Evidence of p38γ and p38δ involvement in cell transformation processes.

The p38 mitogen-activated protein kinase (p38MAPK) signal transduction pathway is an important regulator of cell processes, whose deregulation leads to the development and progression of cancer. Defining the role of each p38MAPK family member in these processes has been difficult. To date, most studies of the p38MAPK pathways focused on function of the p38α isoform, which is widely considered to negatively regulate malignant transformation; nonetheless, few reports address the p38γ and p38δ isoforms. Here, we used embryonic fibroblasts derived from mice lacking p38γ or p38δ and show evidence that these isoforms participate in several processes involved in malignant transformation. We observed that lack of either p38γ or p38δ increased cell migration and metalloproteinase-2 secretion, whereas only p38δ deficiency impaired cell contact inhibition. In addition, lack of p38γ in K-Ras-transformed fibroblasts led to increased cell proliferation as well as tumorigenesis both in vitro and in vivo. Our results indicate that p38γ and p38δ have a role in the suppression of tumor development.

BIRB796 inhibits all p38 MAPK isoforms in vitro and in vivo.

The compound BIRB796 inhibits the stress-activated protein kinases p38alpha and p38beta and is undergoing clinical trials for the treatment of inflammatory diseases. Here we report that BIRB796 also inhibits the activity and the activation of SAPK3/p38gamma. This occurs at higher concentrations of BIRB796 than those that inhibit p38alpha and p38beta and at lower concentrations than those that inhibit the activation of JNK isoforms. We also show that at these concentrations, BIRB796 blocks the stress-induced phosphorylation of the scaffold protein SAP97, further establishing that this is a physiological substrate of SAPK3/p38gamma. Our results demonstrate that BIRB796, in combination with SB203580, a compound that inhibits p38alpha and p38beta, but not the other p38 isoforms, can be used to identify physiological substrates of SAPK3/p38gamma as well as those of p38alpha and p38beta.

p38gamma regulates the localisation of SAP97 in the cytoskeleton by modulating its interaction with GKAP.

Activation of the p38 MAP kinase pathways is crucial for the adaptation of mammalian cells to changes in the osmolarity of the environment. Here we identify SAP97/hDlg, the mammalian homologue of the Drosophila tumour suppressor Dlg, as a physiological substrate for the p38gamma MAP kinase (SAPK3/p38gamma) isoform. SAP97/hDlg is a scaffold protein that forms multiprotein complexes with a variety of proteins and is targeted to the cytoskeleton by its association with the protein guanylate kinase-associated protein (GKAP). The SAPK3/p38gamma-catalysed phosphorylation of SAP97/hDlg triggers its dissociation from GKAP and therefore releases it from the cytoskeleton. This is likely to regulate the integrity of intercellular-junctional complexes, and cell shape and volume in response to osmotic stress.

Identification of glycogen synthase as a new substrate for stress-activated protein kinase 2b/p38beta.

The endogenous glycogen synthase in extracts from mouse skeletal muscle, liver and brain bound specifically to SAPK2b (stress-activated protein kinase 2b)/p38b, but not to other members of the group of SAPK/p38 kinases. Glycogen synthase was phosphorylated in vitro more efficiently by SAPK2b/p38b than by SAPK2a/p38a, SAPK3/p38g or SAPK4/p38d. SAPK2b/p38b phosphorylated glycogen synthase in vitro at residues Ser644, Ser652, Thr718 and Ser724, two of which (Ser644 and Ser652) are also phosphorylated by glycogen synthase kinase 3. Thr718 and Ser724 are novel sites not known to be phosphorylated by other protein kinases. Glycogen synthase becomes phosphorylated at Ser644 in response to osmotic shock; this phosphorylation is prevented by pretreatment of the cells with SB 203580, which inhibits SAPK2a/p38a and SAPK2b/p38b activity. In vitro, phosphorylation of glycogen synthase by SAPK2b/p38b alone had no significant effect on its activity, indicating that phosphorylation at residue Ser644 itself is insufficient to decrease glycogen synthase activity. However, after phosphorylation by SAPK2b/p38b, subsequent phosphorylation at Ser640 by glycogen synthase kinase 3 decreased the activity of glycogen synthase. This decrease was not observed when SAPK2b/p38b activity was blocked with SB 203580. These results suggest that SAPK2b/p38b may be a priming kinase that allows glycogen synthase kinase 3 to phosphorylate Ser640 and thereby inhibit glycogen synthase activity.