Regular articleApoptosis by Par-4 in cancer and neurodegenerative diseases
Introduction
Physiological cell death in animals, especially in development and in the immune system, occurs by the process of apoptosis. It allows the complete elimination of dying cells without causing an inflammatory response [1], [2]. The key defining features of apoptosis are the activation of a series of cysteine aspartyl proteases or caspases, the central engines of apoptosis that orchestrate cell death by cleaving a variety of intracellular substrates and triggering cell demise. They are synthesized as inactive zymogens and are activated by proteolytic cleavage, typically through the action of upstream caspases. Caspase activation is followed by chromatin condensation and the display of phosphatidylserine on the cell surface that marks the cell for phagocytosis by specialized macrophage or neighboring cells, thus avoiding an inflammatory response [3]. Other typical features of apoptosis include cytoplasmic shrinkage, zeiosis, and the formation of apoptotic bodies with nuclear fragments. The underlying death process is designated apoptosis to delineate it clearly from other death programs such as accidental necrosis, apoptosis-like programmed cell death (PCD), and necrosis-like PCD [4].
Two general pathways are thought to be responsible for activation of the caspase cascades. One such pathway is mediated by transmembrane death receptors of the CD95 (Apo-1 or Fas)/TRAIL/tumor-necrosis factor (TNF) receptor 1 family, whose ligation triggers recruitment and assembly of multiprotein complexes to activate the upstream caspase 8 [5]. The other principal death-signaling pathway involves the mitochondria, which act in response to multiple death insults by releasing cytochrome c into the cytosol. Once released, cytochrome c will induce the assembly of an intracellular apoptosome complex that recruits caspase 9 via the adaptor protein Apaf-1 [6]. Activation of caspase 8 or caspase 9 triggers the activation of effector caspases, such as caspase 3, that are involved in survival substrate degradation and nucleosomal DNA fragmentation [7]. The apoptotic pathways are counteracted by survival signaling pathways, which may act by stabilizing the mitochondrial function and integrity and suppressing release of cytochrome c or by interfering with the assembly of the death receptor complexes and inhibiting upstream caspases [8].
Apoptosis is a critical process that evolved to regulate development and immunity and to protect multicellular organisms from the accumulation of damaged cells. Apoptosis is achieved through complex mechanisms that should be tightly regulated because defects in the suppression of programmed cell death can result in an uncontrolled loss of essential cells, giving rise to diverse diseases like neurodegenerative disorders, AIDS, ischemia, and repercussion injury. On the other hand, accumulation of cells harboring serious mutation or unwanted traits by inhibition of apoptosis leads to cancer and autoimmune diseases [9].
Paradoxically, increased cell proliferation driven by activation of oncoproteins (such as Myc, E1A, and E2F) or inactivation of tumor-suppressor proteins (such as retinoblastoma protein) is often associated with accelerated apoptosis. Thus, the coupling between cell division and cell death is thought to act as a barrier that cells must overcome for cancer initiation and progression. This may be the underlying reason why cancer cells often show a high expression of anti-apoptotic proteins such as Bcl-2, Bcl-xL, survivin, or Bcr-Abl along with inactivation of pro-apoptotic tumor-suppressor proteins p53, p19arf, or PTEN that control apoptosis pathways, generating severe defects in the balance between cell division and programmed cell death in cancer settings. The genetic abnormalities that generate defects in apoptotic pathways allow cancer cells to survive. Interestingly, despite the severe disruption of the classic apoptosis pathways, cancer cells retain at least some molecular components necessary for apoptosis [4].
Various chemical, hormonal, and radiation treatments cause irreparable cellular damage that triggers apoptosis in cancer cells. Consequently, the success of cancer treatment depends not only on its ability to induce irreparable cellular damage but also on the ability to respond to the damage by activation of the apoptotic machinery. Mutations in apoptotic pathways may result in resistance to drugs and radiation. Such mutations can be used to predict resistance to different therapeutic approaches and, consequently, serve as new treatment targets [10]. The challenge is to identify and understand the molecular mechanisms involved in tumor progression and to develop anti-cancer therapies that directly attack key survival mechanisms [8].
It is believed that increased apoptosis in one or more populations of neurons is behind the development of neurodegenerative diseases such as Alzheimer’s, Parkinson’s, Huntington’s, and stroke. Studying apoptosis mechanisms in neurons is just as challenging as in other cell types and organs. But contrary to the goal of dissecting these mechanisms in the development of malignancies, in neurons apoptosis inhibitory pathways are sought. Neurons are long-lived cells that do not undergo active regeneration. While apoptosis is a natural process that is required for normal development of the nervous system, its reactivation later in life is pathological. In the nervous system, different neurological disorders arise from degeneration and death of neurons. Indeed, it is suggested that even when apoptosis is activated in neurons, it is counteracted by responses that slow or reverse this process. Neurotrophic factors have been identified that can protect neurons by activation of survival proteins such as NF-κB [11].
Prostate apoptosis response-4, Par-4, a pro-apoptotic protein, was found to play a critical role in a number of cancer and neurodegenerative disease paradigms. While its pro-apoptotic role in cancer cells should be enhanced, approaches to inhibit Par-4 expression or function must be explored in neurons. This review discusses the identification, characterization, and mechanism of action of Par-4 in various disease paradigms and its potential in molecular therapeutics.
Section snippets
Identification and expression of Par-4
Prostate cancer is conventionally treated by androgen ablation, which shows an initial response in about 80% of cases. Unfortunately, only the androgen-dependent cancer cells are affected by this treatment while androgen-independent cancer cells, which may constitute part of the tumor, are not eliminated, leading to relapse of the disease. Par-4 was first identified in an experiment performed to find common apoptotic genes induced in response to apoptotic insults in androgen-dependent and
Structure–function analysis of Par-4
Rat Par-4 is a 332-amino-acid protein (Fig. 2). It has an apparent molecular weight of about 40 kDa on SDS–PAGE. Sequence analysis of the Par-4 sequence revealed a number of interesting sites and domains. One of the most interesting domains of Par-4 is the leucine zipper domain that spans the region between amino acids 292 and 332 (Fig. 2). The primary sequence of a leucine zipper is a roughly 42-residue stretch having a repeated heptad (A-B-C-D-E-F-G-) with nonpolar residues predominating at
Functional role of Par-4
Although the exact physiological role of endogenous Par-4 protein is not known, several functions are uncovered by the interesting array of molecules that Par-4 affects and/or interacts with. All of the partners of Par-4 identified to date are involved with cell survival, transformation, or apoptosis. Human Par-4 was first identified as a binding partner and inhibitor of WT1 and the aPKC [16], [17]. Par-4 also binds and enhances the apoptotic function of DAP-like kinase (Dlk/Zip kinase) [24] (
Mechanism of apoptosis by Par-4
The pro-apoptotic role of Par-4 is apparent from its effect when overexpressed in different cell lines, its effects in cancer and neurodegenerative disease paradigm, and the interesting array of its partner proteins. Consistently, multiple mechanisms are involved in its ability to induce apoptosis (Fig. 4).
Interaction with WT1 may be involved in the inhibition of growth arrest and inhibition of Bcl-2, which is a potent anti-apoptotic protein [16], [29], [48]. Down-regulation of Bcl-2 allows
Potential for Par-4 in molecular therapeutics
Gene therapy has been used to induce apoptotic programs with various degrees of success. The first approach was directed to restore normal p53 functions in cancer cells. Although the initial results have been interesting, refinement of the vectors and delivery concepts is needed. Phase I clinical and pharmacokinetic studies with Bcl-2 antisense oligonucleotide (which effectively degrades messenger RNA) in patients with non-Hodgkin’s lymphoma was well tolerated. Bcl-2 protein level was reduced
Acknowledgements
This work was supported by NIH research grants CA60872 and CA84511. We thank Sushma Gurumurthy for a critical reading of the review.
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