The biochemical mechanism of caspase-2 activation

Stressing out over survival: glutamine as an apoptotic modulator

INTRODUCTION

The amino acid glutamine (GLN) has received considerable attention as a potential therapeutic adjuvant in critical illness and in improving postoperative clinical outcomes. Most studies on the role of GLN in cellular physiology have historically focused on its anabolic roles in specific cell types and its contribution to growth in cancer cells. However, an emerging body of work that examines the consequences of GLN deprivation on cellular survival and gene expression has constructed a new paradigm for this amino acid, namely, that limited extracellular GLN supplies modulate stress and apoptotic responses.

METHODS

A survey of the scientific literature was conducted on GLN in cell survival signaling and apoptosis. Work from our laboratory in liver cancer cells also was included in this review.

RESULTS

Most studies on this topic have used mammalian cell lines derived from the gut, immune system (including hybridomas), and various cancers. GLN limitation, even in the presence of an adequate glucose supply, impacts stress-related gene expression, differentially modulates receptor-mediated apoptosis, and directly elicits apoptosis through signaling mechanisms and caspase cascades that are specific to cell type. To date, GLN transporters, cellular hydration, glutaminyl-tRNA synthetase, ATP levels, mRNA stability, and glutathione economy have been variably implicated in GLN-dependent survival signaling.

CONCLUSION

The cell type-specific mechanisms underlying the regulatory role of GLN in cell survival continue to unfold at a steady pace through in vitro studies. These results have collectively provided testable hypotheses for further in vivo studies into their physiological relevance during GLN "nutritional pharmacology."

Targeting endoplasmic reticulum protein transport: a novel strategy to kill malignant B cells and overcome fludarabine resistance in CLL

Previous studies showed that chronic lymphocytic leukemia (CLL) cells exhibit certain mitochondrial abnormalities including mtDNA mutations, increased superoxide generation, and aberrant mitochondrial biogenesis, which are associated with impaired apoptosis and reduced sensitivity to fludarabine. Here we report that CLL cells and multiple myeloma cells are highly sensitive to brefeldin A, an inhibitor of endoplasmic reticulum (ER) to Golgi protein transport currently being developed as a novel anticancer agent in a prodrug formulation. Of importance, brefeldin A effectively induced apoptosis in fludarabine-refractory CLL cells. Disruption of protein trafficking by brefeldin A caused the sequestration of the prosurvival factors APRIL and VEGF in the ER, leading to abnormal ER swelling and a decrease in VEGF secretion. Such ER stress and blockage of secretory protein traffic eventually resulted in Golgi collapse, activation of caspases, and cell death. Notably, the cellular sensitivity to this compound appeared to be independent of p53 status. Taken together, these findings suggest that malignant B cells may be highly dependent on ER-Golgi protein transport and that targeting this process may be a promising therapeutic strategy for B-cell malignancies, especially for those that respond poorly to conventional treatments.

Shiga toxin 1 induces apoptosis in the human myelogenous leukemia cell line THP-1 by a caspase-8-dependent, tumor necrosis factor receptor-independent mechanism

Shiga toxins (Stxs) induce apoptosis in a variety of cell types. Here, we show that Stx1 induces apoptosis in the undifferentiated myelogenous leukemia cell line THP-1 in the absence of tumor necrosis factor alpha (TNF-alpha) or death receptor (TNF receptor or Fas) expression. Caspase-8 and -3 inhibitors blocked, and caspase-6 and -9 inhibitors partially blocked, Stx1-induced apoptosis. Stx1 induced the mitochondrial pathway of apoptosis, as activation of caspase-8 triggered the (i) cleavage of Bid, (ii) disruption of mitochondrial membrane potential, and (iii) release of cytochrome c into the cytoplasm. Caspase-8, -9, and -3 cleavage and functional activities began 4 h after toxin exposure and peaked after 8 h of treatment. Caspase-6 may also contribute to Stx1-induced apoptosis by directly acting on caspase-8. It appears that functional Stx1 holotoxins must be transported to the endoplasmic reticulum to initiate apoptotic signaling through the ribotoxic stress response. These data suggest that Stxs may activate monocyte apoptosis via a novel caspase-8-dependent, death receptor-independent mechanism.

B-cell antigen receptor-induced apoptosis: looking for clues

Triggering of the B cell antigen receptor (BCR) can initiate divergent responses ranging from activation and cell division to apoptosis, depending on the differentiation stage and additional signals the cell receives. Despite considerable progress in unraveling general apoptosis pathways, the route from the BCR to apoptosis execution is still quite obscure, and there is no consensus yet concerning the mechanism or the players involved. Here, we will summarize current developments in this field and will attempt to pinpoint key questions and perspectives for future research.

Caspase-2 function in response to DNA damage

Caspase-2 is one of the best conserved caspases across species. This enzyme is unique among caspases in that it has features of both initiator and effector caspases. Caspase-2 appears to be necessary for the onset of apoptosis triggered by several insults, including DNA damage, administration of TNF, and different pathogens and viruses. In several experimental systems, a link has been shown between the p53 family proteins and caspase-2 activation leading to cell death. In this review, current knowledge concerning the structure of this protease and its function in cell physiology and cell death, particularly cell death triggered by DNA damage, is summarized and discussed.

Cell size reduction induced by inhibition of the mTOR/S6K-signaling pathway protects Jurkat cells from apoptosis

In Jurkat cells, the decreased cell growth rate associated with a long-lasting deactivation of the mammalian target of rapamycin (mTOR)/p70 ribosomal S6 kinase (S6K)-signaling pathway generates a cell population of progressively reduced cellular mass and size. When promoted by rapamycin as prototype inhibitor, the mTOR deactivation-dependent cell size reduction was associated with slowed, but not suppressed, proliferation. Small-size cells were significantly protected from apoptosis induced by Fas/Apo-1 death-receptor activation (as shown by reduced procaspase cleavage and decreased catalytic activity of relevant caspases) or by stress signals-dependent mitochondrial perturbation (as shown by reduced cleavage of caspase-2, lower dissipation of mitochondrial membrane potential and decreased release of cytochorome c and apoptosis-inducing factor from mitochondria). Protection faded when reactivation of the mTOR/S6K pathway promoted the cell recovery to normal size. These results suggest that cells induced to reduce their mass by the mTOR deactivation-dependent inhibition of cell growth become more resilient to lethal assaults by curbing the cell's suicidal response.

Caspases-2, -3, and -7 are involved in thapsigargin-induced apoptosis of SH-SY5Y neuroblastoma cells

Caspase-2 has been reported to play a role in the cell death observed under a number of different conditions; however, it is unclear whether caspase-2 plays a role in cell death triggered by endoplasmic reticulum (ER) stress. The purpose of this study was to determine whether caspase-2 is involved in SH-SY5Y neuroblastoma cell death caused by thapsigargin-induced ER stress. Thapsigargin treatment (1 microM, 16 hr) stimulated the proteolytic processing of caspases-2, -3, and -7, suggesting that these caspases are activated by ER stress. The role of these caspases in thapsigargin-induced cell death was examined by using cell-permeable caspase inhibitors. In the absence of pretreatment with caspase inhibitors, thapsigargin (0.1 microM, 20 hr) reduced the number of viable cells to 53.9% +/- 3.3% of starting-time control. Pretreatment for 90 min with either the pan-caspase inhibitor Z-VAD-FMK or the caspase-2-selective inhibitor Z-VDVAD-FMK inhibited thapsigargin-stimulated cell death, resulting in the number of viable cells being 115.6% +/- 5.3% (P < 0.001) and 69.3% +/- 2.9% (P < 0.01), respectively, of starting-time control. Neither the caspase-3- and -7-selective inhibitor Z-DEVD-FMK nor the caspase-9-selective inhibitor Z-LEHD-FMK significantly affected thapsigargin-stimulated cell death. An anticaspase-12-reactive protein was also identified in SH-SY5Y cells, but thapsigargin had no effect on proteolysis of this protein. These data demonstrate that caspases-2, -3, and -7 are involved in ER stress-mediated death of SH-SY5Y cells.

Flavone initiates a hierarchical activation of the caspase-cascade in colon cancer cells

There is emerging evidence that dietary factors can prevent cancer by affecting the process of carcinogenesis. Flavonoids present in vegetarian food possess antioxidant activities, have scavenging effects on activated carcinogens and mutagens, affect cell cycle progression and alter gene and protein expression. We report here that flavone, the core structure of the flavone subgroup, potently inhibits proliferation and induces apoptosis in HCT-116 colon cancer cells. Flavone induces the activation of caspases 2, 3, 8, 9 and 10 and a decrease of mitochondrial anti-apoptotic Bcl(2) protein expression. Further analysis revealed that caspase 10 activation is mediated via caspase 1. Additionally, treatment with flavone results in release of the mitochondrial apoptosis-inducing factor (AIF), the key trigger of caspase-independent apoptosis, into the cytosol. In summary, our data show that flavone induces apoptosis in a caspase-dependent and -independent manner.

Caspase-2 is resistant to inhibition by inhibitor of apoptosis proteins (IAPs) and can activate caspase-7

Caspases are a family of cysteine proteases with roles in cytokine maturation or apoptosis. Caspase-2 was the first pro-apoptotic caspase identified, but its functions in apoptotic signal transduction are still being elucidated. This study examined the regulation of the activity of caspase-2 using recombinant proteins and a yeast-based system. Our data suggest that for human caspase-2 to be active its large and small subunits must be separated. For maximal activity its prodomain must also be removed. Consistent with its proposed identity as an upstream caspase, caspase-2 could provoke the activation of caspase-7. Caspase-2 was not subject to inhibition by members of the IAP family of apoptosis inhibitors.

Golgi structure in stress sensing and apoptosis

The Golgi complex in mammalian cells is composed of polarized stacks of flattened cisternal membranes. Stacks are connected by tubules forming a reticular network of membranes closely associated with the microtubule-organizing center. While the Golgi structure is important for the efficient processing of secretory cargo, the organization of the mammalian Golgi complex may indicate potential functions in addition to the processing and sorting of cargo. Similar to the endoplasmic reticulum stress response pathway, the Golgi complex may initiate signaling pathways to alleviate stress, and if irreparable, trigger apoptosis. Here, we review recent experimental evidence suggesting that the elaborate structure of the Golgi complex in mammalian cells may have evolved to sense and transduce stress signals.