Cytosolic and Nuclear Mitogen-Activated Protein Kinases Are Regulated by Distinct Mechanisms (original) (raw)

1996, Experimental Cell Research

ERKs) belong to a group of protein serine/threonine We have investigated the regulation and localization kinases [8]. The most widely studied MAPKs, the of mitogen-activated protein kinase (MAPK) and mito-ubiquitously expressed p42 and p44 MAPK isoforms, gen-activated protein kinase kinase (MAPKK) in both are rapidly phosphorylated and activated in response cytosolic and nuclear fractions of glomerular mesanto stimulation of a wide array of membrane-associated gial cells. p42 MAPK was localized by both immunoblot signaling molecules, including receptor tyrosine kiand kinase activity in both cytosol and nucleus and nases [1-8], protein kinase C (PKC) [9], G-proteins, was rapidly activated, in both fractions, by fetal boand Ras [12][13]. A variety of cellular components have vine serum and TPA. Downregulation of protein kibeen shown to be MAPK substrates, including EGF nase C (PKC) by TPA inhibited stimulation of cytosolic receptor [14], phospholipase A2 16], Rsk 90 [17], p42 MAPK, but unexpectedly had no effect on stimujun, and fos 19]. Therefore, MAPKs appear to be lated p42 MAPK in the nucleus. Next we studied the transducers integrating multiple intracellular signals. upstream kinase p45 MAPKK by indirect immunoflu-p42 and p44 MAPKs localize in both cytoplasm and orescence microscopy, Western blot analysis, and kinucleus in G0-arrested cells [20-23]. They become actinase specific activity. Unlike MAPK, p45 MAPKK is alvated after growth factor stimulation and the cytomost exclusively cytosolic in resting cells and kinase plasmic MAPKs translocate into the nucleus, thereby activity stimulated by TPA is restricted to the cytosol. transmitting the extracellular signals into the nucleus Interestingly, PKC downregulation for 24 h with TPA [5, 20-23]. However, several lines of experimental redramatically enhanced nuclear MAPKK as assessed by sults are not entirely consistent with this theory. First, all three techniques. Cytosolic stimulated MAPKK was the translocation occurs at 5 min and reaches a maxiattenuated in PKC downregulation. Collectively these mum 3 to 4 h after stimulation [20, 23], whereas the results show that in mesangial cells: (i) p42 MAPK and maximal MAPK activity is generally found at 5 min p45 MAPKK localize in both the cytosol and the nufollowed by a rapid decline due to dephosphorylation cleus, and (ii) PKC exerts a negative effect on nuclear MAPKK activity as documented by PKC downregula-of the kinases [1 -8]. Second, the translocation of tion, which augments p45 MAPKK nuclear mass and MAPKs can proceed with kinase-deficient mutants activity. These results indicate that the dual regula- [22]. Third, no lag period between the activation of nution of these two kinases is under differential control clear MAPKs and that of cytosolic MAPKs is found and in the cytosol and the nucleus. ᭧ 1996 Academic Press, Inc.