Radiosensitization by targeting a novel MAP3K negative regulator STK38

丝氨酸苏氨酸激酶38(STK38)是蛋白激酶A(PKA)/蛋白G/蛋白C样蛋白(AGC)家族的成员。然而，人们对其功能或调控机制知之甚少。在各种环境胁迫中，STK38被X射线或H_2O_2特异性激活，而磷脂酰肌醇3-激酶抑制剂Wortmannin或AKT抑制剂IV抑制这种激活。STK38也可被组成活性的AKT1或GSK-3β抑制剂VII激活。免疫共沉淀分析表明GSK-3在细胞内与STK38发生物理相互作用。过表达GSK-3β可抑制氧化应激刺激的STK38活性。我们鉴定GSK-3是一种STK38激酶。GSK-3β在体外对S6和T7残基上的STK38进行磷酸化，这在很大程度上依赖于PKA介导的STK38在S10和S11残基上的启动磷酸化。STK38‘S的氧化应激刺激活性可被其起始位点和/或S6和T7的丙氨酸取代所增强，但可被S6或T7的拟磷突变部分降低。STK38的磷酸化突变体的敲除或过表达增强了氧化应激诱导的细胞死亡。综上所述，我们的结果表明，GSK-3通过磷酸化抑制STK38的完全激活，并提示STK38的激活是防止细胞氧化应激死亡所必需的。

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.