Down-regulation of the PI3-kinase/Akt pathway by ERK MAP kinase in growth factor signaling.

The ERK MAP kinase and PI3-kinase/Akt pathways are major intracellular signaling modules, which are known to regulate diverse cellular processes including cell proliferation, survival and malignant transformation. However, it has not been fully understood how these two pathways interact with each other. Here, we demonstrate that inhibition of the ERK pathway by the MEK inhibitor U0126 or PD98059 significantly potentiates EGF- and FGF-induced Akt phosphorylation at both Thr308 and Ser473. We also show that hyperactivation of the ERK pathway greatly attenuates EGF- and FGF-induced Akt phosphorylation. Furthermore, the enhanced Akt phosphorylation induced by U0126 is inhibited by the PI3-kinase inhibitor LY294002, and is accompanied by the up-regulation of Ras activity. These results suggest that the ERK pathway inhibition enhances Akt phosphorylation through the Ras/PI3-kinase pathway. Thus, our results demonstrate that the ERK pathway negatively modulates the PI3-kinase/Akt pathway in response to growth factor stimulation.

Centaurin-alpha1 is a phosphatidylinositol 3-kinase-dependent activator of ERK1/2 mitogen-activated protein kinases.

Centaurin-alpha1 is known to be a phosphatidylinositol 3,4,5-triphosphate (PIP3)-binding protein that has two pleckstrin homology domains and a putative ADP ribosylation factor GTPase-activating protein domain. However, the physiological function of centaurin-alpha1 is still not understood. Here we have shown that transient expression of centaurin-alpha1 in COS-7 cells results in specific activation of ERK, and the activation is inhibited by co-expression of a dominant negative form of Ras. We have also found that a mutant form of centaurin-alpha1 that is unable to bind PIP3 fails to induce ERK activation and that a phosphatidylinositol 3-kinase inhibitor LY294002 inhibits centaurin-alpha1-dependent ERK activation. Furthermore, transient knockdown of centaurin-alpha1 by small interfering RNAs results in reduced ERK activation after epidermal growth factor stimulation in T-REx 293 cells. These results suggest that centaurin-alpha1 contributes to ERK activation in growth factor signaling, linking the PI3K pathway to the ERK mitogen-activated protein kinase pathway through its ability to interact with PIP3.

AKRL1 and AKRL2 activate the JNK pathway.

BACKGROUND: c-Jun N-terminal kinase (JNK), a member of the mitogen-activated protein kinase (MAPK) family, is activated by specific cytokines and various environmental stresses. MKK4 and MKK7 are shown to be direct activators of JNK. Although several upstream components of the JNK pathway, including members of the MAPKKK family have been described, the components lying between the receptors or sensors and JNK have not been fully characterized. RESULTS: We have identified AKRL1 and AKRL2 (Akr1p-like 1 and 2) as novel activators of the JNK pathway. AKRL1 and AKRL2 proteins have a considerable sequence similarity to Akr1p, a protein essential for endocytosis in Saccharomyces cerevisiae. Expression of AKRL1 or AKRL2 activates JNK and its activators MKK4 and MKK7. This AKRL1/2-induced JNK activation is significantly suppressed by the expression of a kinase-negative mutant of TAK1, a member of the MAPKKK family. AKRL1 and AKRL2 localize to the Golgi. Both the N-terminal half and the C-terminal transmembrane domain of AKRL1/2 are required for the JNK activation. The C-terminal transmembrane domain of AKRL1/2 is required for localization to the Golgi. CONCLUSION: AKRL1 and AKRL2 are localized to Golgi and the novel activators of the JNK pathway.