The role of heat shock proteins in gastrointestinal diseases

Abstract

Heat shock proteins (HSPs) are a highly conserved family of proteins which inhabit almost all subcellular locations and cellular membranes. Depending on their location, these proteins perform a variety of chaperoning functions including folding of newly synthesised polypeptides. HSPs also play a major role in the protection of cells against stressful and injury-inciting stimuli. By virtue of this protective function, HSPs have been shown to prevent acinar cell injury in acute pancreatitis. Also, the levels of HSPs have been shown to be markedly elevated in various forms of cancers when compared with non-transformed cells. Further, inhibition of HSPs has been shown to induce apoptotic cell death in cancer cells suggesting that inhibition of HSPs has a potential to emerge as novel anti-cancer therapy, either as monotherapy or in combination with other chemotherapeutic agents. Several studies have suggested that HSPs can interact with and inhibit both intrinsic and extrinsic pathways of apoptosis at multiple sites. Besides the anti-apoptotic role of HSPs, recent studies suggest that they play a role in the generation of anti-cancer immunity, and attempts have been made to utilise this property of HSPs in the generation of anti-cancer vaccines. The anti-apoptotic function and mechanism of various subtypes of HSPs as well as the current status of anti-HSP therapy are discussed in this review.

Figure 1

Heat shock factor-1 (HSF1) knock-out mice have increased severity of pancreatitis. In the absence of caerulein administration, HSF1 knock-out mice (B) demonstrate similar pancreatic morphology as wild-type mice (A). HSF1 knock-out mice (D) demonstrate increased severity of pancreatitis in response to caerulein administration as compared to HSF1 wild-type mice (C).

Figure 2

Heat shock protein 60 (HSP60) attenuates cytosolic calcium response. Prior water immersion stress-induced HSP60 induction attenuates the cytosolic calcium signal in response to caerulein administration. Cytosolic calcium levels were measured by Fura-2 AM by using spectrophotometry.

Figure 3

A schematic diagram of the possible mechanism by which heat shock protein 70 (HSP70) protects against acinar cell injury in pancreatitis. The events that initiate pancreatitis also increase cytosolic calcium in acinar cells, which in turn causes lysosomes and digestive enzyme zymogen to co-localise. These events bring the inactive trypsinogen into contact with cathepsin B, which then activates trypsinogen. Active trypsin in turn activates other digestive enzyme zymogens, and those active enzymes cause acinar cell injury. HSP70 possibly protects the acinar cell from injury in pancreatitis by attenuating cytosolic calcium increase and thus abrogating all the downstream events in the injury pathway.

Figure 4

Heat shock protein 70 (HSP70) is over-expressed in pancreatic cancer as compared with non-transformed cells. (A) HSP70 is markedly over-expressed in four human pancreatic cancer cell lines when compared with non-transformed pancreatic ductal cells. (B) HSP70 mRNA levels are markedly increased in human pancreatic cancer specimen when compared with normal tissue margins (normal). *p<0.002, n = 7.

Figure 5

Inhibition of heat shock protein 70 (HSP70) expression is highly effective as a therapeutic strategy in animal models of pancreatic cancer. (A) Inhibition of HSP70 expression by triptolide markedly reduces the growth of pancreatic tumours in an orthotopic model of pancreatic cancer in nude mice. Triptolide was administered at a dose of 0.2 mg/kg/day for 60 days. Animals were killed at the 60th day and the tumour volumes in the triptolide group were compared with the control group (treated with vehicle alone). n = 8 in each group, *p<0.05. (B) Representative picture of tumour tissue in the control and triptolide treatment group. (C) Graph demonstrating reduced loco-regional spread in the triptolide treatment group. (p<0.001, n = 8, χ² analysis).

Figure 6

Inhibition of heat shock protein 70 (HSP70) expression by HSP70 siRNA induces caspase-dependent apoptotic cell death in pancreatic cancer cells. (A) Inhibition of HSP70 expression by HSP70 siRNA leads to decreased viability in the MiaPaCa-2 pancreatic cancer cell line at 72 h. Viability was measured by the MTT assay. Two different sequences of HSP70 siRNA were used to rule out any off-target effects of siRNA. n = 3, *p<0.05. (B) Inhibition of HSP70 expression by two different sequences leads to caspase 3 activation. n = 3, *p<0.05. MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide.

Figure 7

A schematic diagram of the possible sites where heat shock protein 70 (HSP70) inhibits intrinsic pathway of apoptosis. It is believed that mitochondrial permeabilisation and subsequent cytochrome c release, along with other apoptosis-inducing factors, constitute central steps in apoptotic cell death. Multiple mechanisms of this mitochondrial permeabilisation have been proposed, including Bax, increased cytosolic calcium, lysosomal enzymes, and cJun N-terminal kinase (JNK). Once cytochrome c is released from the mitochondria it interacts with apoptosis protease-activating factor-1 (APAF-1) and pro-caspase 9 in the cytosol leading to formation of caspase 9 which then activates the effector caspases. These events result ultimately in apoptosis. HSP70 has been shown to inhibit apoptosis by participating in events both before and after mitochondrial membrane permeabilisation, as depicted in the figure.