Cancer-selective apoptotic effects of extracellular and intracellular Par-4

Par-4 as a potential target for cancer therapy

INTRODUCTION

Despite extensive research, cancer continues to be a leading cause of death worldwide and is expected to continue to rise as a result of an aging population. Therefore, new therapies are constantly being developed. Par-4 is a naturally occurring tumor suppressor protein that is capable of inducing apoptosis in cancer, but not normal cells. For this reason, Par-4 offers an attractive target for development of cancer therapy, particularly of difficult to treat cancers.

AREAS COVERED

The mechanisms by which Par-4 induces cell death are summarized. The ways that Par-4 is controlled in cancer cells are discussed. We discuss how different research groups have developed ways to overexpress and/or activate Par-4 in vitro and in vivo. The studies described demonstrate that when Par-4 levels and/or activity are increased, susceptibility to apoptosis is enhanced and tumor growth is inhibited.

EXPERT OPINION

Par-4 is a promising therapeutic protein that can be overexpressed and/or activated to induce apoptosis in a cancer-selective manner. This cancer selectivity is important given that the side-effects of chemotherapeutics can be as debilitating as cancer itself. However, there are key issues that need to be addressed to optimize the effects of Par-4 in patients.

Dermokine-β impairs ERK signaling through direct binding to GRP78

Dermokine-β is abundant in stratified epithelia and in differentiating cultured keratinocytes. In this study, we investigated the role of dermokine-β in differentiation of keratinocytes. Treatment of keratinocytes or skin tumor cells with dermokine-β attenuated phosphorylation of extracellular-signal-regulated kinase (ERK). Exposure of cells to dermokine-β, as well as its carboxyl-terminus domain peptide, interrupted phosphorylation of ERK and stimulated dermokine gene expression. Inhibition of ERK signaling by its specific inhibitor also increased dermokine expression level. A combination of chemical cross-linking and immunoprecipitation, followed by proteomics analyses, identified glucose-regulated protein 78 (GRP78) as a dermokine-β-associated protein. Blockage of GRP78 expression by a specific siRNA abrogated actions of dermokine-β. These findings provide novel insights into the physiological significance of dermokine-β in the epidermis.

Using mice to unveil the genetics of cancer resistance

In the UK, four in ten people will develop some form of cancer during their lifetime, with an individual's relative risk depending on many factors, including age, lifestyle and genetic make-up. Much research has gone into identifying the genes that are mutated in tumorigenesis with the overwhelming majority of genetically-modified (GM) mice in cancer research showing accelerated tumorigenesis or recapitulating key aspects of the tumorigenic process. Yet if six out of ten people will not develop some form of cancer during their lifetime, together with the fact that some cancer patients experience spontaneous regression/remission, it suggests there are ways of 'resisting' cancer. Indeed, there are wildtype, spontaneously-arising mutants and GM mice that show some form of 'resistance' to cancer. Identification of mice with increased resistance to cancer is a novel aspect of cancer research that is important in terms of providing both chemopreventative and therapeutic options. In this review we describe the different mouse lines that display a 'cancer resistance' phenotype and discuss the molecular basis of their resistance.

Farnesoid X receptor protects hepatocytes from injury by repressing miR-199a-3p, which increases levels of LKB1

BACKGROUND & AIMS

Hepatocyte injury occurs during liver fibrogenesis. MicroRNAs (miRNA) regulate some of these processes, and some are regulated by the farnesoid X receptor (FXR). We investigated the effect of repression of specific miRNAs by FXR in hepatocyte injury using fibrotic liver tissue from patients and hepatocytes.

METHODS

We used immunohistochemistry or real-time polymerase chain reaction to analyze proteins and miRNAs in human and mouse liver samples. HepG2 cells were transfected with pre-miRNA, antisense oligonucleotides, small interfering RNAs, the 3'-untranslated region of liver kinase B1 (LKB1) (STK11), or constructs for overexpression, and analyzed.

RESULTS

Liver tissue from patients with severe fibrosis had lower levels of FXR and greater amounts of hepatocyte death than samples from patients with mild disease. Levels of several miRNAs changed when FXR expression was disrupted in the liver; one of these, miR-199a-3p, was significantly up-regulated in patients with severe fibrosis. Activation of FXR by its ligand reduced the level of miR-199a-3p in HepG2 cells. LKB1 messenger RNA was identified as a target of miR-199a-3p, and its expression was reduced in human fibrotic liver tissue. Overexpression of FXR or incubation of cultured hepatocytes with the FXR ligand up-regulated LKB1; LKB1 was not induced in cells transfected with miR-199a-3p. Incubation of HepG2 cells with FXR ligand, or injection of the ligand into mice, protected hepatocytes from injury and increased levels of LKB1; levels of miR-199a-3p were reduced compared with cells that were not incubated with the FXR ligand. Activation of FXR reduced mitochondrial dysfunction and oxidative stress and increased hepatocyte survival.

CONCLUSIONS

In hepatocytes, FXR represses production of miR-199a-3p. In fibrotic livers of humans and mice, FXR expression is reduced, increasing levels of miR-199a-3p, which reduces levels of LKB1. FXR therefore protects hepatocytes from injury by repressing miR-199a-3p and thereby increasing levels of LKB1.

Prostate apoptosis response 4 (Par-4), a novel substrate of caspase-3 during apoptosis activation

Prostate apoptosis response 4 (Par-4) is a ubiquitously expressed proapoptotic tumor suppressor protein. Here, we show for the first time, that Par-4 is a novel substrate of caspase-3 during apoptosis. We found that Par-4 is cleaved during cisplatin-induced apoptosis in human normal and cancer cell lines. Par-4 cleavage generates a C-terminal fragment of ~25 kDa, and the cleavage of Par-4 is completely inhibited by a caspase-3 inhibitor, suggesting that caspase-3 is directly involved in the cleavage of Par-4. Caspase-3-deficient MCF-7 cells do not show Par-4 cleavage in response to cisplatin treatment, and restoration of caspase-3 in MCF-7 cells produces a decrease in Par-4 levels, with the appearance of a cleaved fragment. Additionally, knockdown of Par-4 reduces caspase-3 activation and apoptosis induction. Site-directed mutagenesis reveals that Par-4 cleavage by caspase-3 occurs at an unconventional site, EEPD(131)↓G. Interestingly, overexpression of wild-type Par-4 but not the Par-4 D131A mutant sensitizes cells to cisplatin-induced apoptosis. Upon caspase-3 cleavage, the cleaved fragment of Par-4 accumulates in the nucleus and displays increased apoptotic activity. Overexpression of the cleaved fragment of Par-4 inhibits IκBα phosphorylation and blocks NF-κB nuclear translocation. We have identified a novel specific caspase-3 cleavage site in Par-4, and the cleaved fragment of Par-4 retains proapoptotic activity.

Systemic Par-4 inhibits non-autochthonous tumor growth

The tumor suppressor protein Par-4 (Prostate apoptosis response-4) is spontaneously secreted by normal and cancer cells. Extracellular Par-4 induces caspase-dependent apoptosis in cancer cell cultures by binding, via its effector SAC domain, to cell surface GRP78 receptor. However, the functional significance of extracellular Par-4/SAC has not been validated in animal models. We show that Par-4/SAC-transgenic mice express systemic Par-4/SAC protein and are resistant to the growth of non-autochthonous tumors. Consistently, secretory Par-4/SAC pro-apoptotic activity can be transferred from these cancer-resistant transgenic mice to cancer-susceptible mice by bone marrow transplantation. Moreover, intravenous injection of recombinant Par-4 or SAC protein inhibits metastasis of cancer cells. Collectively, our findings indicate that extracellular Par-4/SAC is systemically functional in inhibition of tumor growth and metastasis progression, and may merit investigation as a therapy.

Beyond the endoplasmic reticulum: atypical GRP78 in cell viability, signalling and therapeutic targeting

GRP78 (glucose-regulated protein of 78 kDa) is traditionally regarded as a major ER (endoplasmic reticulum) chaperone facilitating protein folding and assembly, protein quality control, Ca(2+) binding and regulating ER stress signalling. It is a potent anti-apoptotic protein and plays a critical role in tumour cell survival, tumour progression and angiogenesis, metastasis and resistance to therapy. Recent evidence shows that GRP78 can also exist outside the ER. The finding that GRP78 is present on the surface of cancer but not normal cells in vivo represents a paradigm shift on how GRP78 controls cell homoeostasis and provides an opportunity for cancer-specific targeting. Cell-surface GRP78 has emerged as an important regulator of tumour cell signalling and viability as it forms complexes with a rapidly expanding repertoire of cell-surface protein partners, regulating proliferation, PI3K (phosphoinositide 3-kinase)/Akt signalling and cell viability. Evidence is also emerging that GRP78 serves as a receptor for viral entry into host cells. Additionally, a novel cytosolic form of GRP78 has been discovered prominently in leukaemia cells. These, coupled with reports of nucleus- and mitochondria-localized forms of GRP78, point to the previously unanticipated role of GRP78 beyond the ER that may be critical for cell viability and therapeutic targeting.

Emerging Bcl-2 inhibitors for the treatment of cancer

INTRODUCTION

Bcl-2 family proteins are a component of the antiapoptotic machinery and are overexpressed in different malignancies. Accordingly, their enhanced expression has been attributed to the observed chemoresistance in most of the cancers. Therefore, targeting Bcl-2 family members becomes an important and attractive approach towards cancer therapy and is currently a very rapidly evolving area of research. This article highlights the numerous advancements that have been made in the design and synthesis of small molecule inhibitors (SMI) of pro-survival Bcl-2 proteins.

AREAS COVERED

This review comprehensively describes the progress made over the last 2 decades on this subject, including the clinical status of SMIs of Bcl-2 family proteins. Newer insights on the status of our knowledge on SMIs of Bcl-2 family proteins, their most beneficial application as well as current and future directions in this field are discussed.

EXPERT OPINION

Targeting Bcl-2 family proteins using SMI strategies is gaining momentum, with the emergence of certain new classes of inhibitors in Phase I and II clinical settings. In view of the tremendous progress toward the development of such inhibitors, this innovative approach certainly holds promise and has the potential to become a future mainstay for cancer therapy. The stage is set for the next generation of SMIs, for not only Bcl-2 proteins but also for Mcl-1. Other emerging molecules in the apoptotic machinery will also be explored and targeted.

GRP78 signaling hub a receptor for targeted tumor therapy

Glucose-regulated protein 78 (GRP78) is a potential receptor for targeting therapy in cancer and chronic vascular disease due to its overexpression at the cell surface in tumor cells and in atherosclerotic lesions. Presence of the GRP78 autoantibody in cancer patient sera is generally associated with poor prognosis since it signals a prosurvival mechanism in response to cellular stress. Association of GRP78 with various binding partners involves coordination of multiple signaling pathways that result in either cell survival or cell death. Binding of activated alpha2-macroglobulin to cell-surface GRP78 activates Akt to suppress apoptotic pathways through multiple downstream effectors, and concomitantly upregulates NF-kappaBeta and induces the unfolded protein response (UPR) so that cell proliferation prevails. Interaction of GRP78 with cell-surface T-cadherin promotes endothelial cell survival. Association of oncogenic Cripto with GRP78 nullifies TGF-beta superfamily-dependent signaling through Smad2/3 to promote cell proliferation. In contrast, association of GRP78 with the plasminogen kringle 5 domain or extracellular Par-4 promotes apoptosis. Interaction of GRP78 with microplasminogen induces the UPR while association with tissue factor inhibits procoagulant activity. The diverse and multiple binding proteins of GRP78 and their equally diverse functional outcomes reflect the regulatory cellular functions that GRP78 orchestrates. Several GRP78 targeting peptides have been isolated from different tumors and they show remarkable tumor specificity. Conjugation of GRP78-targeting peptides to an apoptosis-inducing peptide suppresses tumor growth in tumor xenografts, thereby demonstrating that GRP78 is a viable target by which clinical cancer therapies can be successfully developed as well as its potential utility in treating vascular disease.