Hsp27 as a Negative Regulator of Cytochrome c Release

Abstract

We previously showed that Hsp27 protects against apoptosis through its interaction with cytosolic cytochrome c. We have revisited this protective activity in murine cell lines expressing different levels of Hsp27. We report that Hsp27 also interferes, in a manner dependent on level of expression, with the release of cytochrome c from mitochondria. Moreover, a decreased level of endogenous Hsp27, which sensitized HeLa cells to apoptosis, reduced the delay required for cytochrome c release and procaspase 3 activation. The molecular mechanism regulating this function of Hsp27 is unknown. In our cell systems, Hsp27 is mainly cytosolic and only a small fraction of this protein colocalized with mitochondria. Moreover, we show that only a very small fraction of cytochrome c interacts with Hsp27, hence excluding a role of this interaction in the retention of cytochrome c in mitochondria. We also report that Bid intracellular relocalization was altered by changes in Hsp27 level of expression, suggesting that Hsp27 interferes with apoptotic signals upstream of mitochondria. We therefore investigated if the ability of Hsp27 to act as an expression-dependent modulator of F-actin microfilaments integrity was linked to the retention of cytochrome c in mitochondria. We show here that the F-actin depolymerizing agent cytochalasin D rapidly induced the release of cytochrome c from mitochondria and caspase activation. This phenomenon was delayed in cells pretreated with the F-actin stabilizer phalloidin and in cells expressing a high level of Hsp27. This suggests the existence of an apoptotic signaling pathway linking cytoskeleton damages to mitochondria. This pathway, which induces Bid intracellular redistribution, is negatively regulated by the ability of Hsp27 to protect F-actin network integrity. However, this upstream pathway is probably not the only one to be regulated by Hsp27 since, in staurosporine-treated cells, phalloidin only partially inhibited cytochrome c release and caspase activation. Moreover, in etoposide-treated cells, Hsp27 still delayed the release of cytochrome c from mitochondria and Bid intracellular redistribution in conditions where F-actin was not altered.

Inhibition of Neutrophil Apoptosis by TLR Agonists in Whole Blood: Involvement of the Phosphoinositide 3-Kinase/Akt and NF-κB Signaling Pathways, Leading to Increased Levels of Mcl-1, A1, and Phosphorylated Bad

“…It has recently been shown that Hsp27 phosphorylation by Akt leads to disruption of the Akt-Hsp27 interaction, and it has been suggested that released Hsp27 may promote independent survival signals (31). It has also been postulated that Hsp27 inhibits apoptosis through inactivation of caspase-3, caspase-9, and inhibition of cytochrome c release (49,50).…”

Human αA- and αB-crystallins bind to Bax and Bcl-XS to sequester their translocation during staurosporine-induced apoptosis

“…Hsp27 prevents apoptosis by interacting with caspase-3 to modulate its activity, [47][48] by interacting with cytochrome c to prevent procaspase-9 activation, 49 or by regulating Bid intracellular distribution and F actin integrity to block mitochondrial death pathway. 52 In contrast, a-crystallin utilizes a different mechanism to negatively regulate caspase-3 activity. Our recent study shows that aB-crystallin prevents H 2 O 2 -induced apoptosis through interaction with procaspase-3 and partially processed procaspase-3 to prevent caspase-3 activation.…”

“…Its proapoptotic ability can be inhibited by Bcl-2 and Bcl-X L . 38 Another important group of apoptosis regulators are heatshock proteins that include alpha-crystallins (a-crystallins), [39][40][41][42][43][44][45][46] Hsp27, [39][40][47][48][49][50][51][52] Hsp70, [53][54][55][56][57] Hsp90 58 and Hsp60. [59][60] While Hsp60 appears to enhance apoptosis by promoting maturation of procaspase-3, 59,60 the majority of these factors seems to protect cells from induced apoptosis through different mechanisms.…”

“…49 Finally, Hsp27 can abrogate apoptotic pathway through regulation of Bid intracellular distribution and protection of F-actin integrity. 52 Both a-crystallins and Hsp27 are closely related family members. [19][20]61 aA/B-crystallins are initially known as major lens structural proteins that play an essential role in maintaining the transparency of the ocular lens.…”

Overexpression of Heat Shock Protein 27 Reduces Cortical Damage after Cerebral Ischemia

“…The well-documented antiapoptotic properties of HSP27 exhibited in cell culture (Garrido et al, 1999;Bruey et al, 2000;Paul et al, 2002) have also been shown in vivo in the kainate seizure model, in which HSP27 attenuates caspase 3 induction and reduces apoptotic cell death (Akbar et al, 2003). The formation of HSP27 oligomers necessary for reducing oxidative damage in cellular models has also been shown in cardiac ischemia (Hollander et al, 2004).…”