Altered expression and function of P-glycoprotein (170 kDa), encoded by the MDR 1 gene, in T cell subsets from aging humans

Impact of P-glycoprotein on intracellular drug concentration in peripheral blood mononuclear cells and K562 cells

P-glycoprotein (P-gp) expression in lymphocytes is variable and 2-fold higher in rheumatoid arthritis (RA) patients with treatment resistance than in healthy subjects. To date the information on P-gp-mediated drug interaction in lymphocyte is limited. We analyzed the importance on P-gp in lymphocytes using peripheral blood mononuclear cells (PBMCs) together with K562, K562/Adr, and K562/Vin cells, which have various P-gp levels, as cell models, and dexamethasone, nintedanib and apafant as weak to good P-gp substrates. P-gp levels in K562, K562/Adr, and K562/Vin cells were 0.3-, 20-, and 106-fold of healthy PBMCs, respectively. While cell accumulation of apafant and nintedanib decreased in all cells with increasing P-gp levels, dexamethasone accumulation in K562/Adr was comparable to that in healthy PBMCs and K562 cells. Cell accumulations of substrates in cells with low P-gp expression were not significantly changed by the P-gp inhibitors at therapeutic concentrations. However, accumulation increased to 1.4-fold at highest in K562/Adr cells with higher P-gp expression than in PBMCs of the RA patients. These results suggest P-gp controls the cellular concentration of P-gp substrates in PBMCs or K562 cells but cellular concentration of a weak P-gp substrate would not be apparently affected even in cells with a sufficient P-gp expression.

Expression of the Stem Cell Marker ABCB5 in Normal and Tumor Tissues

BACKGROUND/AIM

The ATP-binding cassette subfamily B member 5 (ABCB5) transporter plays a pivotal role in melanocyte progenitor cell fusion and has been identified as a tumor-initiating cell marker. In this study, we determined ABCB5 expression in normal tissues among various species, i.e., Homo sapiens, Mus musculus (mouse), Rattus norvegicus (rat), Sus scrofa domesticus (pig), Gallus gallus (chicken), Anser anser (goose), Poecilia reticulata (Guppy fish), and Lumbricus terrestris (earthworm), as well as 426 biopsies of different human tumor types (colorectal, cervical, endometrium, vaginal, nasopharyngeal, kidney, breast, colon, prostate, pancreas, lung, gallbladder, bladder, brain, liver, skin, small intestine, testis, tonsil, uterus, thyroid, stomach, esophagus, fallopian, parotid, and ovary).

MATERIALS AND METHODS

Using immunohistochemical staining, ABCB5 expression was detected and evaluated in formalin-fixed, paraffin-embedded sections.

RESULTS

High ABCB5 expression was found in normal tissues in specialized cells with secretory and excretory functions, chorionic villi of the placenta, hepatocytes, and blood-tissue barrier sites in the brain and testis. Besides, heterogeneous expression of ABCB5 was also observed in many different tumor types derived from breast, endometrium, ovary, uterus, cervix, prostate, lung, brain, colon, liver, nasopharynx, and others.

CONCLUSION

The localization of ABCB5 in different normal tissues suggests that this protein has an excretory pumping role for physiological metabolites and xenobiotics. This physiological role highlighted its possible impact on the development of multidrug resistance in tumors. Further studies are required to establish the possible clinical significance of ABCB5 as a predictive marker for drug resistance and as a prognostic marker for patient survival.

Renal tubular P-glycoprotein expression is reduced in plasma cell disorders

Background

P-glycoprotein (P-gp) transports many chemicals that vary greatly in their structure and function. It is normally expressed in renal proximal tubular cells. We hypothesized that P-gp expression influences light chain excretion. Therefore, we investigated whether renal tubular P-gp expression is altered in patients with plasma cell disorders.

Methods

We evaluated renal biopsy specimens from patients with plasma cell disorders (n = 16) and primary focal segmental glomerulosclerosis (the control group, n = 17). Biopsies were stained with an anti-P-gp antibody. Loss of P-gp expression was determined semi-quantitatively. Groups were compared for loss of P-gp expression, and clinical variables.

Results

P-gp expression loss was more severe in patients with plasma cell disorders than it was in those with glomerulonephritis (P = 0.021). In contrast, clinical and histological parameters including serum creatinine, level of urinary protein excretion, and interstitial fibrosis/tubular atrophy grade were not significantly different between the groups. P-gp expression loss increased with age in patients with plasma cell disorders (P = 0.071). This expression loss was not associated with serum creatinine, the level of urinary protein excretion or the interstitial fibrosis/tubular atrophy grade. There was no significant association between the severity of P-gp expression loss with the types and serum levels of light chains, isotypes and serum immunoglobulin levels.

Conclusion

Renal tubular P-gp expression is significantly down-regulated in patients with plasma cell disorders characterized by nephrotic range proteinuria. Additional studies are needed to determine whether reintroduction of renal tubular P-gp expression would mitigate the proximal tubular injury that is caused by free-light chains.

A subset of virus-specific CD161+ T cells selectively express the multidrug transporter MDR1 and are resistant to chemotherapy in AML

The establishment of long-lived pathogen-specific T cells is a fundamental property of the adaptive immune response. However, the mechanisms underlying long-term persistence of antigen-specific CD4⁺ T cells are not well-defined. Here we identify a subset of memory CD4⁺ T cells capable of effluxing cellular toxins, including rhodamine (Rho), through the multidrug efflux protein MDR1 (also known as P-glycoprotein and ABCB1). Drug-effluxing CD4⁺ T cells were characterized as CD161⁺CD95⁺CD45RA^-CD127^hiCD28⁺CD25^int cells with a distinct chemokine profile and a Th1-polarized pro-inflammatory phenotype. CD4⁺CD161⁺Rho-effluxing T cells proliferated vigorously in response to stimulation with anti-CD3/CD28 beads and gave rise to CD161^- progeny in vitro. These cells were also capable of self-renewal and maintained their phenotypic and functional characteristics when cultured with homeostatic cytokines. Multidrug-effluxing CD4⁺CD161⁺ T cells were enriched within the viral-specific Th1 repertoire of healthy donors and patients with acute myeloid leukemia (AML) and survived exposure to daunorubicin chemotherapy in vitro. Multidrug-effluxing CD4⁺CD161⁺ T cells also resisted chemotherapy-induced cytotoxicity in vivo and underwent significant expansion in AML patients rendered lymphopenic after chemotherapy, contributing to the repopulation of anti-CMV immunity. Finally, after influenza vaccination, the proportion of influenza-specific CD4⁺ T cells coexpressing CD161 was significantly higher after 2 years compared with 4 weeks after immunization, suggesting CD161 is a marker for long-lived antigen-specific memory T cells. These findings suggest that CD4⁺CD161⁺ T cells with rapid efflux capacity contribute to the maintenance of viral-specific memory T cells. These data provide novel insights into mechanisms that preserve antiviral immunity in patients undergoing chemotherapy and have implications for the development of novel immunotherapeutic approaches.

Aging of the CD4 T Cell Compartment

Higher morbidity and mortality following infections, particularly influenza, is observed in the elderly population. Because of this, people over 65 years old are often targeted for preventive immunization. Many vaccines, however, are not as effective in generating protective antibodies in older individuals. CD4⁺ T cells, through their B cell helper functions, play a central role in the humoral response. Aging has deleterious effects on the immune system, and understanding how aging impairs CD4⁺ T cell functions is of critical importance to design new immunization and treatment strategies targeted to the elderly population. In this paper, we review some of the qualitative and quantitative changes in the CD4⁺ T cell compartment that arise with aging. We also summarize the age-related intrinsic defects that impact naïve, memory and regulatory CD4⁺ T cell functions.

The role of cytokines in the regulation of drug disposition: extended functional pleiotropism?

INTRODUCTION

Drug disposition, metabolism and drug-drug interactions are important considerations for most drugs. Cytokines are integral to the successful resolution of many diseases. Data are emerging on a role for cytokines in regulation of the expression and activity of drug transporters and drug metabolising enzymes. Investigation of the interaction between pharmacological and immunological responses is key to understanding the complex relationships involved in patient response to therapy.

AREAS COVERED

Evidence detailing the ability of cytokines to regulate drug disposition and metabolism is reviewed in the context of different cell and tissue types. The literature search undertaken provides an overview of the current understanding of the interrelationship between pharmacological and immunological factors which may influence successful drug therapy.

EXPERT OPINION

Dysregulation of cytokines and cytokine networks is a hallmark of a number of diseases such as HIV and cancer. The mechanisms by which the immune system can influence drug disposition are relatively understudied but recent work has highlighted the necessity for examining its impact on pharmacokinetics and pharmacodynamics. A more comprehensive approach in clinical studies will allow better determination of the impact of cytokines on drug disposition. In addition, determining the mechanisms that underpin the differential effects of cytokines across different cell types will clarify the responses reported in these studies.

ABC transporters in human lymphocytes: expression, activity and role, modulating factors and consequences for antiretroviral therapies

IMPORTANCE OF THE FIELD

ATP-binding cassette (ABC) transporters are a superfamily of efflux pumps that transport numerous compounds across cell membranes. These transporters are located in various human tissues including peripheral blood cells, in particular lymphocytes, and present a high variability of expression and activity. This variability may affect the intracellular concentrations and efficacy of drugs acting within lymphocytes, such as antiretroviral drugs.

AREAS COVERED IN THIS REVIEW

This review focuses on the current knowledge about the expression, activity, roles and variability of ABC drug transporters in human lymphocytes. The identified modulating factors and their impact on the intracellular pharmacokinetics and efficacy of antiretroviral drugs are also detailed.

WHAT THE READER WILL GAIN

Controversial data regarding the expression, activity and sources of variability of ABC transporters in lymphocytes are discussed. The modulating factors and their pharmacological consequences regarding antiretroviral therapies are also provided.

TAKE HOME MESSAGE

Numerous studies have reported conflicting results regarding the expression and activity of ABC drug transporters in lymphocytes. Despite these discrepancies, which may partly result from heterogeneous analytical methods, ABCC1 appears to have the highest expression in lymphocytes and may thus play a predominant role in the resistance to antiretroviral drugs, particularly to protease inhibitors.

Selective depletion of CD4+CD25+Foxp3+ regulatory T cells by low-dose cyclophosphamide is explained by reduced intracellular ATP levels

CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells have been shown to play important roles in mediating cancer development. Although cyclophosphamide (CY) has shown promise as a drug to selectively target Treg cells with low-dose in vivo, the underlying molecular mechanism remains unclear. In this report, we provide evidence that ATP, the energy molecule and signal element, accounts for the selective depletion of Treg cells by low-dose CY. Relative to conventional T cells or other cell types, ATP levels were much lower in Treg cells. This was due to Treg cells that downregulate one microRNA, miR-142-3p, and upregulate ecto-nucleoside triphosphate diphosphohydrolase CD39. The transfection of miR-142-3p or the blockade of CD39 could increase intracellular ATP levels of Treg cells, consequently decreasing the sensitivity of Treg cells to low-dose CY. On the other hand, the transfection of miR-142-3p inhibitor or the addition of soluble CD39 to the cultured CD4(+)CD25(-) T cells resulted in the decrease of intracellular ATP levels and increase of sensitivity of conventional T cells to low-dose CY. Furthermore, we found that the low levels of ATP attenuated the synthesis of glutathione, leading to the decrease of CY detoxification, thus increasing the sensitivity of Treg cells to low-dose CY. Therefore, we here identify a molecular pathway through which low-dose CY selectively ablates Treg cells. Our findings also imply that low levels of ATP are probably related to Treg cell function.

ABC drug transporters and immunity: novel therapeutic targets in autoimmunity and cancer

ABC transporters were identified originally for their contribution to clinical MDR as a result of their capacity to extrude various unrelated cytotoxic drugs. More recent reports have shown that ABC transporters can play important roles in the development, differentiation, and maturation of immune cells and are involved in migration of immune effector cells to sites of inflammation. Many of the currently identified, endogenous ABC transporter substrates have immunostimulating effects. Increasing the expression of ABC transporters on immune cells and thereby enhancing immune cell development or functionality may be beneficial to immunotherapy in the field of oncology. On the contrary, in the treatment of autoimmune diseases, blockade of these transporters may prove beneficial, as it could dampen disease activity by compromising immune effector cell functions. This review will focus on the expression, regulation, and substrate specificity of ABC transporters in relation to functional activities of immune effector cells and discusses implications for the treatment of cancer on the one hand and autoimmune diseases on the other.

Antigen-independent generation of a unique CD4 T cell-subset with aging and its persistent unresponsiveness

Aging leads to a decline in the reactivity of CD4 T cells, in humans and mice. However, we have reported that not all CD4 T cells in aged mice were hyporesponsive, that is, a particular subset maintained the ability to mount a normal response. In this study, we examined the possibility of recalling reactivity in the hyporesponsive CD4 T cell-subset in aged mice. In vivo experiments revealed the changes in CD4 T cell-subsets in aged mice to be antigen-independent and aging-dependent. Once the CD4 T cells became hyporesponsive, they persistently exhibited a weak response. Furthermore, immunization with a co-stimulatory antibody had no effect on T cell-responses in aged mice, although it had a significant effect in young mice. As this hyporesponsive subset accounts for the majority of CD4 T cells in aged mice, it is important to elucidate the cause of the hyporesponsiveness.

Changes in the transcriptional profile of cardiac myocytes following green fluorescent protein expression

Green fluorescent protein (GFP) and its multiple forms, such as enhanced GFP (EGFP), have been widely used as marker proteins and for tracking purposes in many biological systems, including the heart and cardiac cell systems. Despite some concerns on its toxicity under certain circumstances, GFP remains amongst the most reliable and easy-to-use markers available. Using rat full genome DNA microarrays, we have investigated the broader consequences of adenoviral-driven GFP expression in cardiac myocytes. In our transcriptional profiling analysis, we set a threshold of a twofold change. We removed possible changes resulting from adenoviral infection by comparison with transcriptional profiles of cardiac myocytes with adenoviral-driven expression of an unrelated protein, the kinase MEK. Our analysis revealed changes in the expression of 212 genes. Of these genes, 174 were upregulated and 38 were downregulated following GFP expression. Many of these genes remain unannotated, but an evaluation of those with described functions for their resulting proteins indicated that many were involved in processes, including responses to stimuli/stress and signal transduction. Our analysis thus indicates the broader consequences of GFP expression in altering gene expression profiles in cardiac cells. Care should therefore be taken when using GFP expression as a control in gene expression studies.

ABCB1 (P-glycoprotein) but not ABCC1 (MRP1) is downregulated in peripheral blood mononuclear cells of spontaneously hypertensive rats

Although the kidney is a major target in hypertension, several studies have correlated important immune alterations with the development of hypertension in spontaneously hypertensive rats (SHR), like increased secretion of pro-inflammatory cytokines, inflammatory infiltration in kidneys and thymic atrophy. Because adenosine-triphosphate-binding cassette sub-family B member 1 (ABCB1; P-glycoprotein) and adenosine-triphosphate-binding cassette sub-family C member 1 (ABCC1; multidrug resistance protein 1), two proteins first described in multidrug resistant tumors, physiologically transport several immune mediators and are required for the adequate functioning of the immune system, we aimed to measure the expression and activity of these proteins in peripheral blood mononuclear cells (PBMC), thymocytes, and also kidneys of normotensive Wistar Kyoto rats and SHR. Our results showed that ABCB1, but not ABCC1, activity was diminished (nearly 50%) in PBMC. Moreover, Abcb1b gene was downregulated in PBMC and kidney of SHR and this was not counterbalanced by an upregulation of its homolog Abcb1a, suggesting that the diminished activity is due to downregulation of the gene. No alteration was detected in ABCB1 activity in SHR thymocytes, indicating that this downregulation occurs after lymphocytes leave the primary lymphoid organs. Even though it is not known at present which parameter(s) is(are) responsible for this downregulation, it may contribute for the altered immune response observed in hypertension and to possible altered drug disposition in hypertensive individuals, resulting in greater drug interaction and increased drug toxicity.

Expression of adenosine triphosphate-binding cassette (ABC) drug transporters in peripheral blood cells: relevance for physiology and pharmacotherapy

Adenosine triphosphate-binding cassette (ABC)-type transport proteins were initially described for their ability to reduce intracellular concentrations of anticancer compounds, thereby conferring drug resistance. In recent years, expression of this type of proteins has also been reported in numerous cell types under physiological conditions; here, these transporters are often reported to alter systemic and local drug disposition (e.g. in the brain or the gastrointestinal tract). In this context, peripheral blood cells have also been found to express several ABC-type transporters. While erythrocytes mainly express multidrug resistance protein (MRP) 1, MRP4 and MRP5, which are discussed with regard to their involvement in glutathione homeostasis (MRP1) and in the efflux of cyclic nucleotides (MRP4 and MRP5), leukocytes also express P-glycoprotein and breast cancer resistance protein. In the latter cell types, the main function of efflux transporters may be protection against toxins, as these cells demonstrate a very high turnover rate. In platelets, only two ABC transporters have been described so far. Besides MRP1, platelets express relatively high amounts of MRP4 not only in the plasma membrane but also in the membrane of dense granules, suggesting relevance for mediator storage. In addition to its physiological function, ABC transporter expression in these structures can be of pharmacological relevance since all systemic drugs reach their targets via circulation, thereby enabling interaction of the therapeutic agent with peripheral blood cells. Moreover, both intended effects and unwanted side effects occur in peripheral blood cells, and intracellular micropharmacokinetics can be affected by these transport proteins. The present review summarises the data available on expression of ABC transport proteins in peripheral blood cells.

Independent Regulation of ABCB1 and ABCC Activities in Thymocytes and Bone Marrow Mononuclear Cells during Aging

Aging modifies a number of functional and phenotypic parameters of cells from the immune system. In this study, the activities of two members of the superfamily of ATP-binding cassette (ABC) transport proteins, ABCB1 and ABCC (measured by rhodamine 123 efflux and Fluo-3 efflux respectively), were compared in murine bone marrow cells and thymocytes of young (3-4 weeks old), adult (2-3 months old) and old (18 months old) mice. ABCB1 activity was shown to be age regulated in murine bone marrow mononuclear cells and thymocytes. In the bone marrow, the increased amount of cells with ABCB1 activity observed in old mice was restricted to the c-kit(-)Sca-1(+) and c-kit(+)Sca-1(+) subpopulations. Only a small percentage of c-kit(+) cells in the thymus had ABCB1 activity, and this subpopulation increased with age. In the thymus, old age augmented this activity in the CD4(-) CD8(-) double-negative cells and in the CD4(+) and CD8(+) single-positive populations. The activity of another ABC transporter, the ABCC-related activity, was also modified by age in the bone marrow. However, the age-related increase was observed in the subpopulations were ABCB1 was not modified, namely the non-progenitor population (c-kit(-)Sca-1(-)cells) and c-kit(+)Sca-1(-) cells. Nearly, all thymocytes expressed the ABCC1 molecule in an active form and aging did not affect this pattern. This study demonstrates an independent upregulation of ABCB1 and ABCC activities during the aging process. The increases were observed in different subsets of cells but followed a developmentally regulated pattern. The functions played by these transporters and alterations in aging are discussed.

In vitro evaluation of human cytochrome P450 and P-glycoprotein-mediated metabolism of some phytochemicals in extracts and formulations of African potato

African potato (Hypoxis hemerocallidea, AP) is a traditional herbal medicine widely used as an immune booster and also for the treatment of various ailments such as urinary diseases, prostrate hypertrophy and cancer. Amongst the chemical components contained in AP, the norlignan glycoside, hypoxoside (HYP) is purported to be the most important phytochemical in terms of AP's medicinal value. Additional constituents in AP include the sterols, beta-sitosterol (BSS), stigmasterol (STG), and the stanol, stigmastanol (STN). The potential of extracts of AP, AP formulations as well as HYP, its aglycone rooperol (ROP) and the sterols to inhibit in vitro metabolism of drug marker substrates by human cytochrome P450 (CYP) enzymes such as CYP3A4, 3A5 and CYP19 were investigated. Samples were also assessed for their effect on drug transport proteins such as P-glycoprotein (P-gp). The effects on CYP-mediated metabolism were studied by fluorometric microtitre plate assay. The potential interaction with P-gp was investigated by measuring the efflux of the fluorescent dye rhodamine 123 (Rh 123) in the CaCo-2 (colon carcinoma) cell line. Various extracts of AP, AP formulations, only STG and the norlignans, in particular the aglycone ROP, exhibited inhibitory effects on CYP3A4-, 3A5- and 19-mediated metabolism. The extracts and the formulations that contained a significant amount of HYP showed high induction of P-gp compared to the positive control, ritonavir. Whilst extrapolation of the current in vitro findings to clinical effects may well be considered speculative, these in vitro data should be heeded as a signal of possible in vivo interactions. Appropriate measures are therefore necessary to explore the possibility of such in vitro-in vivo correlations.

Plasma LDL cholesterol has no impact on P-glycoprotein (MDR1/ABCB1) activity in human peripheral blood mononuclear cells

Several studies have demonstrated that the adenosine triphosphate-binding cassette transporter P-glycoprotein (P-gp) is at least partly located in cholesterol- and sphingolipid-enriched parts of the plasma membrane called "lipid rafts" and that modification of cellular cholesterol content has an impact on the activity of P-gp in vitro and ex vivo. Cholesterol modulation in vitro does not closely reflect the in vivo situation. The aim of our study was therefore to investigate whether differences in individual plasma low-density lipoprotein (LDL) cholesterol levels in humans have an impact on cholesterol content in peripheral blood mononuclear cells (PBMCs) and thereby on individual activity of P-gp. PBMCs of 20 ambulatory patients with elevated LDL cholesterol (173.9 +/- 22.4 mg/dl; range 151.0-234.4 mg/dl) and 28 controls (125.2 +/- 16.9 mg/dl; range 74.6-149.6 mg/dl) were isolated. Cellular cholesterol was measured by an enzymatic fluorimetric assay, efflux activity of P-gp in PBMCs was determined by a flow cytometric method (rhodamine123 efflux), and messenger ribonucleic acid expression was quantified by reverse transcriptase real-time polymerase chain reaction (RT-PCR). There was no difference in cellular cholesterol or P-gp activity between the two groups suggesting that high plasma LDL cholesterol concentration as observed in dyslipidemic patients does not correlate with cellular cholesterol content or P-gp activity in PBMCs. There was, however, a significant negative relationship between age and P-gp efflux activity indicating that P-gp activity in PBMCs decreases with advancing age. These results need further confirmation because investigation of age dependency of P-gp activity was not the primary aim of the study.

The impact of advancing age on P-glycoprotein expression and activity: current knowledge and future directions

With the progressive ageing of the population, an increasing number of elderly patients are being exposed to multiple drugs because of co-existing morbidity states requiring pharmacological management. However, the knowledge of the effects of ageing on the pharmacokinetics and pharmacodynamics of individual drugs is far from being optimal as very few elderly subjects are studied in Phase I and II premarketing trials. The efflux transporter P-glycoprotein has recently emerged as a major determinant of drug disposition in humans. This review discusses the current knowledge about the effects of advancing age on P-glycoprotein expression and activity. A critical appraisal of the published literature and an overview on future research directions are also provided.

Pharmacokinetic Considerations Relating to Tacrolimus Dosing in the Elderly

Relaxation of the upper age limits for solid organ transplantation coupled with improvements in post-transplant survival have resulted in greater numbers of elderly patients receiving immunosuppressant drugs such as tacrolimus. Tacrolimus is a potent agent with a narrow therapeutic window and large inter- and intraindividual pharmacokinetic variability. Numerous physiological changes occur with aging that could potentially affect the pharmacokinetics of tacrolimus and, hence, patient dosage requirements. Tacrolimus is primarily metabolised by cytochrome P450 (CYP) 3A enzymes in the gut wall and liver. It is also a substrate for P-glycoprotein, which counter-transports diffused tacrolimus out of intestinal cells and back into the gut lumen. Age-associated alterations in CYP 3A and P-glycoprotein expression and/or activity, along with liver mass and body composition changes, would be expected to affect the pharmacokinetics of tacrolimus in the elderly. However, interindividual variation in these processes may mask any changes caused by aging. More investigation is needed into the impact aging has on CYP and P-glycoprotein activity and expression. No single-dose, intense blood-sampling study has specifically compared the pharmacokinetics of tacrolimus across different patient age groups. However, five population pharmacokinetic studies, one in kidney, one in bone marrow and three in liver transplant recipients, have investigated age as a co-variate. None found a significant influence for age on tacrolimus bioavailability, volume of distribution or clearance. The number of elderly patients included in each study, however, was not documented and may have been only small. It is likely that inter- and intraindividual pharmacokinetic variability associated with tacrolimus increase in elderly populations. In addition to pharmacokinetic differences, donor organ viability, multiple co-morbidity, polypharmacy and immunological changes need to be considered when using tacrolimus in the elderly. Aging is associated with decreased immunoresponsiveness, a slower body repair process and increased drug adverse effects. Elderly liver and kidney transplant recipients are more likely to develop new-onset diabetes mellitus than younger patients. Elderly transplant recipients exhibit higher mortality from infectious and cardiovascular causes than younger patients but may be less likely to develop acute rejection. Elderly kidney recipients have a higher potential for chronic allograft nephropathy, and a single rejection episode can be more devastating. There is a paucity of information on optimal tacrolimus dosage and target trough concentration in the elderly. The therapeutic window for tacrolimus concentrations may be narrower. Further integrated pharmacokinetic-pharmacodynamic studies of tacrolimus are required. It would appear reasonable, based on current knowledge, to commence tacrolimus at similar doses as those used in younger patients. Maintenance dose requirements over the longer term may be lower in the elderly, but the increased variability in kinetics and the variety of factors that impact on dosage suggest that patient care needs to be based around more frequent monitoring in this age group.

The ABC Transporters MDR1 and MRP2: Multiple Functions in Disposition of Xenobiotics and Drug Resistance

ATP-binding cassette (ABC) transporters comprise one of the largest membrane bound protein families. They are involved in transport of numerous compounds. These proteins transport substrates against a concentration gradient with ATP hydrolysis as a driving force across the membrane. Mammalian ABC proteins have important physiological, pharmacological and toxicological functions including the transport of lipids, bile salts, drugs, toxic and environmental agents. The efflux pumps serve both as natural defense mechanisms and influence the bioavailability and disposition of drugs. In general terms, the transporters remove xenobiotics from the cellular environment. For example, in cancer cells, over expression of these molecules may confer to multidrug resistance against cytostatic drugs. In addition, based on diverse structural characteristics and a broad substrate specifity, ABC transport proteins alter the intracellular concentration of a variety of therapeutically used compounds and toxicologically relevant agents. We review the function of the human multidrug resistance protein MDR1, (P-glycoprotein, ABCB1) and the multidrug resistance protein MRP2 (ABCC2). We focus on four topics namely 1) structure and physiological functions of these transporters, 2) substrates e.g., drugs, xenotoxins, and environmental toxicants including their conjugates, 3) drug-drug interactions, and the role of chemosensitizers which may be able to reverse drug resistance, and 4) pharmacologically and toxicologically relevant genetic polymorphisms in transport proteins and their clinical implications.

In silico analysis of gene expression profiles in the olfactory mucosae of aging senescence-accelerated mice

We utilized high-density Affymetrix oligonucleotide arrays to investigate gene expression in the olfactory mucosae of near age-matched aging senescence-accelerated mice (SAM). The senescence-prone (SAMP) strain has a significantly shorter lifespan than does the senescence-resistant (SAMR) strain. To analyze our data, we applied biostatistical methods that included a correlation analysis to evaluate sources of methodologic and biological variability; a two-sided t-test to identify a subpopulation of Present genes with a biologically relevant P-value <0.05; and a false discovery rate (FDR) analysis adjusted to a stringent 5% level that yielded 127 genes with a P-value of <0.001 that were differentially regulated in near age-matched SAMPs (SAMP-Os; 13.75 months) compared to SAMRs (SAMR-Os, 12.5 months). Volcano plots related the variability in the mean hybridization signals as determined by the two-sided t-test to fold changes in gene expression. The genes were categorized into the six functional groups used previously in gene profiling experiments to identify candidate genes that may be relevant for senescence at the genomic and cellular levels in the aging mouse brain (Lee et al. [2000] Nat Genet 25:294-297) and in the olfactory mucosa (Getchell et al. [2003] Ageing Res Rev 2:211-243), which serves several functions that include chemosensory detection, immune barrier function, xenobiotic metabolism, and neurogenesis. Because SAMR-Os and SAMP-Os have substantially different median lifespans, we related the rate constant alpha in the Gompertz equation on aging to intrinsic as opposed to environmental mechanisms of senescence based on our analysis of genes modulated during aging in the olfactory mucosa.

P-glycoprotein decreases with T cell maturation but is not responsible for resistance to CD95-induced apoptosis

P-glycoprotein (Pgp) is a membrane transporter responsible for resistance to chemotherapy in cancer cells. Its presence in T cells is very well documented, but its function in the immune system is still poorly understood. Recent findings suggest that Pgp may be involved in regulating programmed cell death by inhibiting caspase 8 and caspase 3. Utilising antigenically-activated T cells and the physiologically relevant apoptotic ligand, membrane CD95-L, we have previously reported that while T cells are generally resistant to CD95-induced death at early stages of activation, their susceptibility to apoptosis increases with successive activation and clonal expansion. In this study we investigated whether changes in apoptotic susceptibility were related to T cell Pgp function. Results showed that Pgp expression and function in T cells decreases with maturation, with CD8 cells having the highest Pgp function. However, although Pgp function inversely correlated with caspase 3 activity, no difference was observed between apoptotic susceptible CD25- cells and resistant CD25+ cells. In addition sorting of cells with high and low Pgp function showed no correlation with apoptotic capability. Therefore, whilst Pgp modulates caspase activity, it is not responsible for resistance to apoptosis of early activated T cells nor the increased susceptibility observed at the later stages of maturation in antigenically activated cells.

The Effect of Age on P-Glycoprotein Expression and Function in the Fischer-344 Rat

We investigated the effect of age on P-glycoprotein (P-gp) expression and function in rat liver, intestine, kidney, and endothelial cells of the blood-brain barrier (BBB) and lymphocytes. Flow cytometric analysis was used to examine P-gp expression in lymphocytes from male Fischer-344 rats from three age groups (young at 3-4 months, intermediate at 13-14 months, and old at 25-26 months). In addition, P-gp function in lymphocytes was assessed by measuring the ability of the P-gp inhibitor verapamil to limit the efflux of the fluorescent P-gp substrate rhodamine 123. P-gp expression was evaluated in the remaining four tissues by Western blot analysis. The effect of age on P-gp expression was tissue-specific. Although lymphocytic and hepatic P-gp expression increased with age, renal P-gp content was lower in the old kidneys. No statistical difference was observed in P-gp expression in intestinal microsomes or in BBB cell lysates among the three age groups. P-gp function was also increased by 6- to 8-fold in lymphocytes from the old rats. When P-gp expression was compared with CYP3A expression in these rats (reported elsewhere in this journal), we found that P-gp expression increased with age, whereas CYP3A expression and activity declined in the old livers. The converse pattern was observed in the kidney. Thus, age-related changes in P-gp expression and function are likely to be tissue-specific, and these changes may be inversely related to differences in CYP3A expression.

Prolegomenon for Chronic Lymphocytic Leukaemia

Chronic lymphocytic leukaemia (CLL) is a unique lymphoproliferative disorder that scarcely occurs under the age of 40; thereafter the incidence of CLL increases exponentially with age. CLL is characterized by progressive expansion of malignant CD5+ME+ B-cell clone accompanied by a myriad of cellular and humoral immune defects. Each of them might be linked to different clinically manifested complications such as increasing rate of infections, autoimmune disorders and disturbed immune surveillance against tumour cells. We assume that CLL occurs as a consequence of age-dependent, genetically related functional restrictions of the thymic microenvironment in supporting common lymphoid progenitor cells (CD5+ME+CD4-CD8-) to differentiate into mature T-cell and B-cell descendants. In conjunction with genetic abnormalities developing in B-cell progenitors, presumably expressing P glycoprotein (Pgp+), we postulate that developmentally altered T-cell descendants, along with quantitative imbalance among CD4+, their subsets and CD8+ lymphocytes in the peripheral blood, play an important additional role in facilitating the malignant B-cell clone emergence and in modulating the CLL clinical evolution. Namely, imbalance of any of T-cell-mediated cell interactive homeostatic mechanisms accompanied by imbalance in the production of various cytokines might in CLL influence leukaemic B-cell growth by deregulating inducer (c-myc and p53) and/or suppressor (bcl-2 and mutant p53) oncogenes responsible for the promotion or suppression of B-cell mitogenesis that may in turn further contribute to their impaired differentiation and/or differentiation arrest. In conclusion, CLL might be interpreted as a primary immunodeficiency syndrome developing in elderly population due to gradually evolving restriction of genetically controlled programs in the thymic microenvironment responsible for irregular maturation of common lymphoid progenitor cells that constitutively express CD5 antigen and ME receptor into T-cell and B-cell descendants.

Caspase involvement in RIP-associated CD95-induced T cell apoptosis

CD95-induced apoptosis is an important regulatory mechanism in T cells and this complex signalling pathway is now thought to include the protein kinase RIP. Although, RIP is best known for its role in TNF signalling and NF-kappaB activation, it contains a death domain and it is capable of causing apoptosis upon cleavage. In the present study, the role of RIP in CD95-induced apoptosis and its inter-relationship with the caspase cascade was investigated. Studies were performed on both a RIP-/- T cell line and peripheral T lymphocytes, where RIP was degraded through the addition of geldanamycin. Apoptosis was induced by membrane CD95-L, thought to be the most physiological relevant form of CD95-L. Results showed that RIP-/- cells had a decreased susceptibility to death, thus confirming a role for RIP in CD95-induced apoptosis. Furthermore, it was confirmed that RIP is cleaved upon CD95-L stimulation, a process that can be inhibited by Z-VAD. However, only partial inhibition in peripheral T lymphocytes by Z-VAD was observed, suggesting a potential caspase-independent processing of RIP. Studies performed on the activity of effector caspase 3 and on the initiator caspases 2, 8, and 9 revealed that, in the absence of RIP, the activity of these caspases decreases, indicating that RIP-associated apoptosis is caspase-dependent. Hence, these studies support a caspase-related role for RIP in CD95-induced T apoptosis.

Age-related changes of the multidrug resistance P-glycoprotein function in normal human peripheral blood T lymphocytes

The multidrug resistance P-glycoprotein is a transmembrane efflux pump expressed by lymphocytes and is involved in their cytolytic activity. In the present study, we investigated the age-related changes of P-glycoprotein function in normal peripheral blood lymphocytes. Blood samples from 90 normal volunteers (age range, 0 to 86 years) were analyzed. P-glycoprotein function was assessed by the flow cytometric rhodamine 123 assay. P-glycoprotein function was highest in cord blood and progressively declined with age in peripheral blood T CD4+ and CD8+ cells. In contrast, P-glycoprotein function did not vary with age in CD19+ B or CD16+CD56+ natural killer cells. These data suggest that the decline in P-glycoprotein function in T CD4+ and CD8+ lymphocytes as a function of age may contribute to the decrease in T cell cytolytic activity with aging.

Age-related changes in the function of T cells

At the start of the last century in the United Kingdom, only 24% of the 587,830 deaths registered were of individuals over 65, but by the end of the century these figures had changed markedly. Of the 558,052 deaths in 1997, 84% were in the population over 65. This "right shift" in the survival curve is projected to continue. The UK Government Actuary's Department forecast that by 2020, 11.75 million people (19% of the population) will be over 65 rising to 15.1 million people (25% of the population) by 2040. Older members of society show infections of the urinary tract, respiratory tract, skin, soft tissue or intra-abdominal region, infectious endocarditis, bacterial meningitis, tuberculosis, and herpes zoster, at a higher incidence than among younger adults. Moreover, mortality rates for these diseases are often 2-3 times higher among elderly patients than younger individuals with the same disease. The higher morbidity and mortality from these infections, plus the increased prevalence of specific cancers and certain autoimmune diseases point to an immune system deteriorating with age. At the core of the immune system are the T cells and this review analyses possible causes for the changes in T cell function that may account for the deterioration of the immune system. Any intervention to reverse the decline in the immune system must have a rational basis built on a hypothesis-driven inquiry, and one such intervention process is presented here.

Adenosine triphosphate-binding cassette transporter genes in ageing and age-related diseases

The family of adenosine triphosphate (ATP)-binding cassette (ABC) transporters is the largest gene family known. While some ABC transporters translocate single substances across membranes with high specificity, others transport a wide variety of different lipophilic compounds. They are responsible for many physiological processes and are also implicated in a number of diseases. The present review focuses on ABC transporter genes which are involved in ageing and age-related diseases. Expression of ABCB1 (MDR1, P-glycoprotein) increases with age in CD4(+) and CD8(+) T-lymphocytes indicating that P-glycoprotein may be involved in the secretion of cytokines, growth factors, and cytotoxic molecules. As T cells in aged individuals are hyporesponsive leading to a reduced immunodefence capability, a role of ABCB1 in age-related immunological processes is presumed. The ABCA1 (ABC1) gene product translocates intracellular cholesterol and phospholipids out of macrophages. Genetic aberrations in ABCA1 cause perturbations in lipoprotein metabolism and contribute to atherosclerosis. ABCA4 (ABCR) represents a retina-specific ABC transporter expressed in rod photoreceptor cells. The ABCA4 gene product translocates retinyl-derivatives. Mutations in the ABCA4 gene contribute to age-related macular degeneration. Polymorphisms in the sulfonylurea receptor gene (ABCC8, SUR1) are associated with non-insulin-dependent diabetes mellitus (NIDDM). Sulfonylureas inhibit potassium conductance and are used to treat NIDDM by stimulation of insulin secretion across ATP-sensitive potassium channels in pancreatic beta-cell membranes. Possible diagnostic and therapeutic implications of ABC transporters for age-related diseases are discussed.

The impact of efflux transporters in the brain on the development of drugs for CNS disorders

The development of drugs to treat disorders of the CNS requires consideration of achievable brain concentrations. Factors that influence the brain concentrations of drugs include the rate of transport into the brain across the blood-brain barrier (BBB), metabolic stability of the drug, and active transport out of the brain by efflux mechanisms. To date, three classes of transporter have been implicated in the efflux of drugs from the brain: multidrug resistance transporters, monocarboxylic acid transporters, and organic ion transporters. Each of the three classes comprises multiple transporters, each of which has multiple substrates, and the combined substrate profile of these transporters includes a large number of commonly used drugs. This system of transporters may therefore provide a mechanism through which the penetration of CNS-targeted drugs into the brain is effectively minimised. The action of these efflux transporters at the BBB may be reflected in the clinic as the minimal effectiveness of drugs targeted at CNS disorders, including HIV dementia, epilepsy, CNS-based pain, meningitis and brain cancers. Therefore, modulation of these efflux transporters by design of inhibitors and/or design of compounds that have minimal affinity for these transporters may well enhance the treatment of intractable CNS disorders.

Effect of calcineurin inhibitor therapy on P-gp expression and function in lymphocytes of renal transplant patients: a preliminary evaluation

Cyclosporine and tacrolimus are substrates and potent inhibitors of the multidrug transporter, P-glycoprotein, in vitro. The authors have investigated the effect of chronic therapy with these and other immunosuppressive drugs on the expression and function of P-glycoprotein in T lymphocytes. Using a P-gp antibody, the authors studied the level of expression of P-gp in CD4 and CD8 T cells over a period of time in renal transplant patients. For comparison, a group of healthy volunteers and patients who did not receive any calcineurin inhibitors but were maintained on mycophenolate mofetil was included. The P-gp expression on lymphocytes from these two groups remained constant (over several months' time). However, patients who were started on tacrolimus or cyclosporine had an initial decline in expression of P-gp on CD4 T cells. Patients who were initiated on calcineurin therapy on day 1 posttransplant also had a decrease in expression of P-gp on CD4 T lymphocytes. This preliminary analysis suggests that the calcineurin inhibitors might be modulating the expression and function of transporters in lymphocytes, thus changing not only the drug concentration but also the apparent efficacy of these drugs. Further understanding and elucidation of such effects would be important in understanding the relationship between pharmacokinetics and pharmacodynamics of these and other drugs, especially for immunosuppressive and anti-AIDS therapy.

Multidrug resistance gene expression during the murine ontogeny

Multidrug resistance (MDR) was described initially for tumor cells which become resistant not only to the specific drug to which they are submitted, but also to a large range of unrelated drugs. The expression of mdr genes, responsible for the phenotype, and their product P glycoprotein (Pgp), is currently under intensive study due to their ample distribution in different organisms and their possible physiological roles which include protection against xenobiotics. In mice, three mdr isoforms expressed in some normal tissues are known. In this work, we analyzed by RT-PCR the expression of mdr1, mdr2 and mdr3 in several organs of BALB/c and C57BL/6 mice during ontogeny. A considerable variation in mdr expression among individuals of the same strain, as well as among different organs in individuals of the same age group and among different age groups, was detected. We also observed a strong tendency for the expression of a greater number of active isoforms in old mice. The large expression range of the mdr isoforms point to an important role as a natural defense system.

A human lymphocyte based ex vivo assay to study the effect of drugs on P-glycoprotein (P-gp) function

PURPOSE

The effect of drugs on P-glycoprotein (P-gp) is normally studied in transfected or overexpressing cell lines derived from tumor cells or animal tissue. We wanted to develop an assay using normal healthy human tissue to study and characterize the drug-transporter interaction.

METHODS

Lymphocytes were isolated from healthy human blood. The effect of inhibitors of P-gp (cyclosporine, tacrolimus, verapamil, quinidine, vinblastine) and of other transporters (indomethacin, probenecid, sulfinpyrazone) on intracellular accumulation of rhodamine 123 was evaluated by flow cytometry.

RESULTS

The efflux of rhodamine 123 was inhibited by P-gp inhibitors in a saturable, concentration-dependent manner. The potency of inhibition of P-gp was cyclosporine > tacrolimus > quinidine > verapamil > vinblastine. Vinblastine inhibited P-gp at lower concentrations, whereas at high concentrations, there was an activation of rhodamine 123 efflux from lymphocytes. The multidrug resistance associated protein (MRP) inhibitors, sulfinpyrazone and probenecid, did not have any significant effect on intracellular accumulation of rhodamine 123, but indomethacin caused a concentration-dependent increase in retention of rhodamine 123, indicating the involvement of other uncharacterized transporters.

CONCLUSIONS

Lymphocytes can serve as a model tissue for studying modulation of P-gp activity by drugs. Both inhibitors and inducers of P-gp activity can be evaluated.

Abrogation of G(2)/M-phase block enhances the cytotoxicity of daunorubicin, melphalan and cisplatin in TP53 mutant human tumor cells

Irradiation of human melanoma (MeWo, Be11) and squamous cell carcinoma (4451, 4197) cells induces cell cycle blocks from which the cells recover to re-enter mitosis after 40-60 h. In the TP53 mutant cell lines, MeWo and 4451, irradiation induces a G(2)-phase block, where the fraction of cells in G(2) phase reaches a maximum after 18-20 h. In the TP53 wild-type cell lines, 4197 and Be11, a G(1)- and G(2)-phase block is reached 12 and 16 h postirradiation, respectively. Addition of pentoxifylline after irradiation at the time when the number of cells in G(2) phase has reached a maximum shortens the normal recovery from G(2)-phase block to approximately 7 h. Addition of daunorubicin, melphalan and cisplatin under these conditions markedly enhanced drug toxicity. In the TP53-mutated cell lines MeWo and 4451, the survival ratio at 7 Gy measured by colony formation was 2.3-2.8, 8.6-85 and 52-74 for daunorubicin, melphalan and cisplatin, respectively. In the TP53 wild-type cell lines, the corresponding survival ratios were found to be 1.3-1.4, 2.3-3.0 and 1.2-2.6, respectively. The survival ratios are for clonogenic survival after 7 Gy and 2 mM pentoxifylline and measure the influence of drug doses that ensure 95% survival in nonirradiated controls. The results indicate that the G(2)-phase block is a crucial event in the damage response that can be manipulated to achieve a significant enhancement of drug toxicity. These effects are particularly pronounced in TP53 mutant cells and are observed at drug doses well below the clinical range.

mdr1a-Encoded P-Glycoprotein Is Not Required for Peripheral T Cell Proliferation, Cytokine Release, or Cytotoxic Effector Function in Mice

The plasma membrane transport protein P-glycoprotein (P-gp) is expressed by subsets of both CD4+ and CD8+ T cells in mice. The proportion of T cells that express P-gp goes up with age, and the P-gp-expressing subset of the CD4 memory population is hyporesponsive in many in vitro assays. The significance of P-gp expression for T cell function has not been well established, although several reports have suggested that it may promote cytokine export and/or cytotoxic T cell function. To elucidate which T cell functions may require P-gp, we have compared a variety of responses using T cells from wt and P-gp knockout mice. Protein expression and rhodamine-123 efflux studies revealed that peripheral T cells exclusively utilize the mdr1a-encoded isoform rather than the homologous mdr1b or mdr2 isoforms. Comparisons of T cells from mdr1a+/+ and mdr1a−/− mice showed no differences in proliferation or in secretion of IL-2, IL-4, IL-5, IL-10, or IFN-γ in response to polyclonal stimulation. Moreover, mdr1a−/− T cells produced strong allospecific cytotoxic responses comparable to those of wt T cells. Our results show that P-gp is not a necessary component of peripheral T cell functional responses. Further investigation will be needed to determine the significance of P-gp expression in T lymphocytes.

Does the immune system of a mouse age faster than the immune system of a human?

One of the characteristics of all somatic cells is a finite life span. Cells may proliferate until they reach a point after which, although they are metabolically active, they can no longer produce daughter cells. This observation is central to the clonal exhaustion hypothesis, a mechanism cited to explain age-associated immune dysfunction. In this hypothesis, repeated division of lymphocytes leads to a replicative limit, after which they enter the senescent phase but are not lost from the pool of T cells. Advancing age would then be associated with an increase in the number of T cells that are unable to proliferate to a stimulus which induces a proliferative response in T cells from younger individuals. This hypothesis seems both logical and reasonable and is supported by data from both humans and mice with the demonstration of an age-related accumulation of senescent T cells in both species. However, there is an apparent paradox. The paradox arises because the onset of immunosenescence appears to be more closely linked to the life span of the animal rather than the life span of the lymphocyte.

Aging and T-cell-mediated immunity

Changes in the T-lymphocyte compartment represent the most critical component of immunological aging. Recent studies have demonstrated that the age-related decline in T-cell-mediated immunity is a multifactorial phenomenon affecting T-cell subset composition as well as several proximal events such as protein tyrosine phosphorylation, generation of second messengers, calcium mobilization and translocation of protein kinase C, and distal events such as lymphocyte proliferation and cytokine production of the T-cell activation pathway. Age-related T-cell immune deficiency is preceded by thymic involution and is influenced by several intrinsic as well as extrinsic factors. Further, the role of monocytes and macrophages in T-cell activation changes with advancing age. This brief review will summarize the current knowledge of the cellular as well as molecular aspects of immunodeficiency of T cells due to aging, some of the paradoxes of aging as related to T-cell-mediated immunity, and possible factors which contribute to this paradox. Finally, experimental approaches will be suggested that might resolve these controversies and that might provide insights into the diverse and complex mechanisms that contribute to immunodeficiency of T cells. Ultimately these studies may suggest possible therapeutic interventions to enhance immune function in the elderly.