[Small heat shock proteins participate in the regulation of cellular aggregates of misfolded protein]

HSP27 favors ubiquitination and proteasomal degradation of p27 Kip1 and helps S‐phase re‐entry in stressed cells

Stress-inducible HSP27 protects cells from death through various mechanisms. We have recently demonstrated that HSP27 can also enhance the degradation of some proteins through the proteasomal pathway. Here, we show that one of these proteins is the cyclin-dependent kinase (Cdk) inhibitor p27Kip1. The ubiquitination and degradation of this protein that favors progression through the cell cycle was previously shown to involve either a Skp2-dependent mechanism,i.e., at the S-/G2-transition, or a KPC (Kip1 ubiquitination-promoting complex)-dependent mechanism, i.e.,at the G0/G1 transition. In this work, we demonstrate that, in response to serum depletion, p27Kip1 cellular content first increases then progressively decreases as cells begin to die. In this stressful condition, HSP27favors p27Kip1 ubiquitination and degradation by the proteasome. A similar observation was made in response to stress induced by the NO donor glyceryl trinitrate (GTN). HSP27-mediated ubiquitination ofp27Kip1 does not require its phosphorylation on Thr187 or Ser-10, nor does it depend on the SCFSkp2 ubiquitin ligase E3 complex. It facilitates the G1/S transition,which suggests that, in stressful conditions, HSP27might render quiescent cells competent to re-enter the cell cycle.

Redox proteomics analysis of oxidatively modified proteins in G93A-SOD1 transgenic mice--a model of familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron degenerative disease characterized by the loss of neuronal function in the motor cortex, brain stem, and spinal cord. Familial ALS cases, accounting for 10-15% of all ALS disease, are caused by a gain-of-function mutation in Cu,Zn-superoxide dismutase (SOD1). Two hypotheses have been proposed to explain the toxic gain of function of mutant SOD (mSOD). One is that mSOD can directly promote reactive oxygen species and reactive nitrogen species generation, whereas the other hypothesis suggests that mSODs are prone to aggregation due to instability or association with other proteins. However, the hypotheses of oxidative stress and protein aggregation are not mutually exclusive. G93A-SOD1 transgenic mice show significantly increased protein carbonyl levels in their spinal cord from 2 to 4 months and eventually develop ALS-like motor neuron disease and die within 5-6 months. Here, we used a parallel proteomics approach to investigate the effect of the G93A-SOD1 mutation on protein oxidation in the spinal cord of G93A-SOD1 transgenic mice. Four proteins in the spinal cord of G93A-SOD1 transgenic mice have higher specific carbonyl levels compared to those of non-transgenic mice. These proteins are SOD1, translationally controlled tumor protein (TCTP), ubiquitin carboxyl-terminal hydrolase-L1 (UCH-L1), and, possibly, alphaB-crystallin. Because oxidative modification can lead to structural alteration and activity decline, our current study suggests that oxidative modification of UCH-L1, TCTP, SOD1, and possibly alphaB-crystallin may play an important role in the neurodegeneration of ALS.