MAP kinase cascade in astrocytes

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Abstract
We have studied in cultured rat astroglial cells MAP kinases, known for their role in intracellular signal transduction. The MAP kinase activity was stimulated by growth factors (FGFb, FGFa, EGF, PDGF, and IGF1), by a phorbol ester (TPA) activating‐protein kinase C (PKC), by a neuropeptide (endothelin‐1), and by a neuromediator (carbachol). Astrocytes pretreated for 18 h with TPA were still stimulated by growth factors and endothelin, suggesting that down‐regulated isoforms of PKC are not involved in MAP kinase activation. In contrast, the small effect of carbachol was suppressed by TPA pretreatment. Astrocytes contained two proteins (p41 and p44) recognized by MAP kinase antibody. These proteins were phosphorylated on tyrosine residues in the cytosols of stimulated astrocytes. The kinetics of MAP kinase activation by FGFb and IGF1 were very different. FGFb promoted a rapid activation of MAP kinase (about 10 min) plus a prolonged phase that lasted at least 12 h. IGF1 produced only a rapid transient peak of activation at about 20 min. Hence, extracellular signals might generate different effects in astrocytes by differentially modulating the MAP kinase cascade. On a Mono Q column the growth factor‐stimulated MAP kinase activity was separated into two peaks containing p41 and p44. Stimulation of astrocytes altered the elution pattern of p44 as a result of its phosphorylation. An ATP‐dependent MAP kinase activator (MW = 40–45 kDa) was found in fractions of FGFb‐stimulated cells which were not retained on Mono Q column, indicating the existence of a MAP kinase kinase (MEK) in astrocytes. C‐Raf, identified in other cells as a MAP kinase kinase kinase, was also present in astrocytes. Thus astrocytes contain many components of the MAP kinase cascade activated by growth factors that may also be implicated in the action of neuropeptides and neuromediators.