Cdk2-dependent phosphorylation of p21 regulates the role of Cdk2 in cisplatin cytotoxicity

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.

The Therapeutic Potential of CDK4/6 Inhibitors, Novel Cancer Drugs, in Kidney Diseases

Inflammation is a crucial pathological feature in cancers and kidney diseases, playing a significant role in disease progression. Cyclin-dependent kinases CDK4 and CDK6 not only contribute to cell cycle progression but also participate in cell metabolism, immunogenicity and anti-tumor immune responses. Recently, CDK4/6 inhibitors have gained approval for investigational treatment of breast cancer and various other tumors. Kidney diseases and cancers commonly exhibit characteristic pathological features, such as the involvement of inflammatory cells and persistent chronic inflammation. Remarkably, CDK4/6 inhibitors have demonstrated impressive efficacy in treating non-cancerous conditions, including certain kidney diseases. Current studies have identified the renoprotective effect of CDK4/6 inhibitors, presenting a novel idea and potential direction for treating kidney diseases in the future. In this review, we briefly reviewed the cell cycle in mammals and the role of CDK4/6 in regulating it. We then provided an introduction to CDK4/6 inhibitors and their use in cancer treatment. Additionally, we emphasized the importance of these inhibitors in the treatment of kidney diseases. Collectively, growing evidence demonstrates that targeting CDK4 and CDK6 through CDK4/6 inhibitors might have therapeutic benefits in various cancers and kidney diseases and should be further explored in the future.

Ustiloxin A inhibits proliferation of renal tubular epithelial cells in vitro and induces renal injury in mice by disrupting structure and respiratory function of mitochondria

Recently, we found that Ustiloxin A (UA, a mycotoxin) was widely detected in paddy environment and rice samples from several countries, and was also detected in human urine samples from China. However, the current knowledge about the health risks of UA are limited. In this research, the cytotoxicity of UA in mice renal tubular epithelial cells (mRTECs) was evaluated, and the results indicated that UA arrested cell cycle in G2/M phase via altering cellular morphology and microtubule, and inhibited the proliferation and division of mRTECs. Furthermore, UA could inhibit mitochondrial respiration via binding to the CoQ-binding site in dihydro-orotate dehydrogenase (DHODH) protein, and resulted in mitochondrial damage. These adverse effects of UA on mitochondria might be responsible for the cytotoxicity observed in vitro. In vivo, UA at concentrations that were comparable to the realistic concentrations of human exposure induced renal insufficiency in mice, and this might be associated with the renal mitochondrial damage in mice. However, exposure to UA at those realistic concentrations did not promote the progression from renal insufficiency to renal fibrosis and chronic kidney disease was not observed in mice.

PFKFB3 mediates tubular cell death in cisplatin nephrotoxicity by activating CDK4

Nephrotoxicity is a major side effect of cisplatin, a widely used cancer therapy drug. However, the mechanism of cisplatin nephrotoxicity remains unclear and no effective kidney protective strategies are available. Here, we report the induction of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3) in both in vitro cell culture and in vivo mouse models of cisplatin nephrotoxicity. Notably, PFKFB3 was mainly induced in the nucleus of kidney tubular cells, suggesting a novel function other than its canonical role in glycolysis. Both pharmacological inhibition and genetic silencing of PFKFB3 led to the suppression of cisplatin-induced apoptosis in cultured renal proximal tubular cells (RPTCs). Moreover, cisplatin-induced kidney injury or nephrotoxicity was ameliorated in renal proximal tubule-specific PFKFB3 knockout mice. Mechanistically, we demonstrated the interaction of PFKFB3 with cyclin-dependent kinase 4 (CDK4) during cisplatin treatment, resulting in CDK4 activation and consequent phosphorylation and inactivation of retinoblastoma tumor suppressor (Rb). Inhibition of CDK4 reduced cisplatin-induced apoptosis in RPTCs and kidney injury in mice. Collectively, this study unveils a novel pathological role of PFKFB3 in cisplatin nephrotoxicity through the activation of the CDK4/Rb pathway, suggesting a new kidney protective strategy for cancer patients by blocking PFKFB3.

Cisplatin nephrotoxicity: new insights and therapeutic implications

Cisplatin is an effective chemotherapeutic agent for various solid tumours, but its use is limited by adverse effects in normal tissues. In particular, cisplatin is nephrotoxic and can cause acute kidney injury and chronic kidney disease. Preclinical studies have provided insights into the cellular and molecular mechanisms of cisplatin nephrotoxicity, which involve intracellular stresses including DNA damage, mitochondrial pathology, oxidative stress and endoplasmic reticulum stress. Stress responses, including autophagy, cell-cycle arrest, senescence, apoptosis, programmed necrosis and inflammation have key roles in the pathogenesis of cisplatin nephrotoxicity. In addition, emerging evidence suggests a contribution of epigenetic changes to cisplatin-induced acute kidney injury and chronic kidney disease. Further research is needed to determine how these pathways are integrated and to identify the cell type-specific roles of critical molecules involved in regulated necrosis, inflammation and epigenetic modifications in cisplatin nephrotoxicity. A number of potential therapeutic targets for cisplatin nephrotoxicity have been identified. However, the effects of renoprotective strategies on the efficacy of cisplatin chemotherapy needs to be thoroughly evaluated. Further research using tumour-bearing animals, multi-omics and genome-wide association studies will enable a comprehensive understanding of the complex cellular and molecular mechanisms of cisplatin nephrotoxicity and potentially lead to the identification of specific targets to protect the kidney without compromising the chemotherapeutic efficacy of cisplatin.

Interplay between extracellular matrix components and cellular and molecular mechanisms in kidney fibrosis

Chronic kidney disease (CKD) is characterized by pathological accumulation of extracellular matrix (ECM) proteins in renal structures. Tubulointerstitial fibrosis is observed in glomerular diseases as well as in the regeneration failure of acute kidney injury (AKI). Therefore, finding antifibrotic therapies comprises an intensive research field in Nephrology. Nowadays, ECM is not only considered as a cellular scaffold, but also exerts important cellular functions. In this review, we describe the cellular and molecular mechanisms involved in kidney fibrosis, paying particular attention to ECM components, profibrotic factors and cell-matrix interactions. In response to kidney damage, activation of glomerular and/or tubular cells may induce aberrant phenotypes characterized by overproduction of proinflammatory and profibrotic factors, and thus contribute to CKD progression. Among ECM components, matricellular proteins can regulate cell-ECM interactions, as well as cellular phenotype changes. Regarding kidney fibrosis, one of the most studied matricellular proteins is cellular communication network-2 (CCN2), also called connective tissue growth factor (CTGF), currently considered as a fibrotic marker and a potential therapeutic target. Integrins connect the ECM proteins to the actin cytoskeleton and several downstream signaling pathways that enable cells to respond to external stimuli in a coordinated manner and maintain optimal tissue stiffness. In kidney fibrosis, there is an increase in ECM deposition, lower ECM degradation and ECM proteins cross-linking, leading to an alteration in the tissue mechanical properties and their responses to injurious stimuli. A better understanding of these complex cellular and molecular events could help us to improve the antifibrotic therapies for CKD.

Acute Kidney Injury is Aggravated in Aged Mice by the Exacerbation of Proinflammatory Processes

Acute kidney injury (AKI) is more frequent in elderly patients. Mechanisms contributing to AKI (tubular cell death, inflammatory cell infiltration, impaired mitochondrial function, and prolonged cell-cycle arrest) have been linked to cellular senescence, a process implicated in regeneration failure and progression to fibrosis. However, the molecular and pathological basis of the age-related increase in AKI incidence is not completely understood. To explore these mechanisms, experimental AKI was induced by folic acid (FA) administration in young (3-months-old) and old (1-year-old) mice, and kidneys were evaluated in the early phase of AKI, at 48 h. Tubular damage score, KIM-1 expression, the recruitment of infiltrating immune cells (mainly neutrophils and macrophages) and proinflammatory gene expression were higher in AKI kidneys of old than of young mice. Tubular cell death in FA-AKI involves several pathways, such as regulated necrosis and apoptosis. Ferroptosis and necroptosis cell-death pathways were upregulated in old AKI kidneys. In contrast, caspase-3 activation was only found in young but not in old mice. Moreover, the antiapoptotic factor BCL-xL was significantly overexpressed in old, injured kidneys, suggesting an age-related apoptosis suppression. AKI kidneys displayed evidence of cellular senescence, such as increased levels of cyclin dependent kinase inhibitors p16ink4a and p21cip1, and of the DNA damage response marker γH2AX. Furthermore, p21cip1 mRNA expression and nuclear staining for p21cip1 and γH2AX were higher in old than in young FA-AKI mice, as well as the expression of senescence-associated secretory phenotype (SASP) components (Il-6, Tgfb1, Ctgf, and Serpine1). Interestingly, some infiltrating immune cells were p21 or γH2AX positive, suggesting that molecular senescence in the immune cells (“immunosenescence”) are involved in the increased severity of AKI in old mice. In contrast, expression of renal protective factors was dramatically downregulated in old AKI mice, including the antiaging factor Klotho and the mitochondrial biogenesis driver PGC-1α. In conclusion, aging resulted in more severe AKI after the exposure to toxic compounds. This increased toxicity may be related to magnification of proinflammatory-related pathways in older mice, including a switch to a proinflammatory cell death (necroptosis) instead of apoptosis, and overactivation of cellular senescence of resident renal cells and infiltrating inflammatory cells.

Modulation of calcium-binding proteins expression and cisplatin chemosensitivity by calcium chelation in human breast cancer MCF-7 cells

Cisplatin (CDDP) is currently one of the most effective FDA-approved treatments for breast cancer. Previous studies have shown that CDDP-induced cell death in human breast cancer (MCF-7) cells is associated with disruption of calcium homeostasis. However, whether the sensitivity of breast cancer cells to cisplatin is associated with dysregulation of the expression of calcium-binding proteins (CaBPs) remains unknown. In this study, we evaluated the effect of the intracellular calcium chelator (BAPTA-AM) on viability of MCF-7 cells in the presence of toxic and sub-toxic doses of cisplatin. Furthermore, this study assessed the expression of CaBPs, calmodulin, S100A8, and S100A14 in MCF-7 cells treated with cisplatin. Cell viability was determined using MTT-based in vitro toxicity assay. Intracellular calcium imaging was done using Fluo-4 AM, a cell-permeant fluorescent calcium indicator. Expression of CaBPs was tested using real-time quantitative PCR. Exposure of cells to increasing amounts of CDDP correlated with increasing fluorescence of the intracellular calcium indicator, Fluo-4 AM. Conversely, treating cells with cisplatin significantly decreased mRNA levels of calmodulin, S100A8, and S100A14. Treatment of the cells with calcium chelator, BAPTA-AM, significantly enhanced the cytotoxic effects of sub-toxic dose of cisplatin. Our results indicated a statistically significant negative correlation between calmodulin, S100A8, and S100A14 expression and sensitivity of breast cancer cells to a sub-toxic dose of cisplatin. We propose that modulating the activity of calcium-binding proteins, calmodulin, S100A8, and S100A14, could be used to increase cisplatin efficacy, lowering its treatment dosage while maintaining its chemotherapeutic value.

Negative feedback by conserved kinases patterns the degradation of Caenorhabditis elegans Raf in vulval fate patterning

Activation of a canonical EGFR-Ras-Raf-ERK cascade initiates patterning of multipotent vulval precursor cells (VPCs) of Caenorhabditis elegans We have previously shown that this pathway includes a negative-feedback component in which MPK-1/ERK activity targets the upstream kinase LIN-45/Raf for degradation by the SEL-10/FBXW7 E3 ubiquitin ligase. This regulation requires a Cdc4 phosphodegron (CPD) in LIN-45 that is conserved in BRAF. Here, we identify and characterize the minimal degron that encompasses the CPD and is sufficient for SEL-10-mediated, MPK-1-dependent protein degradation. A targeted screen of conserved protein kinase-encoding genes yielded gsk-3 (an ortholog of human GSK3B) and cdk-2 (a CDK2-related kinase) as required for LIN-45 degron-mediated turnover. Genetic analysis revealed that LIN-45 degradation is blocked at the second larval stage due to cell cycle quiescence, and that relief of this block during the third larval stage relies on activation of CDKs. Additionally, activation of MPK-1 provides spatial pattern to LIN-45 degradation but does not bypass the requirement for gsk-3 and cdk-2 This analysis supports a model whereby MPK-1/ERK, GSK-3/GSK3 and CDK-2/CDK2, along with SEL-10/FBXW7, constitute a regulatory network that exerts spatial and temporal control of LIN-45/Raf degradation during VPC patterning.

A glance at molecular mechanisms underlying cisplatin-induced nephrotoxicity and possible renoprotective strategies: a narrative review

Cisplatin is a platinum-based drug that is usually used for the treatment of many carcinomas. However, it comes with several devastating side effects, including nephrotoxicity. Cisplatin toxicity is a very complex process, which is exacerbated by the accumulation of cisplatin in renal tubular cells via passive diffusion and transporter-mediated processes. Once cisplatin enters these cells, it induces the formation of reactive oxygen species that cause cellular damage, including DNA damage, inflammation, and eventually cell death. On a small scale, these damages can be mitigated by cellular antioxidant defense mechanism. However, on a large scale, such as in chemotherapy, this defense mechanism may fail, resulting in nephrotoxicity. The current article reviews the molecular mechanisms underlying cisplatin-induced nephrotoxicity and possible renoprotective strategies to determine novel therapeutic interventions for alleviating this toxicity.

To control or to be controlled? Dual roles of CDK2 in DNA damage and DNA damage response

CDK2 (cyclin-dependent kinase 2), a member of the CDK family, has been shown to play a role in many cellular activities including cell cycle progression, apoptosis and senescence. Recently, accumulating evidence indicates that CDK2 is involved in DNA damage and DNA repair response (DDR). When DNA is damaged by internal or external genotoxic stresses, CDK2 activity is required for proper DNA repair in vivo and in vitro, whereas inactivation of CDK2 by siRNA techniques or by inhibitors could result in DNA damage and stimulate DDR. Hence, CDK2 seems to play dual roles in DNA damage and DDR. On one aspect, it is activated and stimulates DDR to repair DNA damage when DNA damage occurs; on the other hand, its inactivation directly leads to DNA damage and evokes DDR. Here, we describe the roles of CDK2 in DNA damage and DDR, and discuss the potential application of CDK2 inhibitors as anti-cancer agents.

Predicting the Need for a Tier II Ototoxicity Study From Early Renal Function Data

History has established that many drugs, such as the antibiotics, chemotherapies, and loop diuretics, are capable of inducing both nephrotoxicity and ototoxicity. The exact mechanisms by which cellular damage occurs remain to be fully elucidated. Monitoring the indices of renal function conducted in the Food and Drug Administration's prescribed set of early investigational new drug (IND)-enabling studies may be the first signs of ototoxicity properties of the new drug candidate. In developing improved and efficacious new molecular entities, it is critically necessary to understand the cellular and molecular mechanisms underlying the potential ototoxic effects as early in the drug development program as possible. Elucidation of these mechanisms will facilitate the development of safe and effective clinical approaches for the prevention and amelioration of drug-induced ototoxicity prior to the first dose in man. Biomarkers for nephrotoxicity in early tier I or tier II nonclinical IND-enabling studies should raise an inquiry as to the need to conduct a full auditory function assay early in the game to clear the pipeline with a safer candidate that has a higher probability of continued therapeutic compliance once approved for distribution.

Casein kinase 1ε and 1α as novel players in polycystic kidney disease and mechanistic targets for (R)-roscovitine and (S)-CR8

Following the discovery of (R)-roscovitine's beneficial effects in three polycystic kidney disease (PKD) mouse models, cyclin-dependent kinases (CDKs) inhibitors have been investigated as potential treatments. We have used various affinity chromatography approaches to identify the molecular targets of roscovitine and its more potent analog (S)-CR8 in human and murine polycystic kidneys. These methods revealed casein kinases 1 (CK1) as additional targets of the two drugs. CK1ε expression at the mRNA and protein levels is enhanced in polycystic kidneys of 11 different PKD mouse models as well as in human polycystic kidneys. A shift in the pattern of CK1α isoforms is observed in all PKD mouse models. Furthermore, the catalytic activities of both CK1ε and CK1α are increased in mouse polycystic kidneys. Inhibition of CK1ε and CK1α may thus contribute to the long-lasting attenuating effects of roscovitine and (S)-CR8 on cyst development. CDKs and CK1s may constitute a dual therapeutic target to develop kinase inhibitory PKD drug candidates.

A phosphorylation-motif for tuneable helix stabilisation in intrinsically disordered proteins - Lessons from the sodium proton exchanger 1 (NHE1)

Intrinsically disordered proteins (IDPs) are involved in many pivotal cellular processes including phosphorylation and signalling. The structural and functional effects of phosphorylation of IDPs remain poorly understood and difficult to predict. Thus, a need exists to identify motifs that confer phosphorylation-dependent perturbation of the local preferences for forming e.g. helical structures as well as motifs that do not. The disordered distal tail of the Na⁺/H⁺ exchanger 1 (NHE1) is six-times phosphorylated (S693, S723, S726, S771, T779, S785) by the mitogen activated protein kinase 2 (MAPK1, ERK2). Using NMR spectroscopy, we found that two out of those six phosphorylation sites had a stabilizing effect on transient helices. One of these was further investigated by circular dichroism and NMR spectroscopy as well as by molecular dynamic simulations, which confirmed the stabilizing effect and resulted in the identification of a short linear motif for helix stabilisation: [S/T]-P-{3}-[R/K] where [S/T] is the phosphorylation-site. By analysing IDP and phosphorylation site databases we found that the motif is significantly enriched around known phosphorylation sites, supporting a potential wider-spread role in phosphorylation-mediated regulation of intrinsically disordered proteins. The identification of such motifs is important for understanding the molecular mechanism of cellular signalling, and is crucial for the development of predictors for the structural effect of phosphorylation; a tool of relevance for understanding disease-promoting mutations that for example interfere with signalling for instance through constitutive active and often cancer-promoting signalling.

Ochratoxin A induced premature senescence in human renal proximal tubular cells

Ochratoxin A (OTA) has many nephrotoxic effects and is a promising compound for the study of nephrotoxicity. Human renal proximal tubular cells (HKC) are an important model for the study of renal reabsorption, renal physiology and pathology. Since the induction of OTA in renal senescence is largely unknown, whether OTA can induce renal senescence, especially at a sublethal dose, and the mechanism of OTA toxicity remain unclear. In our study, a sublethal dose of OTA led to an enhanced senescent phenotype, β-galactosidase staining and senescence associated secretory phenotype (SASP). Cell cycle arrest and cell shape alternations also confirmed senescence. In addition, telomere analysis by RT-qPCR allowed us to classify OTA-induced senescence as a premature senescence. Western blot assays showed that the p53-p21 and the p16-pRB pathways and the ezrin-associated cell spreading changes were activated during the OTA-induced senescence of HKC. In conclusion, our results demonstrate that OTA promotes the senescence of HKC through the p53-p21 and p16-pRB pathways. The understanding of the mechanisms of OTA-induced senescence is critical in determining the role of OTA in cytotoxicity and its potential carcinogenicity.

The cell cycle biomarkers: promising research, but do not oversell them

This review focuses on the most recent scientific and clinical information on the development and clinical applicability of the cell cycle biomarkers TIMP-2 and IGFBP-7 in the diagnosis and prognosis of patients at risk for and suffering from acute kidney injury (AKI). A number of evaluation studies have demonstrated that compared with existing biomarkers, urinary excretion of the product of both biomarkers, [TIMP-2]•[IGFBP-7], improved diagnostic performance in assessing the risk for AKI, predicting the need for renal replacement therapy, AKI-related complications and short- and long-term prognoses. The reference intervals for these biomarkers, measured by the recently approved NephroCheck test, have been determined in apparently healthy adults and those with stable chronic morbid conditions without AKI. This review recognizes that the combination of these two cell cycle arrest markers for the early detection of AKI is promising but concludes that its clinical impact is still unproved. Clinicians should understand the utility and limitations of this test before deciding whether to make it available at their institution.

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

Cisplatin is a widely used chemotherapeutic agent, yet its efficacy is limited by nephrotoxicity. The severity of nephrotoxicity is associated with the extent of kidney cell death. Previously, we found that cisplatin-induced kidney cell death was dependent on Cdk2 activation, and inhibition of Cdk2 protected cells from cisplatin-induced apoptosis. Using an in vitro kination assay, we showed that Cdk2 phosphorylated Bcl-xL, an anti-apoptotic member of Bcl-2 family proteins, at serine 73. We also found that this phosphorylated Bcl-xL participated in cell death, as a phosphomimetic mutant of Bcl-xL at the serine 73 site (S73D-Bcl-xL) activated caspases. We now find that S73D-Bcl-xL was cleaved at D61 and D76, which are putative caspase cleavage sites, to generate 15-kDa and 12-kDa fragments. Unlike full-length Bcl-xL, these cleavage products of Bcl-xL were previously reported to be pro-apoptotic. We sought to determine whether these Bcl-xL fragments were necessary for the induction of cell death by S73D-Bcl-xL. Mutation of these caspase cleavage sites prevented the formation of the 15-kDa and 12-kDa Bcl-xL cleavage products, but apoptosis still persisted in a S73D modified Bcl-xL. Our findings show that Cdk2 phosphorylation of Bcl-xL at Ser73, but not the Bcl-xL cleavage products, is necessary and sufficient to induce cell death.

Cdk2 phosphorylation of Bcl-xL after stress converts it to a pro-apoptotic protein mimicking Bax/Bak

Apoptosis is a regulated form of cell death that proceeds by defined biochemical pathways. Most apoptosis is controlled by interactions between pro-survival and pro-apoptotic Bcl-2 family proteins in which death is often the consequence of permeabilization of the mitochondrial outer membrane. Many drugs affect this equilibrium to favor apoptosis but this process is not completely understood. We show that the chemotherapeutic drug cisplatin initiates an apoptotic pathway by phosphorylation of a pro-survival Bcl-2 family member, Bcl-xL, by cyclin-dependent kinase 2. The phosphorylation occurred at a previously unreported site and its biologic significance was demonstrated by a phosphomimetic modification of Bcl-xL that was able to induce apoptosis without addition of cisplatin. The mechanism of cell death induction was similar to that initiated by pro-apoptotic Bcl-2 family proteins, that is, phosphorylated Bcl-xL translocated to the mitochondrial membrane, and formed pores in the membrane. This initiated cytochrome c release and caspase activation that resulted in cell death.

DNA damage response in cisplatin-induced nephrotoxicity

Cisplatin and its derivatives are widely used chemotherapeutic drugs for cancer treatment. However, they have debilitating side effects in normal tissues and induce ototoxicity, neurotoxicity, and nephrotoxicity. In kidneys, cisplatin preferentially accumulates in renal tubular cells causing tubular cell injury and death, resulting in acute kidney injury (AKI). Recent studies have suggested that DNA damage and the associated DNA damage response (DDR) are an important pathogenic mechanism of AKI following cisplatin treatment. Activation of DDR may lead to cell cycle arrest and DNA repair for cell survival or, in the presence of severe injury, kidney cell death. Modulation of DDR may provide novel renoprotective strategies for cancer patients undergoing cisplatin chemotherapy.

Tissue Inhibitor Metalloproteinase-2 (TIMP-2)⋅IGF-Binding Protein-7 (IGFBP7) Levels Are Associated with Adverse Long-Term Outcomes in Patients with AKI

Tissue inhibitor metalloproteinase-2 (TIMP-2) and IGF-binding protein-7 (IGFBP7) have been validated for risk stratification in AKI. However, the association of urinary TIMP-2 and IGFBP7 with long-term outcomes is unknown. We evaluated the 9-month incidence of a composite end point of all-cause mortality or the need for RRT in a secondary analysis of a prospective observational international study of critically ill adults. Two predefined [TIMP-2]⋅[IGFBP7] cutoffs (0.3 for high sensitivity and 2.0 for high specificity) for the development of AKI were evaluated. Cox proportional hazards models were used to determine risk for the composite end point. Baseline [TIMP-2]⋅[IGFBP7] values were available for 692 subjects, of whom 382 (55.2%) subjects developed stage 1 AKI (defined by Kidney Disease Improving Global Outcomes guidelines) within 72 hours of enrollment and 217 (31.4%) subjects met the composite end point. Univariate analysis showed that [TIMP-2]⋅[IGFBP7]>2.0 was associated with increased risk of the composite end point (hazard ratio [HR], 2.11; 95% confidence interval [95% CI], 1.37 to 3.23; P<0.001). In a multivariate analysis adjusted for the clinical model, [TIMP-2]⋅[IGFBP7] levels>0.3 were associated with death or RRT only in subjects who developed AKI (compared with levels≤0.3: HR, 1.44; 95% CI, 1.00 to 2.06 for levels>0.3 to ≤2.0; P=0.05 and HR, 2.16; 95% CI, 1.32 to 3.53 for levels>2.0; P=0.002). In conclusion, [TIMP-2]⋅[IGFBP7] measured early in the setting of critical illness may identify patients with AKI at increased risk for mortality or receipt of RRT over the next 9 months.

CDK1-dependent inhibition of the E3 ubiquitin ligase CRL4CDT2 ensures robust transition from S Phase to Mitosis

Replication-coupled destruction of a cohort of cell cycle proteins ensures efficient and precise genome duplication. Three proteins destroyed during replication via the CRL4(CDT2) ubiquitin E3 ligase, CDT1, p21, and SET8 (PR-SET7), are also essential or important during mitosis, making their reaccumulation after S phase a critical cell cycle event. During early and mid-S phase and during DNA repair, proliferating cell nuclear antigen (PCNA) loading onto DNA (PCNA(DNA)) triggers the interaction between CRL4(CDT2) and its substrates, resulting in their degradation. We have discovered that, beginning in late S phase, PCNA(DNA) is no longer sufficient to trigger CRL4(CDT2)-mediated degradation. A CDK1-dependent mechanism that blocks CRL4(CDT2) activity by interfering with CDT2 recruitment to chromatin actively protects CRL4(CDT2) substrates. We postulate that deliberate override of replication-coupled destruction allows anticipatory accumulation in late S phase. We further show that (as for CDT1) de novo SET8 reaccumulation is important for normal mitotic progression. In this manner, CDK1-dependent CRL4(CDT2) inactivation contributes to efficient transition from S phase to mitosis.

Cell cycle arrest in a model of colistin nephrotoxicity

Colistin (polymixin E) is an antibiotic prescribed with resurging frequency for multidrug resistant gram negative bacterial infections. It is associated with nephrotoxicity in humans in up to 55% of cases. Little is known regarding genes involved in colistin nephrotoxicity. A murine model of colistin-mediated kidney injury was developed. C57/BL6 mice were administered saline or colistin at a dose of 16 mg/kg/day in 2 divided intraperitoneal doses and killed after either 3 or 15 days of colistin. After 15 days, mice exposed to colistin had elevated blood urea nitrogen (BUN), creatinine, and pathologic evidence of acute tubular necrosis and apoptosis. After 3 days, mice had neither BUN elevation nor substantial pathologic injury; however, urinary neutrophil gelatinase-associated lipocalin was elevated (P = 0.017). An Illumina gene expression array was performed on kidney RNA harvested 72 h after first colistin dose to identify differentially expressed genes early in drug treatment. Array data revealed 21 differentially expressed genes (false discovery rate < 0.1) between control and colistin-exposed mice, including LGALS3 and CCNB1. The gene signature was significantly enriched for genes involved in cell cycle proliferation. RT-PCR, immunoblot, and immunostaining validated the relevance of key genes and proteins. This murine model offers insights into the potential mechanism of colistin-mediated nephrotoxicity. Further studies will determine whether the identified genes play a causative or protective role in colistin-induced nephrotoxicity.

Assembly, activation, and substrate specificity of cyclin D1/Cdk2 complexes

Previous studies have shown conflicting data regarding cyclin D1/cyclin-dependent kinase 2 (Cdk2) complexes, and considering the widespread overexpression of cyclin D1 in cancer, it is important to fully understand their relevance. While many have shown that cyclin D1 and Cdk2 form active complexes, others have failed to show activity or association. Here, using a novel p21-PCNA fusion protein as well as p21 mutant proteins, we show that p21 is a required scaffolding protein, with cyclin D1 and Cdk2 failing to complex in its absence. These p21/cyclin D1/Cdk2 complexes are active and also bind the trimeric PCNA complex, with each trimer capable of independently binding distinct cyclin/Cdk complexes. We also show that increased p21 levels due to treatment with chemotherapeutic agents result in increased formation and kinase activity of cyclin D1/Cdk2 complexes, and that cyclin D1/Cdk2 complexes are able to phosphorylate a number of substrates in addition to Rb. Nucleophosmin and Cdh1, two proteins important for centrosome replication and implicated in the chromosomal instability of cancer, are shown to be phosphorylated by cyclin D1/Cdk2 complexes. Additionally, polypyrimidine tract binding protein-associated splicing factor (PSF) is identified as a novel Cdk2 substrate, being phosphorylated by Cdk2 complexed with either cyclin E or cyclin D1, and given the many functions of PSF, it could have important implications on cellular activity.

Acute kidney injury and chronic kidney disease: an integrated clinical syndrome

The previous conventional wisdom that survivors of acute kidney injury (AKI) tend to do well and fully recover renal function appears to be flawed. AKI can cause end-stage renal disease (ESRD) directly, and increase the risk of developing incident chronic kidney disease (CKD) and worsening of underlying CKD. In addition, severity, duration, and frequency of AKI appear to be important predictors of poor patient outcomes. CKD is an important risk factor for the development and ascertainment of AKI. Experimental data support the clinical observations and the bidirectional nature of the relationships between AKI and CKD. Reductions in renal mass and nephron number, vascular insufficiency, cell cycle disruption, and maladaptive repair mechanisms appear to be important modulators of progression in patients with and without coexistent CKD. Distinction between AKI and CKD may be artificial. Consideration should be given to the integrated clinical syndrome of diminished GFR, with acute and chronic stages, where spectrum of disease state and outcome is determined by host factors, including the balance of adaptive and maladaptive repair mechanisms over time. Physicians must provide long-term follow-up to patients with first episodes of AKI, even if they presented with normal renal function.