Khoshnan Ali, Ko Jan, Tescu Simona, Brundin Patrick, Patterson Paul H
Biology Division 216-76, California Institute of Technology, Pasadena, California, USA.
PLoS One. 2009 Jun 2;4(6):e5768. doi: 10.1371/journal.pone.0005768.
Proteolysis of huntingtin (Htt) plays a key role in the pathogenesis of Huntington's disease (HD). However, the environmental cues and signaling pathways that regulate Htt proteolysis are poorly understood. One stimulus may be the DNA damage that accumulates in neurons over time, and the subsequent activation of signaling pathways such as those regulated by IkappaB kinase (IKK), which can influence neurodegeneration in HD.
METHODOLOGY/PRINCIPAL FINDINGS: We asked whether DNA damage induces the proteolysis of Htt and if activation of IKK plays a role. We report that treatment of neurons with the DNA damaging agent etoposide or gamma-irradiation promotes cleavage of wild type (WT) and mutant Htt, generating N-terminal fragments of 80-90 kDa. This event requires IKKbeta and is suppressed by IKKalpha. Elevated levels of IKKalpha, or inhibition of IKKbeta expression by a specific small hairpin RNA (shRNA) or its activity by sodium salicylate, prevents Htt proteolysis and increases neuronal resistance to DNA damage. Moreover, IKKbeta phosphorylates the anti-apoptotic protein Bcl-xL, a modification known to reduce Bcl-xL levels, and activates caspases that can cleave Htt. When IKKbeta expression is blocked, etoposide treatment does not decrease Bcl-xL and activation of caspases is diminished. Similar to silencing of IKKbeta, increasing the level of Bcl-xL in neurons prevents etoposide-induced caspase activation and Htt proteolysis.
CONCLUSIONS/SIGNIFICANCE: These results indicate that DNA damage triggers cleavage of Htt and identify IKKbeta as a prominent regulator. Moreover, IKKbeta-dependent reduction of Bcl-xL is important in this process. Thus, inhibition of IKKbeta may promote neuronal survival in HD as well as other DNA damage-induced neurodegenerative disorders.
亨廷顿蛋白（Htt）的蛋白水解在亨廷顿舞蹈症（HD）的发病机制中起关键作用。然而，调节Htt蛋白水解的环境线索和信号通路却知之甚少。一种刺激因素可能是随着时间推移在神经元中积累的DNA损伤，以及随后诸如由IκB激酶（IKK）调节的信号通路的激活，这可能影响HD中的神经退行性变。
方法/主要发现：我们研究了DNA损伤是否诱导Htt的蛋白水解以及IKK的激活是否起作用。我们报告，用DNA损伤剂依托泊苷或γ射线照射处理神经元可促进野生型（WT）和突变型Htt的切割，产生80 - 90 kDa的N端片段。这一过程需要IKKβ且被IKKα抑制。IKKα水平升高，或通过特异性小发夹RNA（shRNA）抑制IKKβ表达，或用水杨酸钠抑制其活性，均可阻止Htt蛋白水解并增加神经元对DNA损伤的抗性。此外，IKKβ使抗凋亡蛋白Bcl - xL磷酸化，这种修饰已知会降低Bcl - xL水平，并激活可切割Htt的半胱天冬酶。当IKKβ表达被阻断时，依托泊苷处理不会降低Bcl - xL水平且半胱天冬酶的激活减弱。与沉默IKKβ类似，增加神经元中Bcl - xL的水平可阻止依托泊苷诱导的半胱天冬酶激活和Htt蛋白水解。
结论/意义：这些结果表明DNA损伤触发Htt的切割，并确定IKKβ是一个重要的调节因子。此外，IKKβ依赖的Bcl - xL减少在此过程中很重要。因此，抑制IKKβ可能促进HD以及其他DNA损伤诱导的神经退行性疾病中的神经元存活。
