HSP27: Function & Regulation

Characteristically, Hsp27 is ubiquitously expressed and involved in the regulation of main cellular physiologic functions⁵⁸ as explained below and these are: a) inhibition of apoptosis, b) protection against oxidative stress, c) binding erroneously folded proteins for transfer to ATP-dependent chaperones and to the proteasome for further degradation⁵⁸ and d) the regulation of the cytoskeleton. Moreover, it has been implicated in promotion, generation and/or cytoprotection in few pathologies.

a) Inhibition of Apoptosis
Apoptosis signaling is one of the main regulatory functions of Hsp27. Apoptosis comprises two pathways, intrinsic and extrinsic (Figure 2).

The intrinsic pathway is based on the cytochrome c-dependent activation of caspases, special proteases which are involved in cleavage and processing of several proteins⁵⁹. Different signals (such as γ-irradiation, oxidative stress, or antineoplastic agents) lead to liberation of cytochrome c from mitochondria. In the cytosol, cytochrome c interacts with apoptotic protease activating factor-1 (Apaf-1), procaspase-9, and dATP forming the so-called apoptosome. Formation of the apoptosome leads to activation of procaspase-9, which in turn cleaves procaspase-3, the main caspase effector during apoptosis, (Figure 2)⁵⁸.

It is supposed that Hsp27 can affect different stages of this process. First, Hsp27 seems to be able to prevent the release of cytochrome c from mitochondria⁶⁰. As mentioned earlier, indirect data indicate that Hsp27 interacts with and modulates activity of many protein kinases, and among them phosphatidylinositol 3-kinase (PI3-K). This protein kinase activates protein kinase Akt, phosphorylating Bax which prevents pore formation in the outer mitochondria membrane and thus prevents liberation of cytochrome c from mitochondria⁶¹. In addition, Hsp27 seems to inhibit the apoptosis-signal regulated kinase 1 (Ask1)-JNK pathway which through Bax leads to cytochrome c leakage⁶². Second, the Hsp27 directly interacts with cytochrome c leaving the mitochondria and by this means prevents apoptosome formation¹¹. It is still controversial whether Hsp27 affects both routes of caspase-3 activation or interacting with procaspase-3 inhibits its activation without affecting apoptosome formation and activation of caspase-9⁶². Activity of caspases is regulated by inhibitor of apoptosis proteins (IAPs), which in turn are inhibited by the so-called second mitochondria- derived activator of caspases (Smac/Diablo). Hsp27 seems to be able to inhibit the release of Smac/Diablo from mitochondria and by this means can also induce inhibition of caspases, preventing apoptosis⁶³.

The extrinsic pathway of apoptosis is dependent on special membrane receptors. For instance, Fas receptor plays an important role in induction of apoptosis activating two different signal pathways. On one hand, binding of special ligand (FasL) promotes interaction of Fas receptor with Fas associated-death-domain (FADD) protein which activates procaspase-8 and downstream caspases (caspases 3, 6, and 7)^64,65 and modulates interaction of Bax with the mitochondrial membrane⁶⁶. On the other hand, binding of Fas-ligand promotes interaction of Fas receptor with another protein, called Daxx. Daxx-dependent induction of apoptosis relies on translocation of Daxx from the nucleus to the cytosol, and its interaction with Fas receptor is probably followed by activation of apoptosis signal related kinase 1 (Ask1). Upon signal-induced phosphorylation, Hsp27 migrates from cytosol to nucleus, i.e., in the direction that is opposite to the direction of DAXX movement. Phosphorylated Hsp27 is supposed to interact with Daxx, preventing its translocation to the cytosol, and thus inhibits binding of Daxx to Fas receptor and Daxx-dependent apoptosis⁶⁵. Thus, Hsp27 protects against apoptosis by the interaction with components of the death cell pathway.

b) Protection against Oxidative Stress
During oxidative stress, Hsp27 functions as an antioxidant, lowering the levels of reactive oxygen species (ROS) by raising the levels of intracellular glutathione and lowering the levels of intracellular iron and by presenting oxidized proteins to the proteasome for processing and degradation⁵.

c) Chaperonin Activity
Functions as chaperone by binding misfolded proteins for transfer to ATP-dependent chaperones and to the proteasome for further degradation in the proteasome⁵⁸ and facilitates the refolding of partially denatured proteins into active conformations.^67,29

d) Regulation of the Cytoskeleton
Hsp27 has been characterized with the ability to regulate actin cytoskeletal dynamics during heat shock and other stress conditions, functioning to promote actin polymerization therefore, another role of Hsp27 is to regulate the conformation of cytoskeleton; It is thought that transiently formed small Hsp27 oligomers somehow prevent F-actin disruption and interact with special protein targets and F-actin modulation, inhibiting F-actin polymerization^68,69.