Radiosensitization by targeting a novel MAP3K negative regulator STK38

丝氨酸-苏氨酸激酶38（STK 38）是蛋白激酶A（PKA）/PKG/PKC样家族（AGC）的成员。然而，对其功能或调节机制知之甚少。在各种环境胁迫中，STK 38被X射线或H_2O_2特异性激活，磷脂酰肌醇3-激酶抑制剂wortmannin或AKT抑制剂IV抑制这种激活。STK 38也被组成型活性AKT 1或GSK-3 β抑制剂VII激活。免疫共沉淀分析显示GSK-3在细胞中与STK 38发生物理相互作用。GSK-3 β过表达抑制氧化应激刺激的STK 38活性。我们将GSK-3鉴定为STK 38激酶。GSK-3 β在体外使STK 38在残基S6和T7上磷酸化，这在很大程度上取决于PKA介导的STK 38分别在残基S10和S11上的引发磷酸化。STK 38的氧化应激刺激活性通过在其引发位点和/或在S6和T7处的丙氨酸取代而增强，但通过在S6或T7处的拟磷酸化突变而部分降低。敲低或过表达STK 38的拟磷酸化突变体增强氧化应激诱导的细胞死亡。总之，我们的结果表明GSK-3通过磷酸化抑制STK 38的完全激活，并表明STK 38的激活是防止细胞因氧化应激而死亡所必需的。

Serine-threonine kinase 38 (STK38) is a member of the protein kinase A (PKA)/PKG/PKC-like family (AGC). However, little is known about its functions or regulatory mechanisms. Among various environmental stresses, STK38 was specifically activated by X-irradiation or H_2O_2, and the phosphatidylinositol 3-kinase inhibitor wortmannin or AKT inhibitor IV suppressed this activation. STK38 was also activated by a constitutively active AKT1 or by GSK-3beta inhibitor VII. Co-immunoprecipitation analysis revealed that GSK-3 physically interacted with STK38 in cells. Overexpression of GSK-3beta inhibited the oxidative stress-stimulated STK38 activity. We identified GSK-3 as an STK38 kinase. GSK-3beta phosphorylated STK38 on residues S6 and T7 in vitro, largely depending on a PKA-mediated priming phosphorylation of STK38 on residues S10 and S11, respectively. STK38's oxidative stress-stimulated activity was enhanced by alanine substitution at its priming sites and/or at S6 and T7, but was partially reduced by a phosphomimetic mutation at S6 or T7. Knockdown or overexpression of the phosphomimetic mutant of STK38 enhanced oxidative stress-induced cell death. Taken together, our results indicate that that GSK-3 inhibits STK38 full activation through phosphorylation, and suggest that the activation of STK38 is required for preventing cell death against oxidative stress.