Inactivation of farnesyltransferase and geranylgeranyltransferase I by caspase-3: cleavage of the common alpha subunit during apoptosis

Regulation of protein prenylation

Prenyltransferases (PTases) are known to play a role in embryonic development, normal tissue homeostasis and cancer by posttranslationally modifying proteins involved in these processes. They are being discussed as potential drug targets in an increasing number of diseases, ranging from Alzheimer's disease to malaria. Protein prenylation and the development of specific PTase inhibitors (PTIs) have been subject to intense research in recent decades. Recently, the FDA approved lonafarnib, a specific farnesyltransferase inhibitor that acts directly on protein prenylation; and bempedoic acid, an ATP citrate lyase inhibitor that might alter intracellular isoprenoid composition, the relative concentrations of which can exert a decisive influence on protein prenylation. Both drugs represent the first approved agent in their respective substance class. Furthermore, an overwhelming number of processes and proteins that regulate protein prenylation have been identified over the years, many of which have been proposed as molecular targets for pharmacotherapy in their own right. However, certain aspects of protein prenylation, such as the regulation of PTase gene expression or the modulation of PTase activity by phosphorylation, have attracted less attention, despite their reported influence on tumor cell proliferation. Here, we want to summarize the advances regarding our understanding of the regulation of protein prenylation and the potential implications for drug development. Additionally, we want to suggest new lines of investigation that encompass the search for regulatory elements for PTases, especially at the genetic and epigenetic levels.

Scaffold-based analysis of nonpeptide oncogenic FTase inhibitors using multiple similarity matching, binding affinity scoring and enzyme inhibition assay

Oncogenic protein farnesyltransferase (FTase) is a key enzyme responsible for the lipid modification of a large and important number of proteins including Ras, which has been recognized as a druggable target of diverse cancers. Here, we report a systematic scaffold-based analysis to investigate the affinity, selectivity and cross-reactivity of nonpeptide inhibitors across ontology-enriched, disease-associated FTase mutants, by integrating multiple similarity matching, binding affinity scoring and enzyme inhibition assay. It is revealed that nonpeptide inhibitors are generally insensitive to FTase mutations; many of them cannot definitely select for wild-type target over mutant enzymes. Therefore, off-target is observed as a common phenomenon for the untargeted consequence of targeted therapies with FTase inhibition. This is not unexpected if considering that the enzyme active site is highly conserved in composition, configuration and function. The off-target, on the one hand, causes nonpeptide inhibitors with adverse drug reactions and, on the other hand, makes the inhibitors as promising candidates for the new use of old drugs. To practice the latter, a number of unexpected mutant-inhibitor interactions involved in cancer signaling pathways are uncovered in the created profile, from which several nonpeptide inhibitors are identified as insensitive to a drug-resistant mutation. Structural analysis suggests that the inhibitor ligands can bind to the mutant active site in a similar manner with wild-type target, although their nonbonded interactions appear to be impaired moderately upon the mutation.

Role of G-proteins in islet function in health and diabetes

Glucose-stimulated insulin secretion (GSIS) involves interplay between metabolic and cationic events. Seminal contributions from multiple laboratories affirm essential roles for small G-proteins (Rac1, Cdc42, Arf6, Rab27A) in GSIS. Activation of these signalling proteins promotes cytoskeletal remodeling, transport and docking of insulin granules on the plasma membrane for exocytotic secretion of insulin. Evidence in rodent and human islets suggests key roles for lipidation (farnesylation and geranylgeranylation) of these G-proteins for their targeting to appropriate cellular compartments for optimal regulation of effectors leading to GSIS. Interestingly, however, inhibition of prenylation appears to cause mislocalization of non-prenylated, but (paradoxically) activated G-proteins, in "inappropriate" compartments leading to activation of stress kinases and onset of mitochondrial defects, loss in GSIS and apoptosis of the islet β-cell. This review highlights our current understanding of roles of G-proteins and their post-translational lipidation (prenylation) signalling networks in islet function in normal health, metabolic stress (glucolipotoxicity and ER stress) and diabetes. Critical knowledge gaps that need to be addressed for the development of therapeutics to halt defects in these signalling steps in β-cells in models of impaired insulin secretion and diabetes are also highlighted and discussed.

Metabolic Stress Induces Caspase-3 Mediated Degradation and Inactivation of Farnesyl and Geranylgeranyl Transferase Activities in Pancreatic β-Cells

BACKGROUND/AIMS

At least 300 prenylated proteins are identified in the human genome; the majority of which partake in a variety of cellular processes including growth, differentiation, cytoskeletal organization/dynamics and vesicle trafficking. Aberrant prenylation of proteins is implicated in human pathologies including cancer; neurodegenerative diseases, retinitis pigmentosa, and premature ageing syndromes. Original observations from our laboratory have demonstrated that prenylation of proteins [small G-proteins and γ-subunits of trimeric G-proteins] is requisite for physiological insulin secretion. Herein, we assessed the impact of metabolic stress [gluco-, lipotoxicity and ER-stress] on the functional status of protein prenylation pathway in pancreatic β-cells.

METHODS

Farnesyltransferase [FTase] and geranylgeranyltransferase [GGTase] activities were quantified by radioisotopic methods. Caspase-3 activation and FTase/GGTase-α subunit degradation were determined by Western blotting.

RESULTS

We observed that metabolic stress activates caspase-3 and induces degradation of the common α-subunit of FTase and GGTase-I in INS-1 832/13 cells, normal rodent islets and human islets leading to functional defects [inactivation] in FTase and GGTase activities. Caspase-3 activation and FTase/GGTase-α degradation were also seen in islets from the Zucker diabetic fatty [ZDF] rat, a model for Type 2 diabetes. Consequential to defects in FTase/GGTase-α signaling, we observed significant accumulation of unprenylated proteins [Rap1] in β-cells exposed to glucotoxic conditions. These findings were replicated in β-cells following pharmacological inhibition of generation of prenylpyrophosphate substrates [Simvastatin] or catalytic activity of prenylating enzymes [GGTI-2147].

CONCLUSIONS

Our findings provide the first evidence to suggest that metabolic stress induced dysfunction of the islet β-cell may, in part, be due to defective protein prenylation signaling pathway.

Protein prenylation in islet β-cell function in health and diabetes: Putting the pieces of the puzzle together

Post-translational prenylation involves incorporation of 15-(farnesyl) or 20-(geranylgeranyl) carbon derivatives of mevalonic acid into highly conserved C-terminal cysteines of proteins. The farnesyl transferase (FTase) and the geranylgeranyl transferase (GGTase) mediate incorporation of farnesyl and geranylgeranyl groups, respectively. At least 300 proteins are prenylated in the human genome; the majority of these are implicated in cellular processes including growth, differentiation, cytoskeletal function and vesicle trafficking. From a functional standpoint, isoprenylation is requisite for targeting of modified proteins to relevant cellular compartments for regulation of effector proteins. Pharmacological and molecular biological studies have provided compelling evidence for key roles of this signaling pathway in physiological insulin secretion in normal rodent and human islets. Recent evidence indicates that inhibition of prenylation results in mislocalization of unprenylated proteins, and surprisingly, they remain in active (GTP-bound) conformation. Sustained activation of G proteins has been reported in mice lacking GGTase, suggesting alternate mechanisms for the activation of non-prenylated G proteins. These findings further raise an interesting question if mislocalized, non-prenylated and functionally active G proteins cause cellular pathology since aberrant protein prenylation has been implicated in the onset of cardiovascular disease and diabetes. Herein, we overview the existing evidence to implicate prenylation in islet function and potential defects in this signaling pathways in the diabetic β-cell. We will also identify critical knowledge gaps that need to be addressed for the development of therapeutics to halt defects in these signaling steps in β cells in models of impaired insulin secretion, metabolic stress and diabetes.

Nifedipine prevents etoposide-induced caspase-3 activation, prenyl transferase degradation and loss in cell viability in pancreatic β-cells

Emerging evidence implicates novel roles for post-translational prenylation (i.e., farnesylation and geranylgeranylation) of various signaling proteins in a variety of cellular functions including hormone secretion, survival and apoptosis. In the context of cellular apoptosis, it has been shown previously that caspase-3 activation, a hallmark of mitochondrial dysregulation, promotes hydrolysis of several key cellular proteins. We report herein that exposure of insulin-secreting INS 832/13 cells or normal rat islets to etoposide leads to significant activation of caspase-3 and subsequent degradation of the common α-subunit of farnesyl/geranylgeranyl transferases (FTase/GGTase). Furthermore, the above stated signaling steps were prevented by Z-DEVD-FMK, a known inhibitor of caspase-3. In addition, treatment of cell lysates with recombinant caspase-3 also caused FTase/GGTase α-subunit degradation. Moreover, nifedipine, a calcium channel blocker, markedly attenuated etoposide-induced caspase-3 activation, FTase/GGTase α-subunit degradation in INS 832/13 cells and normal rat islets. Further, nifedipine significantly restored etoposide-induced loss in metabolic cell viability in INS 832/13 cells. Based on these findings, we conclude that etoposide induces loss in cell viability by inducing mitochondrial dysfunction, caspase-3 activation and degradation of FTase/GGTase α-subunit. Potential significance of these findings in the context of protein prenylation and β-cell survival are discussed.

Glucotoxic conditions induce endoplasmic reticulum stress to cause caspase 3 mediated lamin B degradation in pancreatic β-cells: protection by nifedipine

Nuclear lamins form the lamina on the interior of the nuclear envelope, and are involved in the regulation of various cellular processes, including DNA replication and chromatin organization. Despite this evidence, little is known about potential alterations in nuclear metabolism, specifically lamin structure and integrity in isolated β-cells subjected to stress conditions, including chronic exposure to hyperglycemia (i.e., glucotoxicity). Herein, we investigated effects of glucotoxic conditions on the catalytic activation of caspase 3 and the associated degradation of one of its substrate proteins, namely lamin-B. We report that incubation of insulin-secreting INS-1 832/13 cells, normal rat islets or human islets under glucotoxic conditions (20 mM; 12-48 h) results in the degradation of native lamin B leading to accumulation of the degraded products in non-relevant cellular compartments, including cytosol. Moreover, the effects of high glucose on caspase 3 activation and lamin B degradation were mimicked by thapsigargin, a known inducer of endoplasmic reticulum stress (ER stress). Nifedipine, a known blocker of calcium channel activation, inhibited high glucose-induced caspase 3 activation and lamin B degradation in these cells. 4-Phenyl butyric acid, a known inhibitor of ER stress, markedly attenuated glucose-induced CHOP expression (ER stress marker), caspase 3 activation and lamin B degradation. We conclude that glucotoxic conditions promote caspase 3 activation and lamin B degradation, which may, in part, be due to increased ER stress under these conditions. We also provide further evidence to support beneficial effects of calcium channel blockers against metabolic dysfunction of the islet β-cell induced by hyperglycemic conditions.

Targeting protein prenylation for cancer therapy

Protein farnesylation and geranylgeranylation, together referred to as prenylation, are lipid post-translational modifications that are required for the transforming activity of many oncogenic proteins, including some RAS family members. This observation prompted the development of inhibitors of farnesyltransferase (FT) and geranylgeranyl-transferase 1 (GGT1) as potential anticancer drugs. In this Review, we discuss the mechanisms by which FT and GGT1 inhibitors (FTIs and GGTIs, respectively) affect signal transduction pathways, cell cycle progression, proliferation and cell survival. In contrast to their preclinical efficacy, only a small subset of patients responds to FTIs. Identifying tumours that depend on farnesylation for survival remains a challenge, and strategies to overcome this are discussed. One GGTI has recently entered the clinic, and the safety and efficacy of GGTIs await results from clinical trials.

Phagocyte-like NADPH oxidase promotes cytokine-induced mitochondrial dysfunction in pancreatic β-cells: evidence for regulation by Rac1

Reactive oxygen species (ROS) are important mediators of cellular signal transduction cascades such as proliferation, migration, and apoptosis. Chronic exposure of isolated β-cells to proinflammatory cytokines elevates intracellular oxidative stress leading to the demise of pancreatic β-cells culminating in the onset of diabetes. Although the mitochondrial electron transport chain is felt to be the primary source of ROS, several lines of recent evidence suggest that phagocyte-like NADPH oxidase plays a central role in cytokine-mediated ROS generation and apoptosis of β-cells. However, the precise mechanisms underlying the regulation of NADPH oxidase remain unknown. To address this, insulin-secreting INS 832/13 cells were treated with cytomix (IL-1β, IFN-γ, and TNF-α; 10 ng/ml each) for different time intervals (0-24 h). A significant, time-dependent increase in NADPH oxidase activation/intracellular ROS production, p47(phox) subunit, but not p67(phox) subunit, expression of the phagocyte-like NADPH oxidase were demonstrable under these conditions. Furthermore, siRNA-p47(phox) transfection or exposure of INS 832/13 cells to apocynin, a selective inhibitor of NADPH oxidase, markedly attenuated cytomix-induced ROS generation in these cells. Cytomix-mediated mitochondrial dysfunction in INS 832/13 cells was evident by a significant loss of mitochondrial membrane potential (MMP) and upregulated caspase 3 activity. Cytomix treatment also caused a transient (within 15 min) activation of Rac1, a component of the NADPH oxidase holoenzyme. Furthermore, GGTI-2147 and NSC23766, known Rac1 inhibitors, not only attenuated the cytomix-induced Rac1 activation but also significantly prevented loss of MMP (NSC23766 > GGTI-2147). However, NSC23766 had no effect on cytomix-induced NO generation or caspase 3 activation, suggesting additional regulatory mechanisms might underlie these signaling steps. Together, these findings suggested that Rac1-mediated regulation of phagocyte-like NADPH oxidase contributes to cytokine-mediated mitochondrial dysfunction in the β-cell.

Identification and characterization of genes associated with tapping panel dryness from Hevea brasiliensis latex using suppression subtractive hybridization

BACKGROUND

Tapping panel dryness (TPD) is one of the most serious threats to natural rubber production. Although a great deal of effort has been made to study TPD in rubber tree, the molecular mechanisms underlying TPD remain poorly understood. Identification and systematical analyses of the genes associated with TPD are the prerequisites for elucidating the molecular mechanisms involved in TPD. The present study is undertaken to generate information about the genes related to TPD in rubber tree.

RESULTS

To identify the genes related to TPD in rubber tree, forward and reverse cDNA libraries from the latex of healthy and TPD trees were constructed using suppression subtractive hybridization (SSH) method. Among the 1106 clones obtained from the two cDNA libraries, 822 clones showed differential expression in two libraries by reverse Northern blot analyses. Sequence analyses indicated that the 822 clones represented 237 unique genes; and most of them have not been reported to be associated with TPD in rubber tree. The expression patterns of 20 differentially expressed genes were further investigated to validate the SSH data by reverse transcription PCR (RT-PCR) and real-time PCR analysis. According to the Gene Ontology convention, 237 unique genes were classified into 10 functional groups, such as stress/defense response, protein metabolism, transcription and post-transcription, rubber biosynthesis, etc. Among the genes with known function, the genes preferentially expressed were associated with stress/defense response in the reverse library, whereas metabolism and energy in the forward one.

CONCLUSIONS

The genes associated with TPD were identified by SSH method in this research. Systematic analyses of the genes related to TPD suggest that the production and scavenging of reactive oxygen species (ROS), ubiquitin proteasome pathway, programmed cell death and rubber biosynthesis might play important roles in TPD. Therefore, our results not only enrich information about the genes related to TPD, but also provide new insights into understanding the TPD process in rubber tree.

Protein and m-RNA expression of farnesyl-transferases, RhoA and RhoB in rat liver hepatocytes: action of perillyl alcohol and vitamin A in vivo

We analysed the action, in rats in vivo, of the protein isoprenylation inhibitor perillyl alcohol (POH) and that of vitamin A, alone or in association, on m-RNA and protein expression of farnesyltransferases (FTases alpha and beta subunits) and their protein substrates RhoA and RhoB, in isolated hepatocytes. Combined administration of POH and vitamin A induced a sharp decrease in FTase alpha protein after 96 h, suggesting an involvement not only of farnesyltransferases but also of geranylgeranyltransferases, which share the FTase alpha protein. FTase beta protein did not decrease. POH plus vitamin A, in contrast with POH or vitamin A alone, induced a decrease in RhoB protein, probably because of different cleavages. No modification was observed in RhoA protein. Vitamin A alone increased RhoB m-RNA and protein expression. As one of the functions of RhoB is cell polarisation, these data support our previous hypothesis of a polarised transport of vitamin A from hepatocytes to hepatic stellate cells. As the behaviours of m-RNAs and proteins in this study were often different, cytoplasmic metabolic pathways must be considered for the parameters studied. The behaviour of Rho B, which is thought to have an antioncogene function, is discussed in view of its isoprenylated forms in the membranes. These preliminary findings stress the need, when studying the association of two isoprenoids in cancer therapy, to consider normal as well as tumour-bearing animals.

Therapeutic efficacy of prenylation inhibitors in the treatment of myeloid leukemia

Farnesyltransferase inhibitors (FTIs) represent a new class of anticancer agents that specifically target post-translational farnesylation of various proteins that mediate several cellular processes such as signal transduction, growth, differentiation, angiogenesis and apoptosis. These compounds were originally designed to block oncogenic RAS-induced tumor growth by impeding RAS localization to the membrane, but it is now evident that FTIs also affect processing of several other proteins. The need for novel therapies in myeloid leukemia is underscored by the high rate of treatment failure due to high incidences of relapse- and treatment-related toxicities. As RAS deregulation is important in the pathogenesis of myeloid leukemias, targeting of RAS signaling may provide a new therapeutic strategy. Several FTIs (eg BMS-214662, L-778,123, R-115777 and SCH66336) have entered phase I and phase II clinical trials in myeloid leukemias. This review discusses recent clinical results, potential combination therapies, mechanisms of resistance and the clinical challenges of toxicities associated with prenylation inhibitors.

Many cuts to ruin: a comprehensive update of caspase substrates

Apoptotic cell death is executed by the caspase-mediated cleavage of various vital proteins. Elucidating the consequences of this endoproteolytic cleavage is crucial for our understanding of cell death and other biological processes. Many caspase substrates are just cleaved as bystanders, because they happen to contain a caspase cleavage site in their sequence. Several targets, however, have a discrete function in propagation of the cell death process. Many structural and regulatory proteins are inactivated by caspases, while other substrates can be activated. In most cases, the consequences of this gain-of-function are poorly understood. Caspase substrates can regulate the key morphological changes in apoptosis. Several caspase substrates also act as transducers and amplifiers that determine the apoptotic threshold and cell fate. This review summarizes the known caspase substrates comprising a bewildering list of more than 280 different proteins. We highlight some recent aspects inferred by the cleavage of certain proteins in apoptosis. We also discuss emerging themes of caspase cleavage in other forms of cell death and, in particular, in apparently unrelated processes, such as cell cycle regulation and cellular differentiation.