol were absent in LPS-hyporesponsive C3H/HeJ mice. In recombinant inbred mice, the genes controlling responses to LPS and to taxol were closely linked. Dexamethasone blocked release of TNF-alpha by both stimuli but did not block the decrease in TNF-alpha receptors. Thus, a protein associated with microtubules may be a cellular target of LPS.

The protein phosphatase 2A (PP2A) acts on several kinases in the extracellular signal-regulated kinase (ERK) signaling pathway but whether a specific holoenzyme dephosphorylates ERK and whether this activity is controlled during mitogenic stimulation is unknown. By using both RNA interference and overexpression of PP2A B regulatory subunits, we show that B56, but not B, family members of PP2A increase ERK dephosphorylation, without affecting its activation by MEK. Induction of the early gene product and ERK substrate IEX-1 (ier3) by growth factors leads to opposite effects and reverses B56-PP2A-mediated ERK dephosphorylation. IEX-1 binds to B56 subunits and pERK independently, enhances B56 phosphorylation by ERK at a conserved Ser/Pro site in this complex and triggers dissociation from the catalytic subunit. This is the first demonstration of the involvement of B56-containing PP2A in ERK dephosphorylation and of a B56-specific cellular protein inhibitor regulating its activity in an ERK-dependent fashion. In addition, our results raise a new paradigm in ERK signaling in which ERK associated to a substrate can transphosphorylate nearby proteins.