Abstract
In this study, we investigated the mechanism by which UVB irradiation activates Akt (also known as protein kinase B (PKB)) in mouse epidermal JB6 cells. Treatment with a phosphatidylinositol 3-kinase inhibitor, LY 294002, or expression of a dominant negative mutant of p85 (regulatory component of phosphatidylinositol 3-kinase) inhibited UVB-induced Akt activation. Interestingly, Akt activation by UVB was attenuated by treatment with PD 98059, a specific mitogen-activated protein kinase/extracellular signal-regulated protein kinase (Erk) kinase 1 inhibitor, or SB 202190, a specific p38 kinase inhibitor. Furthermore, the expression of a dominant negative mutant of Erk2 or p38 kinase, but not that of c-Jun N-terminal kinase 1 (JNK1), blocked UVB-induced Akt activation. The expression of a dominant negative mutant of p85 or treatment with LY 294002 also inhibited UVB-induced Erk phosphorylation. The UVB-activated mitogen-activated protein kinase members, which were immunoprecipitated from cells exposed to UVB, did not phosphorylate Akt. Instead, Akt was phosphorylated at both threonine 308 and serine 473 and activated by UVB-activated mitogen- and stress-activated protein kinase 1 (Msk1). The expression of a Msk1 C-terminal kinase-dead mutant inhibited UVB-induced phosphorylation and activation of Akt. These data thus suggested that UVB-induced Akt activation was mediated through Msk1, which is a downstream kinase of the Erk and p38 kinase signaling pathways.
Highlights
Ultraviolet irradiation, especially in the UVB range (290 – 320 nm), accounts for most of the harmful biological effects associated with sunlight, including cancer in mammals and malformations in amphibians [1, 2]
We demonstrated that activation of Akt is induced by UVB irradiation and that the activation of Akt is mediated by extracellular signal-regulated protein kinases (Erks) and p38 kinase through their downstream kinase, mitogen- and stress-activated protein kinase 1 (Msk1), in addition to the phosphatidylinositol 3-kinase (PI3-K)/PDK pathway
Akt Phosphorylation Assay—Cells were treated with UVB (4 kJ/m2), lysates were prepared from the cells, and the immunoprecipitation was carried out using the phospho-specific Erk, p38 kinase, or Jun N-terminal kinases (JNKs) antibody or the specific Msk1, MAPKAP-K2, or Rsk1 antibody as described above
Summary
Ultraviolet irradiation, especially in the UVB range (290 – 320 nm), accounts for most of the harmful biological effects associated with sunlight, including cancer in mammals and malformations in amphibians [1, 2]. The expression of a Msk1 C-terminal kinase-dead mutant inhibited UVB-induced phosphorylation and activation of Akt. These data suggested that UVB-induced Akt activation was mediated through Msk1, which is a downstream kinase of the Erk and p38 kinase signaling pathways. We demonstrated that activation of Akt is induced by UVB irradiation and that the activation of Akt is mediated by Erks and p38 kinase through their downstream kinase, mitogen- and stress-activated protein kinase 1 (Msk1), in addition to the PI3-K/PDK pathway.
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