Mimicking Cdk2 phosphorylation of Bcl-xL at Ser73 results in caspase activation and Bcl-xL cleavage

Repurposing AZD5438 and Dabrafenib for Cisplatin-Induced AKI

SIGNIFICANCE STATEMENT

To combat both untoward effects of nephrotoxicity and ototoxicity in cisplatin-treated patients, two potential therapeutic oral anticancer drugs AZD5438 and dabrafenib, a phase-2 clinical trial protein kinase CDK2 inhibitor and an US Food and Drug Administration-approved drug BRAF inhibitor, respectively, were tested in an established mouse AKI model. Both drugs have previously been shown to protect significantly against cisplatin-induced hearing loss in mice. Each drug ameliorated cisplatin-induced increases in the serum biomarkers BUN, creatinine, and neutrophil gelatinase-associated lipocalin. Drugs also improved renal histopathology and inflammation, mitigated cell death by pyroptosis and necroptosis, and significantly enhanced overall survival of cisplatin-treated mice.

BACKGROUND

Cisplatin is an effective chemotherapy agent for a wide variety of solid tumors, but its use is dose-limited by serious side effects, including AKI and hearing loss. There are no US Food and Drug Administration-approved drugs to treat both side effects. Recently, two anticancer oral drugs, AZD5438 and dabrafenib, were identified as protective against cisplatin-induced hearing loss in mice. We hypothesize that similar cell stress and death pathways are activated in kidney and inner ear cells when exposed to cisplatin and tested whether these drugs alleviate cisplatin-induced AKI.

METHODS

The HK-2 cell line and adult FVB mice were used to measure the protection from cisplatin-induced cell death and AKI by these drugs. Serum markers of kidney injury, BUN, creatinine, and neutrophil gelatinase-associated lipocalin as well as histology of kidneys were analyzed. The levels of markers of kidney cell death, including necroptosis and pyroptosis, pERK, and proliferating cell nuclear antigen, were also examined by Western blotting and immunofluorescence. In addition, CDK2 knockout (KO) mice were used to confirm AZD5438 protective effect is through CDK2 inhibition.

RESULTS

The drugs reduced cisplatin-induced cell death in the HK-2 cell line and attenuated cisplatin-induced AKI in mice. The drugs reduced serum kidney injury markers, inhibited cell death, and reduced the levels of pERK and proliferating cell nuclear antigen, all of which correlated with prolonged animal survival. CDK2 KO mice were resistant to cisplatin-induced AKI, and AZD5438 conferred no additional protection in the KO mice.

CONCLUSIONS

Cisplatin-induced damage to the inner ear and kidneys shares similar cellular beneficial responses to AZD5438 and dabrafenib, highlighting the potential therapeutic use of these agents to treat both cisplatin-mediated kidney damage and hearing loss.

Burn-Induced Apoptosis in the Livers of Aged Mice Is Associated With Caspase Cleavage of Bcl-xL

INTRODUCTION

Older adult burn victims have poorer outcomes than younger burn victims. The liver is critical for the recovery of patients with burns. Postburn hepatic apoptosis in young individuals compromises liver integrity; however, this pathway has not yet been studied in older individuals. Because aged animals with burns suffer significant liver damage, we hypothesized that apoptosis is altered in these animals and may affect liver function. Understanding postburn hepatic apoptosis and its effects on liver function in aged animals may help improve outcomes in older patients.

METHODS

We compared the protein and gene expression levels in young and aged mice after a 15% total-body-surface-area burn. Liver and serum samples were collected at different time points after injury.

RESULTS

Caspase-9 expression in liver tissue was downregulated by 47% in young animals and upregulated by 62% in aged animals 9 h postburn (P < 0.05). The livers of aged mice showed a Bcl-extra-large (Bcl-xL) transcription increase only after 6 h; however, the livers of young mice exhibited 4.3-fold, 14.4-fold, and 7.8-fold Bcl-xL transcription increases at 3, 6, and 9 h postburn, respectively (P < 0.05). The livers of young mice showed no changes in Caspase-9, Caspase-3, or Bcl-xL protein levels during the early postburn period. In contrast, the livers of aged mice contained cleaved caspase-9, reduced full-length caspase-3, and an accumulation of ΔN-Bcl-x at 6 and 9 h postburn (P < 0.05). p21 expression decreased in aged mice; however, it was significantly increased in the liver tissue of young mice postburn (P < 0.05). Serum amyloid A1 and serum amyloid A2 serum protein levels were 5.2- and 3.1-fold higher in young mice than in aged mice, respectively, at 6 and 9 h postburn (P < 0.05).

CONCLUSIONS

Livers of aged mice exhibited different apoptotic processes compared to those of young mice early after burn injury. Collectively, burn-induced liver apoptosis in aged mice compromises hepatic serum protein production.

Coprophagy Prevention Decreases the Reproductive Performance and Granulosa Cell Apoptosis via Regulation of CTSB Gene in Rabbits

Coprophagy is an instinctive behavior in rabbit with important effects on growth and reproductive performance. The underlying mechanism of this effect in rabbit is unknown. Here, we used Elizabeth circle as a coprophagy preventing model in female rabbits and assess feed intake, growth, and reproductive performance. We found that preventing coprophagy did not affect feed intake but decreased body weight and weight of several organs and tissues and resulted in complete reproductive failure during the late pregnancy period, accompanied by reduced levels of plasma progesterone. RNA-seq analysis of rabbit ovarian tissues revealed that preventing coprophagy affected significantly 241 genes (DEGs), with the large majority being downregulated. Bioinformatic analyses revealed that those DEGs are mostly involved in apoptosis, immune response, and metabolic pathways. Among DEGs, the lysosomal cysteine protease cathepsin B (CTSB) was significantly downregulated in the coprophagy prevention group. Further studies using siRNA and adenovirus overexpression systems revealed that CTSB promotes the proliferation of rabbit granulosa cells (GCS) and prevents apoptosis. Measurement of transcripts coding for proteins related to apoptosis revealed a minor transcriptomic effect of CTSB, indicating that its effect is likely post-transcriptional. Overexpression of CTSB increased secretion of progesterone and estradiol, partly via upregulation of CYP19A1 while inhibition of CTSB decreased progesterone secretion partly via downregulation of the StAR gene. In conclusion, our study demonstrated the detrimental effect on reproduction by preventing coprophagy with a main role for this response played by CTSB on the granulosa cells of the ovary.

The interplay between apoptosis and cellular senescence: Bcl-2 family proteins as targets for cancer therapy

Cell death by apoptosis and permanent cell cycle arrest by senescence serve as barriers to the development of cancer. Chemotherapeutic agents not only induce apoptosis, they can also induce senescence known as therapy-induced senescence (TIS). There are, however, controversies whether TIS improves or worsens therapeutic outcome. Unlike apoptosis, which permanently removes cancer cells, senescent cells are metabolically active, and can contribute to tumor progression and relapse. If senescent cells are not cleared by the immune system or if cancer cells escape senescence, they may acquire resistance to apoptotic stimuli and become highly aggressive. Thus, there have been significant efforts in developing senolytics, drugs that target these pro-survival molecules to eliminate senescent cells. The anti-apoptotic Bcl-2 family proteins not only protect against cell death by apoptosis, but they also allow senescent cells to survive. While combining senolytics with chemotherapeutic drugs is an attractive approach, there are also limitations. Moreover, members of the Bcl-2 family have distinct effects on apoptosis and senescence. The purpose of this review article is to discuss recent literatures on how members of the Bcl-2 family orchestrate the interplay between apoptosis and senescence, and the challenges and progress in targeting these Bcl-2 family proteins for cancer therapy.

Development of Conformational Antibodies to Detect Bcl-xL's Amyloid Aggregates in Metal-Induced Apoptotic Neuroblastoma Cells

Bcl-xL, a member of the Bcl-2 family, is a pro-survival protein involved in apoptosis regulation. We have previously reported the ability of Bcl-xL to form various types of fibers, from native to amyloid conformations. Here, we have mimicked the effect of apoptosis-induced caspase activity on Bcl-xL by limited proteolysis using trypsin. We show that cleaved Bcl-xL (ΔN-Bcl-xL) forms fibers that exhibit the features of amyloid structures (BclxLcf37). Moreover, three monoclonal antibodies (mAbs), produced by mouse immunization and directed against ΔN-Bcl-xL or Bcl-xL fibers, were selected and characterized. Our results show that these mAbs specifically target ΔN-Bcl-xL in amyloid fibers in vitro. Upon metal-stress-induced apoptosis, these mAbs are able to detect the presence of Bcl-xL in amyloid aggregates in neuroblastoma SH-SY5Y cell lines. In conclusion, these specific mAbs directed against amyloidogenic conformations of Bcl-xL constitute promising tools for studying, in vitro and in cellulo, the contribution of Bcl-xL in apoptosis. These mAbs may further help in developing new diagnostics and therapies, considering Bcl-xL as a strategic target for treating brain lesions relevant to stroke and neurodegenerative diseases.

Alpha-Tocotrienol Prevents Oxidative Stress-Mediated Post-Translational Cleavage of Bcl-xL in Primary Hippocampal Neurons

B-cell lymphoma-extra large (Bcl-xL) is an anti-apoptotic member of the Bcl2 family of proteins, which supports neurite outgrowth and neurotransmission by improving mitochondrial function. During excitotoxic stimulation, however, Bcl-xL undergoes post-translational cleavage to ∆N-Bcl-xL, and accumulation of ∆N-Bcl-xL causes mitochondrial dysfunction and neuronal death. In this study, we hypothesized that the generation of reactive oxygen species (ROS) during excitotoxicity leads to formation of ∆N-Bcl-xL. We further proposed that the application of an antioxidant with neuroprotective properties such as α-tocotrienol (TCT) will prevent ∆N-Bcl-xL-induced mitochondrial dysfunction via its antioxidant properties. Primary hippocampal neurons were treated with α-TCT, glutamate, or a combination of both. Glutamate challenge significantly increased cytosolic and mitochondrial ROS and ∆N-Bcl-xL levels. ∆N-Bcl-xL accumulation was accompanied by intracellular ATP depletion, loss of mitochondrial membrane potential, and cell death. α-TCT prevented loss of mitochondrial membrane potential in hippocampal neurons overexpressing ∆N-Bcl-xL, suggesting that ∆N-Bcl-xL caused the loss of mitochondrial function under excitotoxic conditions. Our data suggest that production of ROS is an important cause of ∆N-Bcl-xL formation and that preventing ROS production may be an effective strategy to prevent ∆N-Bcl-xL-mediated mitochondrial dysfunction and thus promote neuronal survival.

CDK2 inhibitors as candidate therapeutics for cisplatin- and noise-induced hearing loss

Hearing loss caused by aging, noise, cisplatin toxicity, or other insults affects 360 million people worldwide, but there are no Food and Drug Administration-approved drugs to prevent or treat it. We screened 4,385 small molecules in a cochlear cell line and identified 10 compounds that protected against cisplatin toxicity in mouse cochlear explants. Among them, kenpaullone, an inhibitor of multiple kinases, including cyclin-dependent kinase 2 (CDK2), protected zebrafish lateral-line neuromasts from cisplatin toxicity and, when delivered locally, protected adult mice and rats against cisplatin- and noise-induced hearing loss. CDK2-deficient mice displayed enhanced resistance to cisplatin toxicity in cochlear explants and to cisplatin- and noise-induced hearing loss in vivo. Mechanistically, we showed that kenpaullone directly inhibits CDK2 kinase activity and reduces cisplatin-induced mitochondrial production of reactive oxygen species, thereby enhancing cell survival. Our experiments have revealed the proapoptotic function of CDK2 in postmitotic cochlear cells and have identified promising therapeutics for preventing hearing loss.

Inhibition of Bcl-xL prevents pro-death actions of ΔN-Bcl-xL at the mitochondrial inner membrane during glutamate excitotoxicity

ABT-737 is a pharmacological inhibitor of the anti-apoptotic activity of B-cell lymphoma-extra large (Bcl-xL) protein; it promotes apoptosis of cancer cells by occupying the BH3-binding pocket. We have shown previously that ABT-737 lowers cell metabolic efficiency by inhibiting ATP synthase activity. However, we also found that ABT-737 protects rodent brain from ischemic injury in vivo by inhibiting formation of the pro-apoptotic, cleaved form of Bcl-xL, ΔN-Bcl-xL. We now report that a high concentration of ABT-737 (1 μM), or a more selective Bcl-xL inhibitor WEHI-539 (5 μM) enhances glutamate-induced neurotoxicity while a low concentration of ABT-737 (10 nM) or WEHI-539 (10 nM) is neuroprotective. High ABT-737 markedly increased ΔN-Bcl-xL formation, aggravated glutamate-induced death and resulted in the loss of mitochondrial membrane potential and decline in ATP production. Although the usual cause of death by ABT-737 is thought to be related to activation of Bax at the outer mitochondrial membrane due to sequestration of Bcl-xL, we now find that low ABT-737 not only prevents Bax activation, but it also inhibits the decline in mitochondrial potential produced by glutamate toxicity or by direct application of ΔN-Bcl-xL to mitochondria. Loss of mitochondrial inner membrane potential is also prevented by cyclosporine A, implicating the mitochondrial permeability transition pore in death aggravated by ΔN-Bcl-xL. In keeping with this, we find that glutamate/ΔN-Bcl-xL-induced neuronal death is attenuated by depletion of the ATP synthase c-subunit. C-subunit depletion prevented depolarization of mitochondrial membranes in ΔN-Bcl-xL expressing cells and substantially prevented the morphological change in neurites associated with glutamate/ΔN-Bcl-xL insult. Our findings suggest that low ABT-737 or WEHI-539 promotes survival during glutamate toxicity by preventing the effect of ΔN-Bcl-xL on mitochondrial inner membrane depolarization, highlighting ΔN-Bcl-xL as an important therapeutic target in injured brain.

Bcl-xL deamidation and cancer: Charting the fame trajectories of legitimate child and hidden siblings

Bcl-2 family proteins control programmed cell death through a complex network of interactions within and outside of this family, that are modulated by post-translational modifications (PTM). Bcl-x_L, an anti-apoptotic member of this family, is overexpressed in a number of cancers, plays an important role in tumorigenesis and is correlated with drug resistance. Bcl-x_L is susceptible to a number of different PTMs. Here, we focus on deamidation. We will first provide an overview of protein deamidation. We will then review how the apoptotic and autophagic functions of Bcl-x_L are modified by this PTM, and how this impacts on its oncogenic properties. Possible therapeutic outcomes will also be discussed. Finally, we will highlight how the specific case of Bcl-x_L deamidation provides groundings to revisit some concepts related to protein deamidation in general.