P glycoprotein-mediated cholesterol cycling determines proximal tubular cell viability

Preparation and anti-tumor effects of mesoporous silica nanoparticles loaded with trifluoperazine

We have developed a targeted nano-drug delivery system that effectively harnesses the anti-tumor properties of trifluoperazine (TFP), while concurrently mitigating its side effects on the central nervous system. The manufacturing process entailed the preparation of mesoporous silica nanoparticles (MSN-NH2), followed by the loading of trifluoperazine into the pores of MSN-NH2 and then surface modification with polyethylene glycol (PEG) and anisamide (AA), resulting in the formation of TFP@MSN@PEG-AA (abbreviated as TMPA) nanoparticles. In vitro and in vivo anti-tumor activity and hemolysis experiments showed that TMPA had an excellent safety profile and a good anti-tumor effect. Importantly, the drug content of the TMPA nanoparticle group was found to be significantly lower than that of the TFP group in the mouse brain tissue as determined by High Performance Liquid Chromatography (HPLC) detection. Therefore, the developed drug delivery system achieved the goal of maintaining TFP's anti-tumor action while avoiding its negative effects on the central nervous system.

'Biologic memory' in response to acute kidney injury: cytoresistance, toll-like receptor hyper-responsiveness and the onset of progressive renal disease

Following the induction of ischemic or toxin-mediated acute kidney injury (AKI), cellular adaptations occur that 're-program' how the kidney responds to future superimposed insults. This re-programming is not simply a short-lived phenomenon; rather it can persist for many weeks, implying that a state of 'biologic memory' has emerged. These changes can be both adaptive and maladaptive in nature and they can co-exist in time. A beneficial adaptation is the emergence of acquired cytoresistance, whereby a number of physiologic responses develop that serve to protect the kidney against further ischemic or nephrotoxic attack. Conversely, some changes are maladaptive, such as a predisposition to Gram-negative or Gram-positive bacteremia due to a renal tubular up-regulation of toll-like receptor responses. This latter change culminates in exaggerated cytokine production, and with efflux into the systemic circulation, extra-renal tissue injury can result (so-called 'organ cross talk'). Another maladaptive response is a persistent up-regulation of pro-inflammatory, pro-fibrotic and vasoconstrictive genes, culminating in progressive renal injury and ultimately end-stage renal failure. The mechanisms by which this biologic re-programming, or biologic memory, is imparted remain subjects for considerable debate. However, injury-induced, and stable, epigenetic remodeling at pro-inflammatory/pro-fibrotic genes seems likely to be involved. The goal of this editorial is to highlight that the so-called 'maintenance phase' of acute renal failure is not a static one, somewhere between injury induction and the onset of repair. Rather, this period is one in which the induction of 'biologic memory' can ultimately impact renal functional recovery, extra-renal injury and the possible transition of AKI into chronic, progressive renal disease.

Efflux pump-mediated resistance in chemotherapy

Efflux pump mechanisms perform important physiological functions such as prevention of toxin absorption from the gastrointestinal tract, elimination of bile from the hepatocytes, effective functioning of the blood-brain barrier and placental barrier, and renal excretion of drugs. They exist in all living cells, but those in the bacterial and mammalian cells are more important to the clinician and pharmacologist, as they constitute an important cause of antimicrobial drug resistance, which contributes to treatment failure, high medical bills, and increased mortality / morbidity. This review was aimed at highlighting the role of efflux pump mechanisms in microbial resistance to chemotherapeutic agents. It was also aimed to elucidate their structure and mechanisms of action so as to integrate the efflux pump mechanisms in the design and development of novel antimicrobial agents. Findings from previous studies and research on this subject assessed through Google search, Pubmed, Hinari websites, as well as standard textbooks on chemotherapy, provided the needed information in the process of this review. Efflux pump inhibitors are promising strategies for preventing and reverting efflux-mediated resistance to chemotherapeutic agents. They are usually employed as adjuncts in antimicrobial and cancer chemotherapy. Toxicity, more common with the older-generation inhibitors such as verapamil and reserpine, constitutes the greatest impediment to their clinical applications. No efflux pump inhibitor has been approved for routine clinical use, as a result of doubtful clinical efficacy and unacceptably high incidence of adverse effects, particularly inhibition of the P-450 drug metabolizing enzyme. At present, their applications are mainly restricted to epidemiological studies. Nonetheless, the search for efficacious and tolerable efflux pump inhibitors continues because of the potential benefits. There is a need to consider efflux pump substrate selectivity in the design and development of novel chemotherapeutic agents.

HMG-CoA reductase activation and urinary pellet cholesterol elevations in acute kidney injury

BACKGROUND AND OBJECTIVES

Experimental acute kidney injury (AKI) activates the HMG-CoA reductase (HMGCR) gene, producing proximal tubule cholesterol loading. AKI also causes sloughing of proximal tubular cell debris into tubular lumina. This study tested whether these two processes culminate in increased urinary pellet cholesterol content, and whether the latter has potential AKI biomarker utility.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Urine samples were collected from 29 critically ill patients with (n = 14) or without (n= 15) AKI, 15 patients with chronic kidney disease, and 15 healthy volunteers. Centrifuged urinary pellets underwent lipid extraction, and the extracts were assayed for cholesterol content (factored by membrane phospholipid phosphate content). In vivo HMGCR activation was sought by measuring levels of RNA polymerase II (Pol II), and of a gene activating histone mark (H3K4m3) at exon 1 of the HMGCR gene (chromatin immunoprecipitation assay of urine chromatin samples).

RESULTS

AKI+ patients had an approximate doubling of urinary pellet cholesterol content compared with control urine samples (versus normal; P < 0.001). The values significantly correlated (r, 0.5; P < 0.01) with serum, but not urine, creatinine concentrations. Conversely, neither critical illness without AKI nor chronic kidney disease raised pellet cholesterol levels. Increased HMGCR activity in the AKI+ patients was supported by three- to fourfold increased levels of Pol II, and of H3K4m3, at the HMGCR gene (versus controls or AKI- patients).

CONCLUSIONS

(1) Clinical AKI, like experimental AKI, induces HMGCR gene activation; (2) increased urinary pellet cholesterol levels result; and (3) urine pellet cholesterol levels may have potential AKI biomarker utility. The latter will require future testing in a large prospective trial.

Renal ischemia-induced cholesterol loading: transcription factor recruitment and chromatin remodeling along the HMG CoA reductase gene

Acute kidney injury evokes renal tubular cholesterol synthesis. However, the factors during acute kidney injury that regulate HMG CoA reductase (HMGCR) activity, the rate-limiting step in cholesterol synthesis, have not been defined. To investigate these factors, mice were subjected to 30 minutes of either unilateral renal ischemia or sham surgery. After 3 days, bilateral nephrectomy was performed and cortical tissue extracts were prepared. The recruitment of RNA polymerase II (Pol II), transcription factors (SREBP-1, SREBP-2, NF-kappaB, c-Fos, and c-Jun), and heat shock proteins (HSP-70 and heme oxygenase-1) to the HMGCR promoter and transcription region (start/end exons) were assessed by Matrix ChIP assay. HMGCR mRNA, protein, and cholesterol levels were determined. Finally, histone modifications at HMGCR were assessed. Ischemia/reperfusion (I/R) induced marked cholesterol loading, which corresponded with elevated Pol II recruitment to HMGCR and increased expression levels of both HMGCR protein and mRNA. I/R also induced the binding of multiple transcription factors (SREBP-1, SREBP-2, c-Fos, c-Jun, NF-kappaB) and heat shock proteins to the HMGCR promoter and transcription regions. Significant histone modifications (increased H3K4m3, H3K19Ac, and H2A.Z variant) at these loci were also observed but were not identified at either the 5' and 3' HMGCR flanking regions (+/-5000 bps) or at negative control genes (beta-actin and beta-globin). In conclusion, I/R activates the HMGCR gene via multiple stress-activated transcriptional and epigenetic pathways, contributing to renal cholesterol loading.

‘Endotoxin tolerance’: TNF-α hyper-reactivity and tubular cytoresistance in a renal cholesterol loading state

The term 'endotoxin tolerance' defines a state in which prior endotoxin (lipopolysaccharide (LPS)) exposure induces resistance to subsequent LPS attack. However, its characteristics within kidney have not been well defined. Hence, this study tested the impact of LPS 'preconditioning' (LPS-PC; 18 or 72 h earlier) on: (i) selected renal inflammatory mediators (tumor necrosis factor (TNF)-alpha, interleukin-10 (IL-10), monocyte chemotactic protein-1 (MCP-1), inducible nitric oxide synthase (iNOS), Toll-like receptor 4 (TLR4); protein or mRNA); (ii) cholesterol homeostasis (a stress reactant); and (iii) isolated proximal tubule (PT) vulnerability to hypoxia or membrane cholesterol (cholesterol oxidase/esterase) attack. Two hours post LPS injection, LPS-PC mice manifested reduced plasma TNF-alpha levels, consistent with systemic LPS tolerance. However, in kidney, paradoxical TNF-alpha hyper-reactivity (protein/mRNA) to LPS existed, despite normal TLR4 protein levels. PT TNF-alpha levels paralleled renal cortical results, implying that PTs were involved. LPS-PC also induced: (i) renal cortical iNOS, IL-10 (but not MCP-1) mRNA hyper-reactivity; (ii), PT cholesterol loading, and (iii) cytoresistance to hypoxia and plasma membrane cholesterol attack. A link between cholesterol homeostasis and cell LPS responsiveness was suggested by observations that cholesterol reductions in HK-2 cells (methylcyclodextrin), or reductions in HK-2 membrane fluidity (A2C), blunted LPS-mediated TNF-alpha/MCP-1 mRNA increases. In sum: (i) systemic LPS tolerance can be associated with renal hyper-responsiveness of selected components within the LPS signaling cascade (e.g., TNF-alpha, iNOS, IL-10); (ii) PT cytoresistance against hypoxic/membrane injury coexists; and (iii) LPS-induced renal/PT cholesterol accumulation may mechanistically contribute to each of these results.

Modulation of P-glycoprotein activity by cannabinoid molecules in HK-2 renal cells

1. Endogenous and synthetic cannabinoid molecules have been investigated as possible MDR-1/P-glycoprotein (P-gp) modulators in HK-2-immortalized renal cells, using calcein acetoxymethylester (calcein-AM) as a P-gp substrate. 2. Among the endocannabinoid molecules tested, anandamide (AEA), but not 2-arachidonoyl-glycerol (2-AG) or palmitoyl-ethanolamide (PEA), increased the intracellular fluorescence emitted by calcein, a metabolic derivative of the P-gp substrate calcein-AM, indicative of a reduction in transport capacity. 3. All the three synthetic cannabimimetics tested, that is, R-(+)-methanandamide (R(+)-MET), AM 251 and CP55,940 significantly increased calcein accumulation in the cytosol. 4. RT-PCR demonstrated that HK-2 cells do not express CB1 or CB2 cannabinoid receptors. 5. R(+)-MET, AM251 and CP55,940 were also evaluated as modulators of P-gp expression, by Western blot analysis. Only AM251 weakly enhanced the protein levels (by 1.2-fold) after a 4-day-long incubation with the noncytotoxic drug concentration 2 microM. 6. The present data provide the first evidence that the endocannabinoid AEA and different synthetic cannabinoids may inhibit the P-gp activity in vitro via a cannabinoid receptor-independent mechanism.

Role of multiple drug resistance protein 1 in neutral but not acidic glycosphingolipid biosynthesis

Transfection studies have implicated the multiple drug resistance pump, MDR1, as a glucosyl ceramide translocase within the Golgi complex (Lala, P., Ito, S., and Lingwood, C. A. (2000) J. Biol. Chem. 275, 6246-6251). We now show that MDR1 inhibitors, cyclosporin A or ketoconazole, inhibit neutral glycosphingolipid biosynthesis in 11 of 12 cell lines tested. The exception, HeLa cells, do not express MDR1. Microsomal lactosyl ceramide and globotriaosyl ceramide synthesis from endogenous or exogenously added liposomal glucosyl ceramide was inhibited by cyclosporin A, consistent with a direct role for MDR1/glucosyl ceramide translocase activity in their synthesis. In contrast, cellular ganglioside synthesis in the same cells, was unaffected by MDR1 inhibition, suggesting neutral and acid glycosphingolipids are synthesized from distinct precursor glycosphingolipid pools. Metabolic labeling in wild type and knock-out (MDR1a, 1b, MRP1) mouse fibroblasts showed the same loss of neutral glycosphingolipid (glucosyl ceramide, lactosyl ceramide) but not ganglioside (GM3) synthesis, confirming the proposed role for MDR1 translocase activity. Cryo-immunoelectron microscopy showed MDR1 was predominantly intracellular, largely in rab6-containing Golgi vesicles and Golgi cisternae, the site of glycosphingolipid synthesis. These studies identify MDR1 as the major glucosyl ceramide flippase required for neutral glycosphingolipid anabolism and demonstrate a previously unappreciated dichotomy between neutral and acid glycosphingolipid synthesis.

Proximal tubular cholesterol loading after mitochondrial, but not glycolytic, blockade

Diverse forms of injury cause proximal tubular cholesterol accumulation. However, underlying mechanisms in general, and those involved with ATP depletion injury in particular, remain poorly defined. To help elucidate this issue, cholesterol homeostasis and its determinants were assessed after partial ATP depletion states. Serum-exposed HK-2 cells were subjected to mild ATP depletion, induced by mitochondrial inhibition (antimycin A; AA) or glycolytic blockade (2-deoxyglucose; DG). Four or 18 h later, cell cholesterol levels, hydroxymethylglutaryl (HMG)-CoA reductase (HMGCR), the LDL receptor (LDL-R), and ABCA1/SR-B1 cholesterol transporters were assessed. AA and DG each induced mild, largely sublethal ATP depletion injury. Each also caused significant HMGCR increments and SR-B1 decrements and left ABCA1 intact. In contrast, only AA increased the LDL-R, and only AA evoked a cholesterol-loading state (approximately 25% up). One-half of this increase was statin inhibitable, and one-half could be blocked by serum deletion, implying that both synthetic and nonsynthetic (e.g., LDL-R transport) pathways were involved. The AA-induced HMGCR and LDL-R protein changes were paralleled by their mRNAs, suggesting the presence of altered transcriptional events. We conclude that 1) sublethal ATP depletion, whether induced by mitochondrial or glycolytic blockade, can upregulate HMGCR and decrease SR-B1, and these changes represent a previously unrecognized ATP depletion "phenotype"; 2) mitochondrial blockade can also upregulate the LDL-R and evoke a cholesterol-loading state; 3) the latter likely occurs via synthetic and transport pathways; and 4) the mitochondrion may be a critical, and previously unrecognized, determinant of postinjury cell cholesterol homeostasis, potentially by impacting the LDL-R.

Sepsis syndrome stimulates proximal tubule cholesterol synthesis and suppresses the SR-B1 cholesterol transporter

BACKGROUND

Previous studies demonstrate that renal cortical/proximal tubule cholesterol accumulation is part of the renal "stress response." The present study was performed to help define underlying mechanisms, using experimental sepsis as a test model.

METHODS

Male CD-1 mice and female low-density lipoprotein receptor (LDLR) knockout mice were injected with a heat-killed Escherichia coli suspension. Renal cortex and serum were obtained from these and control mice either 4, 6, or 18 hours later. Tissues samples were assayed for free cholesterol (FC), cholesteryl esters (CE), HMG CoA reductase (HMGCR) mRNA, and SR-B1 [the high-density lipoprotein (HDL) receptor/cholesterol transporter]. Statin effects on renal cortical HMGCR mRNA and FC/CE levels also were assessed. Finally, the impact of serum from septic versus normal mice on cultured proximal tubule (HK-2) cell cholesterol levels was assessed.

RESULTS

Sepsis induced approximately 30% and 300 to 500% increases in renal FC and CE content, respectively. Cholesterol accumulation was not blunted in LDLR-/- mice versus their controls. Statin therapy also did not alter sepsis-induced renal FC/CE accumulation. However, statin treatment exerted no discernible intra-renal activity (for example, no rise in renal HMGCR mRNA), despite significant extra-renal activity (25% reduction in serum cholesterol; 400% increase in hepatic HMGCR mRNA). HK-2 cells exposed to septic serum sustained a 40% cholesterol increase, compared to cells exposed to control serum. This response was completely statin inhibited, proving that de novo synthesis was involved. Sepsis markedly suppressed renal levels of SR-B1 (an FC efflux protein). Renal HMGCR mRNA did not fall despite sepsis triggered cholesterol loading, indicating a failure of negative feedback activity.

CONCLUSIONS

Sepsis-induced renal cholesterol accumulation is not simply an intrinsic renal response, since it can be enhanced by circulating "stress factors" that drive HMGCR activity. Sepsis also down-regulates SR-B1. Thus, decreased cell FC efflux, coupled with increased synthesis, may synergistically induce the post-sepsis cholesterol overload state.

The mevalonate pathway during acute tubular injury: selected determinants and consequences

Renal injury evokes tubular cholesterol accumulation, mediated in part by increased HMG CoA reductase (HMGCR) levels. The present study was undertaken to define potential molecular determinants of these changes and to ascertain the relative importance of increased cholesterol production versus mevalonate pathway-driven protein prenylation, on the emergence of the so-called postrenal injury "cytoresistant state." Cultured proximal tubule (HK-2) cells were subjected to Fe or ATP depletion injury, followed 1 to 24 hours later by assessments of: 1) sterol transcription factor expression (SREBP)-1 and -2); 2) HMGCR mRNA levels; and 3) Ras/Rho prenylation. HMGCR mRNA and Ras/Rho prenylation were also assessed after in vivo ischemic and Fe-mediated renal damage. Using specific inhibitors, the relative importance of protein prenylation versus terminal cholesterol synthesis on HK-2 cell susceptibility to injury was also assessed. Acute injury induced HK-2 cell SREBP disruption and reductions in HMGCR mRNA. Renal cortical HMGCR mRNA also fell in response to either in vivo ischemic or Fe-mediated oxidant damage. At 24 hours after in vitro/in vivo injury, a time of cholesterol buildup, no increase in Ras/Rho prenylation was observed. Prenylation inhibitors did not sensitize HK-2 cells to injury. Conversely, squalene synthase (terminal cholesterol synthesis) blockade sensitized HK-2 cells to both Fe and ATP depletion attack. We concluded that: 1) acute tubular cell injury can destroy SREBPs and lower HMGCR mRNA. This suggests that posttranscriptional/translational events are responsible for HMGCR enzyme and cholesterol accumulation after renal damage. 2) Injury-induced cholesterol accumulation appears dissociated from increased protein prenylation. 3) Cholesterol accumulation, per se, seems to be the dominant mechanism by which the mevalonate pathway contributes to the postrenal injury cytoresistant state.

Altered cholesterol localization and caveolin expression during the evolution of acute renal failure

BACKGROUND

Renal cortical/proximal tubule cholesterol accumulation, with preferential localization within plasma membrane "detergent resistant microdomains" (DRMs: rafts/caveolae), is a hallmark of the maintenance phase of acute renal failure (ARF). This study addressed two related issues: (1) Are maintenance-phase cholesterol increases accompanied by an up-regulation of caveolin, a DRM/caveolar-associated cholesterol binding protein? (2) Is DRM cholesterol/caveolin homeostasis acutely altered during the induction phase of ARF?

METHODS

Mouse kidneys were subjected to ischemia +/- reperfusion (I/R) followed by assessment of cholesterol DRM partitioning. Acute cell injury effects on potential caveolin release from isolated proximal tubules or into urine also were assessed. Finally, renal cortical/isolated proximal tubule caveolin levels were determined 18 hours after I/R or myoglobinuric ARF.

RESULTS

Acute ischemia causes a rapid shift of cholesterol into cortical DRMs (>22%). Cholesterol migration into DRMs also was observed in ATP-depleted cultured proximal tubule (HK-2) cells. Acute hypoxic or toxic tubule injury induced plasma membrane caveolin release (Western blot). By the maintenance phase of ARF, marked renal cortical/proximal tubule caveolin increases resulted.

CONCLUSIONS

Acute proximal tubular injury damages caveolar/DRM structures, as determined by cholesterol maldistribution and caveolin release. Post-injury, there is a dramatic up-regulation of renal cortical/proximal tubule caveolin, suggesting an increased caveolar mass. These findings indicate, to our knowledge for the first time, that dysregulation of caveolae/raft microdomain expression is a correlate of, and potential participant in, the induction and maintenance phases of ischemic and toxic forms of experimental ARF.