C H Wilson

Caspase-2 deficiency promotes aberrant DNA-damage response and genetic instability.

Caspase-2 is an initiator caspase, which has been implicated to function in apoptotic and non-apoptotic signalling pathways, including cell-cycle regulation, DNA-damage signalling and tumour suppression. We previously demonstrated that caspase-2 deficiency enhances E1A/Ras oncogene-induced cell transformation and augments lymphomagenesis in the EμMyc mouse model. Caspase-2−/− mouse embryonic fibroblasts (casp2−/− MEFs) show aberrant cell-cycle checkpoint regulation and a defective apoptotic response following DNA damage. Disruption of cell-cycle checkpoints often leads to genomic instability (GIN), which is a common phenotype of cancer cells and can contribute to cellular transformation. Here we show that caspase-2 deficiency results in increased DNA damage and GIN in proliferating cells. Casp2−/− MEFs readily escape senescence in culture and exhibit increased micronuclei formation and sustained DNA damage during cell culture and following γ-irradiation. Metaphase analyses demonstrated that a lack of caspase-2 is associated with increased aneuploidy in both MEFs and in EμMyc lymphoma cells. In addition, casp2−/− MEFs and lymphoma cells exhibit significantly decreased telomere length. We also noted that loss of caspase-2 leads to defective p53-mediated signalling and decreased trans-activation of p53 target genes upon DNA damage. Our findings suggest that loss of caspase-2 serves as a key function in maintaining genomic integrity, during cell proliferation and following DNA damage.

Impaired antioxidant defence and accumulation of oxidative stress in caspase-2-deficient mice

Caspase-2 has been implicated in apoptosis and in non-apoptotic processes such as cell cycle regulation, tumor suppression and ageing. Using caspase-2 knockout (casp2−/−) mice, we show here that the putative anti-ageing role of this caspase is due in part to its involvement in the stress response pathway. The old casp2−/− mice show increased cellular levels of oxidized proteins, lipid peroxides and DNA damage, suggesting enhanced oxidative stress. Furthermore, murine embryonic fibroblasts from casp2−/− mice showed increased reactive oxygen species generation when challenged with pro-oxidants. Reduced activities of antioxidant enzymes glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) were observed in the old casp2−/− mice. Interestingly, in the old casp2−/− animals expression of FoxO1 and FoxO3a was significantly reduced, whereas p21 levels and the number of senescent hepatocytes were elevated. In contrast to young wild-type mice, the casp2−/− animals fed an on ethanol-based diet failed to show enhanced GSH-Px and SOD activities. Thus, caspase-2, most likely via FoxO transcription factors, regulates the oxidative stress response in vivo.

Loss of caspase-2 augments lymphomagenesis and enhances genomic instability in Atm-deficient mice

Significance The cysteine protease caspase-2 has been implicated in the suppression of oncogene-mediated tumor formation. However, the mechanisms underlying the function of caspase-2 as a tumor suppressor are not well defined. In this study, we use a well-characterized mouse lymphoma model and demonstrate a critical role for caspase-2 in maintaining genome stability and in the suppression of tumorigenesis following loss of the essential DNA repair gene ataxia telangiectasia mutated (Atm). Our findings suggest that caspase-2 cooperates with ATM to suppress genomic instability, oxidative stress, and tumor progression. Caspase-2, the most evolutionarily conserved member of the caspase family, has been shown to be involved in apoptosis induced by various stimuli. Our recent work indicates that caspase-2 has putative functions in tumor suppression and protection against cellular stress. As such, the loss of caspase-2 enhances lymphomagenesis in Eµ-Myc transgenic mice, and caspase-2 KO (Casp2−/−) mice show characteristics of premature aging. However, the extent and specificity of caspase-2 function in tumor suppression is currently unclear. To further investigate this, ataxia telangiectasia mutated KO (Atm−/−) mice, which develop spontaneous thymic lymphomas, were used to generate Atm−/−Casp2−/− mice. Initial characterization revealed that caspase-2 deficiency enhanced growth retardation and caused synthetic perinatal lethality in Atm−/− mice. A comparison of tumor susceptibility demonstrated that Atm−/−Casp2−/− mice developed tumors with a dramatically increased incidence compared with Atm−/− mice. Atm−/−Casp2−/− tumor cells displayed an increased proliferative capacity and extensive aneuploidy that coincided with elevated oxidative damage. Furthermore, splenic and thymic T cells derived from premalignant Atm−/−Casp2−/− mice also showed increased levels of aneuploidy. These observations suggest that the tumor suppressor activity of caspase-2 is linked to its function in the maintenance of genomic stability and suppression of oxidative damage. Given that ATM and caspase-2 are important components of the DNA damage and antioxidant defense systems, which are essential for the maintenance of genomic stability, these proteins may synergistically function in tumor suppression by regulating these processes.

Age-related proteostasis and metabolic alterations in Caspase-2-deficient mice

Ageing is a complex biological process for which underlying biochemical changes are still largely unknown. We performed comparative profiling of the cellular proteome and metabolome to understand the molecular basis of ageing in Caspase-2-deficient (Casp2−/−) mice that are a model of premature ageing in the absence of overt disease. Age-related changes were determined in the liver and serum of young (6–9 week) and aged (18–24 month) wild-type and Casp2−/− mice. We identified perturbed metabolic pathways, decreased levels of ribosomal and respiratory complex proteins and altered mitochondrial function that contribute to premature ageing in the Casp2−/− mice. We show that the metabolic profile changes in the young Casp2−/− mice resemble those found in aged wild-type mice. Intriguingly, aged Casp2−/− mice were found to have reduced blood glucose and improved glucose tolerance. These results demonstrate an important role for caspase-2 in regulating proteome and metabolome remodelling during ageing.

Caspase-2 protects against oxidative stress in vivo.

Caspase-2 belongs to the caspase family of cysteine proteases with established roles in apoptosis. Recently, caspase-2 has been implicated in nonapoptotic functions including maintenance of genomic stability and tumor suppression. Our previous studies demonstrated that caspase-2 also regulates cellular redox status and delays the onset of several ageing-related traits. In the current study, we tested stress tolerance ability in caspase-2-deficient (Casp2−/−) mice by challenging both young and old mice with a low dose of the potent reactive oxygen species (ROS) generator, PQ that primarily affects lungs. In both groups of mice, PQ induced pulmonary damage. However, the lesions in caspase-2 knockout mice were consistently and reproducibly more severe than those in wild-type (WT) mice. Furthermore, serum interleukin (IL)-1β and IL-6 levels were higher in PQ-exposed aged Casp2−/− mice indicating increased inflammation. Interestingly, livers from Casp2−/− mice displayed karyomegaly, a feature commonly associated with ageing and aneuploidy. Given that Casp2−/− mice show impaired antioxidant defense, we tested oxidative damage in these mice. Protein oxidation significantly increased in PQ-injected old Casp2−/− mice. Moreover, FoxO1, SOD2 and Nrf2 expression levels were reduced and induction of superoxide dismutase (SOD) and glutathione peroxidase activity was not observed in PQ-treated Casp2−/− mice. Strong c-Jun amino-terminal kinase (JNK) activation was observed in Casp2−/− mice, indicative of increased stress. Together, our data strongly suggest that caspase-2 deficiency leads to increased cellular stress largely because these mice fail to respond to oxidative stress by upregulating their antioxidant defense mechanism. This makes the mice more vulnerable to exogenous challenges and may partly explain the shorter lifespan of Casp2−/− mice.

Caspase-2 deficiency accelerates chemically induced liver cancer in mice

Aberrant cell death/survival has a critical role in the development of hepatocellular carcinoma (HCC). Caspase-2, a cell death protease, limits oxidative stress and chromosomal instability. To study its role in reactive oxygen species (ROS) and DNA damage-induced liver cancer, we assessed diethylnitrosamine (DEN)-mediated tumour development in caspase-2-deficient (Casp2−/−) mice. Following DEN injection in young animals, tumour development was monitored for 10 months. We found that DEN-treated Casp2−/− mice have dramatically elevated tumour burden and accelerated tumour progression with increased incidence of HCC, accompanied by higher oxidative damage and inflammation. Furthermore, following acute DEN injection, liver injury, DNA damage, inflammatory cytokine release and hepatocyte proliferation were enhanced in mice lacking caspase-2. Our study demonstrates for the first time that caspase-2 limits the progression of tumourigenesis induced by an ROS producing and DNA damaging reagent. Our findings suggest that after initial DEN-induced DNA damage, caspase-2 may remove aberrant cells to limit liver damage and disease progression. We propose that Casp2−/− mice, which are more susceptible to genomic instability, are limited in their ability to respond to DNA damage and thus carry more damaged cells resulting in accelerated tumourigenesis.

An unexpected role for caspase-2 in neuroblastoma

Caspase-2 has been implicated in various cellular functions, including cell death by apoptosis, oxidative stress response, maintenance of genomic stability and tumor suppression. The loss of the caspase-2 gene (Casp2) enhances oncogene-mediated tumorigenesis induced by E1A/Ras in athymic nude mice, and also in the Eμ-Myc lymphoma and MMTV/c-neu mammary tumor mouse models. To further investigate the function of caspase-2 in oncogene-mediated tumorigenesis, we extended our studies in the TH-MYCN transgenic mouse model of neuroblastoma. Surprisingly, we found that loss of caspase-2 delayed tumorigenesis in the TH-MYCN neuroblastoma model. In addition, tumors from TH-MYCN/Casp2−/− mice were predominantly thoracic paraspinal tumors and were less vascularized compared with tumors from their TH-MYCN/Casp2+/+ counterparts. We did not detect any differences in the expression of neuroblastoma-associated genes in TH-MYCN/Casp2−/− tumors, or in the activation of Ras/MAPK signaling pathway that is involved in neuroblastoma progression. Analysis of expression array data from human neuroblastoma samples showed a correlation between low caspase-2 levels and increased survival. However, caspase-2 levels correlated with clinical outcome only in the subset of MYCN-non-amplified human neuroblastoma. These observations indicate that caspase-2 is not a suppressor in MYCN-induced neuroblastoma and suggest a tissue and context-specific role for caspase-2 in tumorigenesis.

Sex-specific alterations in glucose homeostasis and metabolic parameters during ageing of caspase-2-deficient mice

Gender-specific differences are commonly found in metabolic pathways and in response to nutritional manipulation. Previously, we identified a role for caspase-2 in age-related glucose homeostasis and lipid metabolism using male caspase-2-deficient (Casp2−/−) mice. Here we show that the resistance to age-induced glucose tolerance does not occur in female Casp2−/− mice and it appears to be independent of insulin sensitivity in males. Using fasting (18u2009h) as a means to further investigate the role of caspase-2 in energy and lipid metabolism, we identified sex-specific differences in the fasting response and lipid mobilization. In aged (18–22 months) male Casp2−/− mice, a significant decrease in fasting liver mass, but not total body weight, was observed while in females, total body weight, but not liver mass, was reduced when compared with wild-type (WT) animals. Fasting-induced lipolysis of adipose tissue was enhanced in male Casp2−/− mice as indicated by a significant reduction in white adipocyte cell size, and increased serum-free fatty acids. In females, white adipocyte cell size was significantly smaller in both fed and fasted Casp2−/− mice. No difference in fasting-induced hepatosteatosis was observed in the absence of caspase-2. Further analysis of white adipose tissue (WAT) indicated that female Casp2−/− mice may have enhanced fatty acid recycling and metabolism with expression of genes involved in glyceroneogenesis and fatty acid oxidation increased. Loss of Casp2 also increased fasting-induced autophagy in both male and female liver and in female skeletal muscle. Our observations suggest that caspase-2 can regulate glucose homeostasis and lipid metabolism in a tissue and sex-specific manner.

Caspase-2 deficiency enhances whole-body carbohydrate utilisation and prevents high-fat diet-induced obesity.

Caspase-2 has been shown to be involved in metabolic homeostasis. Here, we show that caspase-2 deficiency alters basal energy metabolism by shifting the balance in fuel choice from fatty acid to carbohydrate usage. At 4 weeks of age, whole-body carbohydrate utilisation was increased in Casp2−/− mice and was maintained into adulthood. By 17 weeks of age, Casp2−/− mice had reduced white adipose mass, smaller white adipocytes decreased fasting blood glucose and plasma triglycerides but maintained normal insulin levels. When placed on a 12-week high-fat diet (HFD), Casp2−/− mice resisted the development of obesity, fatty liver, hyperinsulinemia and insulin resistance. In addition, HFD-fed Casp2−/− mice had reduced white adipocyte hypertrophy, apoptosis and expansion of both subcutaneous and visceral adipose depots. Increased expression of UCP1 and the maintenance of adiponectin levels in white adipose tissue of HFD-fed Casp2−/− mice indicated increased browning and adipocyte hyperplasia. We found that while the preference for whole-body carbohydrate utilisation was maintained, HFD-fed Casp2−/− mice were not impaired in their ability to switch to utilising fats as a fuel source. Our findings suggest that caspase-2 impacts basal energy metabolism by regulating adipocyte biology and fat expansion, most likely via a non-apoptotic function. Furthermore, we show that caspase-2 deficiency shifts the balance in fuel choice towards increased carbohydrate utilisation and propose that this is due to mild energy stress. As a consequence, Casp2−/− mice show an adaptive remodelling of adipose tissue that protects from HFD-induced obesity and improves glucose homeostasis while paradoxically increasing their susceptibility to oxidative stress induced damage and premature ageing.

The glucagon-like peptide-1 analogue exendin-4 reverses impaired intracellular Ca(2+) signalling in steatotic hepatocytes.

The release of Ca(2+) from the endoplasmic reticulum (ER) and subsequent replenishment of ER Ca(2+) by Ca(2+) entry through store-operated Ca(2+) channels (SOCE) play critical roles in the regulation of liver metabolism by adrenaline, glucagon and other hormones. Both ER Ca(2+) release and Ca(2+) entry are severely inhibited in steatotic hepatocytes. Exendin-4, a slowly-metabolised glucagon-like peptide-1 (GLP-1) analogue, is known to reduce liver glucose output and liver lipid, but the mechanisms involved are not well understood. The aim of this study was to determine whether exendin-4 alters intracellular Ca(2+) homeostasis in steatotic hepatocytes, and to evaluate the mechanisms involved. Exendin-4 completely reversed lipid-induced inhibition of SOCE in steatotic liver cells, but did not reverse lipid-induced inhibition of ER Ca(2+) release. The action of exendin-4 on Ca(2+) entry was rapid in onset and was mimicked by GLP-1 or dibutyryl cyclic AMP. In steatotic liver cells, exendin-4 caused a rapid decrease in lipid (half time 6.5min), inhibited the accumulation of lipid in liver cells incubated in the presence of palmitate plus the SOCE inhibitor BTP-2, and enhanced the formation of cyclic AMP. Hormone-stimulated accumulation of extracellular glucose in glycogen replete steatotic liver cells was inhibited compared to that in non-steatotic cells, and this effect of lipid was reversed by exendin-4. It is concluded that, in steatotic hepatocytes, exendin-4 reverses the lipid-induced inhibition of SOCE leading to restoration of hormone-regulated cytoplasmic Ca(2+) signalling. The mechanism may involve GLP-1 receptors, cyclic AMP, lipolysis, decreased diacylglycerol and decreased activity of protein kinase C.