MicroRNA in diabetic nephropathy: renin angiotensin, aGE/RAGE, and oxidative stress pathway

MicroRNAs (miRNA) are a novel class of small, noncoding RNA molecules that have gained the attention of many researchers in recent years due to their ability to posttranscriptionally regulate the expression of families of genes simultaneously. Their role in normal physiology and pathobiology is intriguing and their regulation in normal and disease states is fascinating. That the cells can return to a state of homeostasis when these small molecules are perturbed is truly remarkable given the multiple cellular targets of each miRNA and that many mRNAs are targeted by multiple miRNAs. Several reviews have covered aspects of miRNA function in biology and disease. Here, we review the role of miRNA in regulating the renin-angiotensin system, AGE/RAGE signalling, and under conditions of oxidative stress in the context of diabetic nephropathy.

Advanced glycation end products as environmental risk factors for the development of type 1 diabetes

The globally rising incidence of Type 1 diabetes (T1D) is no longer restricted to individuals with higher risk genotypes, but is now significantly increasing in a population with lower risk genotypes, likely as the result of environmental factors. In this review, we discuss the potential of advanced glycation end products (AGEs) as environmental contributors to the development of T1D. AGEs are nonenzymatically formed protein modifications found in the body, as well as, consumed in our daily diets. To date, many studies have provided evidence of AGE involvement in β cell dysfunction, whether by AGE modification itself or via interaction with AGE receptors. The receptor for AGE (RAGE) and AGE-receptor-1 (AGE-R1) are of particular interest, given that studies have demonstrated the deleterious effects of RAGE modulation and the protection afforded by AGE-R1 in the context of diabetes. More interestingly, we have recently found that two RAGE polymorphism are predictive of T1D in humans while the third is protective. Moreover, soluble RAGE (sRAGE) levels (a circulating competitive inhibitor of RAGE) were greatly reduced at seroconversion to autoantibodies in both children on high risk of T1D background and in an animal model of autoiummune diabetes. Taken together with the fact that AGEs have also shown to be involved in immunomodulation, it is tempting to postulate that dietary AGEs, RAGE and even AGE-R1 could be working synergistically or independently to breach the tightly regulated immune system, providing a missing link in the development of T1D.

Suppression of microRNA-29 expression by TGF-β1 promotes collagen expression and renal fibrosis

Synthesis and deposition of extracellular matrix (ECM) within the glomerulus and interstitium characterizes renal fibrosis, but the mechanisms underlying this process are incompletely understood. The profibrotic cytokine TGF-β1 modulates the expression of certain microRNAs (miRNAs), suggesting that miRNAs may have a role in the pathogenesis of renal fibrosis. Here, we exposed proximal tubular cells, primary mesangial cells, and podocytes to TGF-β1 to examine its effect on miRNAs and subsequent collagen synthesis. TGF-β1 reduced expression of the miR-29a/b/c/family, which targets collagen gene expression, and increased expression of ECM proteins. In both resting and TGF-β1-treated cells, ectopic expression of miR-29 repressed the expression of collagens I and IV at both the mRNA and protein levels by targeting the 3'untranslated region of these genes. Furthermore, we observed low levels of miR-29 in three models of renal fibrosis representing early and advanced stages of disease. Administration of the Rho-associated kinase inhibitor fasudil prevented renal fibrosis and restored expression of miR-29. Taken together, these data suggest that TGF-β1 inhibits expression of the miR-29 family, thereby promoting expression of ECM components. Pharmacologic modulation of these miRNAs may have therapeutic potential for progressive renal fibrosis.

Dedifferentiation of immortalized human podocytes in response to transforming growth factor-β: a model for diabetic podocytopathy

OBJECTIVE

Diabetic nephropathy is associated with dedifferentiation of podocytes, losing the specialized features required for efficient glomerular function and acquiring a number of profibrotic, proinflammatory, and proliferative features. These result from tight junction and cytoskeletal rearrangement, augmented proliferation, and apoptosis.

RESEARCH DESIGN AND METHODS

Experiments were performed in conditionally immortalized human podocytes developed by transfection with the temperature-sensitive SV40-T gene. Cells were then cultured in the presence of transforming growth factor (TGF)-β1 or angiotensin II in the presence or absence of a selective inhibitor of the TGF-β type I receptor kinase, SB-431542. Gene and protein expression were then examined by real-time RT-PCR and immunofluorescence, and correlated with changes observed in vivo in experimental diabetes.

RESULTS

Treatment of cells with TGF-β1 resulted in dynamic changes in their morphology, starting with retraction and shortening of foot processes and finishing with the formation of broad and complex tight junctions between adjacent podocytes. This dedifferentiation was also associated with dose- and time-dependent reduction in the expression of glomerular epithelial markers (nephrin, p-cadherin, zonnula occludens-1) and increased expression of mesenchymal markers (α-smooth muscle actin, vimentin, nestin), matrix components (fibronectin, collagen I, and collagen IV α3), cellular proliferation, and apoptosis. The induction of diabetes in mice was also associated with similar changes in morphology, protein expression, and proliferation in glomerular podocytes.

CONCLUSIONS

In response to TGF-β and other TGF-dependent stimuli, mature podocytes undergo dedifferentiation that leads to effacement of foot processes, morphologic flattening, and increased formation of intercellular tight junctions. This simplification of their phenotype to a more embryonic form is also associated with reentry of mature podocytes into the cell cycle, which results in enhanced proliferation and apoptosis. These "pathoadaptive" changes are seen early in the diabetic glomerulus and ultimately contribute to albuminuria, glomerulosclerosis, and podocytopenia.

Osteoprotegerin promotes vascular fibrosis via a TGF-β1 autocrine loop

BACKGROUND

This study was designed to evaluate the potential role of osteoprotegerin (OPG) in arterial fibrosis.

METHODS

Aortic samples were analyzed after in vivo treatment of ApoE(-/-) mice with recombinant human OPG. Mouse vascular smooth muscle cells (VSMC) were exposed in vitro to recombinant OPG and analyzed for markers of inflammation and fibrosis, such as fibronectin, collagen I, III, IV and transforming growth factor-β1 (TGF-β1). Conversely, the potential modulation of endogenous OPG expression and release by VSMC was analyzed in response to different pro-atherosclerotic cytokines, TGF-β1, platelet derived growth factor (PDGF) and angiogensin II (Ang II).

RESULTS

In vivo treatment with human OPG induced signs of fibrosis and up-regulated the arterial expression of TGF-β1. Consistently, in vitro treatment of VSMC with human OPG induced the expression of fibronectin, collagen type I, III, IV, metalloprotein-2 (MMP-2) and MMP-9, as well as of TGF-β1. On the other hand, exposure to recombinant TGF-β1 promoted the expression/release of endogenous OPG and mediated the increase of OPG release induced by PDGF and Ang II in VSMC.

CONCLUSIONS

Taken together, these data support a pathogenic role for OPG in the development and progression of atherosclerotic lesions and suggest the existence of a vicious circle between TGF-β1 and OPG.

TGF-beta regulates miR-206 and miR-29 to control myogenic differentiation through regulation of HDAC4

MicroRNAs (miRs) are emerging as prominent players in the regulation of many biological processes, including myogenic commitment and skeletal muscle formation. Members of the TGF-β family can influence the proliferation and myogenic differentiation of cells, although it is presently not clear what role miRNAs play in the TGF-β-mediated control of myogenic differentiation. Here, we demonstrate in the myogenic C2C12 cell line, and in primary muscle cells, that miR-206 and miR-29-two miRs that act on transcriptional events implicated in muscle differentiation are down-regulated by TGF-β. We further demonstrate that TGF-β treatment of myogenic cells is associated with increased expression of histone deacetylase 4 (HDAC4), a key inhibitor of muscle differentiation that has been identified as a target for regulation by miR-206 and miR-29. We confirmed that increased expression of miR-206 and miR-29 resulted in the translational repression of HDAC4 in the presence or absence of TGF-β via interaction with the HDAC4 3'-untranslated region. Importantly, we found that miR-206 and miR-29 can attenuate the inhibitory actions of TGF-β on myogenic differentiation. Furthermore, we present evidence that the mechanism by which miR-206 and miR-29 can inhibit the TGF-β-mediated up-regulation of HDAC4 is via the inhibition of Smad3 expression, a transducer of TGF-β signaling. These findings identify a novel mechanism of interaction between TGF-β and miR-206 and -29 in the regulation of myogenic differentiation through HDAC4.

miR-200a Prevents renal fibrogenesis through repression of TGF-β2 expression

OBJECTIVE

Progressive fibrosis in the diabetic kidney is driven and sustained by a diverse range of profibrotic factors. This study examines the critical role of microRNAs (miRNAs) in the regulation of the key fibrotic mediators, TGF-β1 and TGF-β2.

RESEARCH DESIGN AND METHODS

Rat proximal-tubular epithelial cells (NRK52E) were treated with TGF-β1 and TGF-β2 for 3 days, and expression of markers of epithelial-to-mesenchymal transition (EMT) and fibrogenesis were assessed by RT-PCR and Western blotting. The expression of miR-141 and miR-200a was also assessed, as was their role as translational repressors of TGF-β signaling. Finally, these pathways were explored in two different mouse models, representing early and advanced diabetic nephropathy.

RESULTS

Both TGF-β1 and TGF-β2 induced EMT and fibrogenesis in NRK52E cells. TGF-β1 and TGF-β2 also downregulated expression of miR-200a. The importance of these changes was demonstrated by the finding that ectopic expression miR-200a downregulated smad-3 activity and the expression of matrix proteins and prevented TGF-β-dependent EMT. miR-200a also downregulated the expression of TGF-β2, via direct interaction with the 3' untranslated region of TGF-β2. The renal expression of miR-141 and miR-200a was also reduced in mouse models representing early and advanced kidney disease.

CONCLUSIONS

miR-200a and miR-141 significantly impact on the development and progression of TGF-β-dependent EMT and fibrosis in vitro and in vivo. These miRNAs appear to be intricately involved in fibrogenesis, both as downstream mediators of TGF-β signaling and as components of feedback regulation, and as such represent important new targets for the prevention of progressive kidney disease in the context of diabetes.

E-cadherin expression is regulated by miR-192/215 by a mechanism that is independent of the profibrotic effects of transforming growth factor-beta

OBJECTIVE

Increased deposition of extracellular matrix (ECM) within the kidney is driven by profibrotic mediators including transforming growth factor-beta (TGF-beta) and connective tissue growth factor (CTGF). We investigated whether some of their effects may be mediated through changes in expression of certain microRNAs (miRNAs).

RESEARCH DESIGN AND METHODS

Proximal tubular cells, primary rat mesangial cells, and human podocytes were analyzed for changes in the expression of key genes, ECM proteins, and miRNA after exposure to TGF-beta (1-10 ng/microl). Tubular cells were also infected with CTGF-adenovirus. Kidneys from diabetic apoE mice were also analyzed for changes in gene expression and miRNA levels.

RESULTS

TGF-beta treatment was associated with morphologic and phenotypic changes typical of epithelial-mesenchymal transition (EMT) including increased fibrogenesis in all renal cell types and decreased E-cadherin expression in tubular cells. TGF-beta treatment also modulated the expression of certain miRNAs, including decreased expression of miR-192/215 in tubular cells, mesangial cells, which are also decreased in diabetic kidney. Ectopic expression of miR-192/215 increased E-cadherin levels via repressed translation of ZEB2 mRNA, in the presence and absence of TGF-beta, as demonstrated by a ZEB2 3'-untranslated region luciferase reporter assay. However, ectopic expression of miR-192/215 did not affect the expression of matrix proteins or their induction by TGF-beta. In contrast, CTGF increased miR-192/215 levels, causing a decrease in ZEB2, and consequently increased E-cadherin mRNA.

CONCLUSIONS

These data demonstrate the linking role of miRNA-192/215 and ZEB2 in TGF-beta/CTGF-mediated changes in E-cadherin expression. These changes appear to occur independently of augmentation of matrix protein synthesis, suggesting that a multistep EMT program is not necessary for fibrogenesis to occur.

Quantitative gene expression analysis in kidney tissues

Quantitative gene expression analysis is fundamental to many experimental protocols and hypothesis testing in scientific research. The most popular currently used method to measure the expression level of specific genes in biological samples is real-time quantitative polymerase chain reaction (PCR). The method itself has become routine in many laboratories however stringent protocols and careful planning are required to for the generation of meaningful data. Many variations to these protocols are described in the literature. We describe here the methods used in our laboratory that have been compiled following many hours of troubleshooting and in our view they are robust protocols, providing solid data. The protocols are applicable to tissue culture cells where acute changes in gene expression are routinely observed following exposure to chemical or environmental stimuli, as well as tissue samples where gene expression is altered as a result of disease processes and interventions.

The role of tubular epithelial-mesenchymal transition in progressive kidney disease

The accumulation of interstitial matrix represents the final common pathway of most forms of kidney disease. Much of this matrix is synthesized by interstitial myofibroblasts, recruited from resident fibroblasts and circulating precursors. In addition, a significant proportion is derived from epithelial-mesenchymal transition (EMT) of tubuloepithelial cells. The importance of EMT has been demonstrated in experimental models, where blockade of EMT attenuates renal fibrosis. Although a number of factors may initiate EMT in the kidney, the most potent is transforming growth factor-beta1 (TGF-beta1). Moreover, many other prosclerotic factors have effects on EMT indirectly, via induction of TGF-beta1. Signaling events in this pathway include activation of Smad/integrin-linked kinase (ILK) and connective tissue growth factor (CTGF). Basement membrane integrity is also a key regulator of EMT. In particular, overexpression of matrix metalloproteinase-2 has a key role in the initiation of EMT, membrane dissolution, and the interstitial transit of transformed mesenchymal cells. Endogenous inhibitors of EMT also play an important counterregulatory role both to prevent EMT and stimulate uncommitted cells to regain their tubular phenotype (mesenchymal-epithelial transition). Such inhibitors represent a potential therapeutic approach, offering a mechanism to slow or even redress established renal fibrosis.

Connective tissue growth factor plays an important role in advanced glycation end product-induced tubular epithelial-to-mesenchymal transition: implications for diabetic renal disease

Epithelial-to-mesenchymal transition (EMT) of tubular cells contributes to the renal accumulation of matrix protein that is associated with diabetic nephropathy. Both TGF-beta1 and advanced glycation end products (AGE) are able to induce EMT in cell culture. This study examined the role of the prosclerotic growth factor connective tissue growth factor (CTGF) as a downstream mediator of these processes. EMT was assessed by the expression of alpha-smooth muscle actin, vimentin, E-cadherin, and matrix proteins and the induction of a myofibroblastic phenotype. CTGF, delivered in an adenovirus or as recombinant human CTGF (250 ng/ml), was shown to induce a partial EMT. This was not blocked by neutralizing anti-TGF-beta1 antibodies, suggesting that this action was TGF-beta1 independent. NRK-52E cells that were exposed to AGE-modified BSA (AGE-BSA; 40 microM) or TGF-beta1 (10 ng/ml) also underwent EMT. This was associated with the induction of CTGF gene and protein expression. Transfection with siRNA to CTGF was able to attenuate EMT-associated phenotypic changes after treatment with AGE or TGF-beta1. These in vitro effects correlate with the in vivo finding of increased CTGF expression in the diabetic kidney, which co-localizes on the tubular epithelium with sites of EMT. In addition, inhibition of AGE accumulation was able to reduce CTGF expression and attenuate renal fibrosis in experimental diabetes. These findings suggest that CTGF represents an important independent mediator of tubular EMT, downstream of the actions of AGE or TGF-beta1. This interaction is likely to play an important role in progressive diabetic nephropathy and strengthens the rationale to consider CTGF as a potential target for the treatment of diabetic nephropathy.

C-terminal domain of insulin-like growth factor (IGF) binding protein-6: structure and interaction with IGF-II

IGFs are important mediators of growth. IGF binding proteins (IGFBPs) 1-6 regulate IGF actions and have IGF-independent actions. The C-terminal domains of IGFBPs contribute to high-affinity IGF binding and modulation of IGF actions and confer some IGF-independent properties, but understanding how they achieve this has been constrained by the lack of a three-dimensional structure. We therefore determined the solution structure of the C-domain of IGFBP-6 using nuclear magnetic resonance (NMR). The domain consists of a thyroglobulin type 1 fold comprising an alpha-helix followed by a loop, a three-stranded antiparallel beta-sheet incorporating a second loop, and finally a disulfide-bonded flexible third loop. The IGF-II binding site on the C-domain was identified by examining NMR spectral changes upon complex formation. It consists of a largely hydrophobic surface patch involving the alpha-helix, the first beta-strand, and the first and second loops. The site was confirmed by mutagenesis of several residues, which resulted in decreased IGF binding affinity. The IGF-II binding site lies adjacent to surfaces likely to be involved in glycosaminoglycan binding of IGFBPs, which might explain their decreased IGF affinity when bound to glycosaminoglycans, and nuclear localization. Our structure provides a framework for understanding the roles of IGFBP C-domains in modulating IGF actions and conferring IGF-independent actions, as well as ultimately for the development of therapeutic IGF inhibitors for diseases including cancer.

The role of advanced glycation in reduced organic cation transport associated with experimental diabetes

Tubular dysfunction is an important early manifestation of diabetic nephropathy. Reduced renal expression of organic cation transporters (OCTs) potentially contributes to impaired cation clearance in diabetes. This study examines the role of advanced glycation end-products (AGEs) in mediating these changes. Experimental diabetes was induced with streptozotocin (55 mg/kg). Rats were randomly treated with the AGE inhibitor aminoguanidine for 32 weeks. In a second protocol, diabetic rats were followed with and without low-dose insulin therapy (2 U/day) for 4 weeks. Expression of OCTs was determined by real-time RT-PCR (reverse transcription-polymerase chain reaction) and Western blotting. As a marker of cation transport, the fractional clearance of endogenous N-methylnicotinamide (NMN) was determined by high-performance liquid chromatography. Both short- and long-term diabetes was associated with reduced gene and protein expression of the three renal OCT isotypes. This was associated with a reduction in the fractional clearance of NMN compared with control animals by over 50%. These changes correlated with the accumulation of renal and plasma AGEs. Treatment with the AGE inhibitor aminoguanidine restored the expression of OCT-2 and OCT-3 in diabetic animals and normalized renal NMN clearance. NMN clearance was also improved in diabetic animals receiving low-dose insulin, correlating with a reduction in AGEs and improvement in effective renal plasma flow. These studies demonstrate an early impairment of expression of OCTs and cation clearance associated with diabetes. These changes correlate with the accumulation of AGEs and may be partly attenuated by an AGE inhibitor, implicating a role for AGEs in organic cation transport.

Reduced tubular cation transport in diabetes: prevented by ACE inhibition

BACKGROUND

The renal clearance of organic cations is important for the homeostasis of a number of exogenous and endogenous compounds. The organic cation transporters (OCTs) situated on the basolateral surface of proximal tubular cells mediate active cation excretion. Alterations of cation transport may occur in diabetes, although the role of the OCTs has not been previously assessed.

METHODS

Experimental diabetes was induced in rats with streptozotocin (55 mg/kg) and animals were randomly assigned to receive ramipril (3 mg/mL) in drinking water for 24 weeks. In a second protocol, rats were infused with angiotensin II (Ang II) at a dose of 58.3 ng/kg/min for 2 weeks via an implanted osmotic pump. Expression of the OCTs and renal clearance of the endogenous cation N-methyl-nicotinamide (NMN) was assessed.

RESULTS

Diabetes was associated with a reduction in gene and protein expression of both OCT-1 and OCT-2 and a reduction in NMN clearance. These effects were prevented by ramipril, associated with the prevention of albuminuria and tubular injury as demonstrated by the expression of osteopontin and glutathione peroxidase 3 (GPX-3). An infusion of Ang II also reduced NMN clearance but without altering the renal expression of OCTs.

CONCLUSION

We hypothesize that reduced expression of OCTs in diabetes may be a marker of tubular injury. However, Ang II may also directly augment renal cation clearance independent of changes in transporter expression. Together these effects may provide additional mechanism to explain treatment-related improvements in creatinine clearance and renoprotection in diabetes following blockade of the renin-angiotensin system (RAS).

Precipitous release of methyl-CpG binding protein 2 and histone deacetylase 1 from the methylated human multidrug resistance gene (MDR1) on activation

Overexpression of the human multidrug resistance gene 1 (MDR1) is a negative prognostic factor in leukemia. Despite intense efforts to characterize the gene at the molecular level, little is known about the genetic events that switch on gene expression in P-glycoprotein-negative cells. Recent studies have shown that the transcriptional competence of MDR1 is often closely associated with DNA methylation. Chromatin remodeling and modification targeted by the recognition of methylated DNA provide a dominant mechanism for transcriptional repression. Consistent with this epigenetic model, interference with DNA methyltransferase and histone deacetylase activity alone or in combination can reactivate silent genes. In the present study, we used chromatin immunoprecipitation to monitor the molecular events involved in the activation and repression of MDR1. Inhibitors of DNA methyltransferase (5-azacytidine [5aC]) and histone deacetylase (trichostatin A [TSA]) were used to examine gene transcription, promoter methylation status, and the chromatin determinants associated with the MDR1 promoter. We have established that methyl-CpG binding protein 2 (MeCP2) is involved in methylation-dependent silencing of human MDR1 in cells that lack the known transcriptional repressors MBD2 and MBD3. In the repressed state the MDR1 promoter is methylated and assembled into chromatin enriched with MeCP2 and deacetylated histone. TSA induced significant acetylation of histones H3 and H4 but did not activate transcription. 5aC induced DNA demethylation, leading to the release of MeCP2, promoter acetylation, and partial relief of repression. MDR1 expression was significantly increased following combined 5aC and TSA treatments. Inhibition of histone deacetylase is not an overriding mechanism in the reactivation of methylated MDR1. Our results provide us with a clearer understanding of the molecular mechanism necessary for repression of MDR1.

MRP1 not MDR1 gene expression is the predominant mechanism of acquired multidrug resistance in two prostate carcinoma cell lines

Multidrug resistant prostate cancer cell lines DU 0.03 and PC 0.03 were established from the parental prostate cancer cell lines DU145 and PC-3 respectively by stepwise selection in doxorubicin (DOX) from 0.001 to 0.03 &mgr;g/ml. As cells adapted to each concentration of DOX. the drug concentration was increased by 0.001 &mgr;g/ml. The chemosensitivity of each line was determined by growth inhibition assay. The DU 0.03 and PC 0.03 lines exhibit a 5-10-fold and 1.3-2.8-fold increase in resistance to anthracyclines, vinblastine (VLB) and mitozantrone (Mito), respectively. Verapamil (5 &mgr;M) partially reversed the resistance to the anthracycline and completely reversed the resistance to VLB and Mito. Drug kinetic studies measured by intracellular accumulation of (3)H-daunorubicin demonstrated a 3 fold decrease in the level of intracellular (3)H-daunorubicin in the PC 0.03 and DU 0.03 resistant lines compared with their respective parental line. This effect was partially reversed by 5 &mgr;M verapamil. The expression of MDR1 and MRP genes was analysed by Northern blotting and RT-PCR. P-glycoprotein (Pgp) and MRP protein were tested by immunocytochemistry staining using the monoclonal antibodies J-SB1. C219 and MRK16 (Pgp) and MRPm6 and MRPr1 (MRP). Neither Northern blot analysis nor the more sensitive RT-PCR demonstrated detectable MDR1 transcripts in any of the prostate cancer cell lines and the three Pgp monoclonal antibodies failed to reveal expression of Pgp. A 2-4-fold increase in MRP1 mRNA levels in the drug resistant DU 0.03 and PC 0.03 lines were demonstrated by both Northern blotting and RT-PCR consistent with the findings observed after staining by the two specific monoclonal antibodies, MRPm6 and MRPr1. Southern blot analysis demonstrated a 2-fold increase in the MRP1 gene copy number in the PC 0.03 line but not in the DU 0.03 line, suggesting that the overexpression of the MRP gene was regulated at the level of transcription in the latter line. We conclude that MRP1 not MDR1 overexpression. contributes to acquired drug resistance in these two prostate cancer cell lines. Prostate Cancer and Prostatic Diseases (2000) 3, 66-75

Regulation of MDR1 gene expression: emerging concepts

Drug resistance genes, such as MDR1, involved in drug efflux, and their regulation have been the subject of intense research efforts in the past 10 years. Many factors and cellular signalling pathways play a role in the regulation of MDR1 gene expression. Commonly used chemotherapeutic agents activate in vitro and in vivo general stress response pathways, potential targets of which include MDR1 and other drug resistance genes. The contribution of these agents to the emergence of drug-resistant tumour cells is of concern. Recent evidence points to a role for the epigenetic regulation of MDR1 gene expression. The identification of key components in the DNA methylation/chromatin system of gene regulation may in time lead to more informed and targeted approaches to treating drug-resistant tumours. Copyright 2000 Harcourt Publishers Ltd.

Altered multidrug resistance phenotype caused by anthracycline analogues and cytosine arabinoside in myeloid leukemia

The expression of P-glycoprotein (Pgp) is often increased in acute myeloid leukemia (AML). However, little is known of the regulation of Pgp expression by cytotoxics in AML. We examined whether Pgp expression and function in leukemic blasts was altered after a short exposure to cytotoxics. Blasts were isolated from 19 patients with AML (15 patients) or chronic myeloid leukemia in blastic transformation (BT-CML, 4 patients). Pgp expression and function were analyzed by flow cytometric analysis of MRK 16 binding and Rhodamine 123 retention, respectively. At equitoxic concentrations, ex vivo exposure for 16 hours to the anthracyclines epirubicin (EPI), daunomycin (DAU), idarubicin (IDA), or MX2 or the nucleoside analogue cytosine arabinoside (AraC) differentially upregulated MDR1/Pgp expression in Pgp-negative and Pgp-positive blast cells. In Pgp-negative blasts, all four anthracyclines and AraC significantly increased Pgp expression (P =.01) and Pgp function (P =.03). In contrast, MX2, DAU, and AraC were the most potent in inducing Pgp expression and function in Pgp positive blasts (P <.05). A good correlation between increased Pgp expression and function was observed in Pgp-negative (r =.90, P =.0001) and Pgp-positive blasts (r =.77, P =.0002). This increase in Pgp expression and function was inhibited by the addition of 1 micromol/L PSC 833 to blast cells at the time of their exposure to these cytotoxics. In 1 patient with AML, an increase in Pgp levels was observed in vivo at 4 and 16 hours after the administration of standard chemotherapy with DAU/AraC. Upregulation of Pgp expression was also demonstrated ex vivo in blasts harvested from this patient before the commencement of treatment. In 3 other cases (1 patient with AML and 2 with BT-CML) in which blasts were Pgp negative at the time of initial clinical presentation, serial samples at 1 to 5 months after chemotherapy showed the presence of Pgp-positive blasts. All 3 patients had refractory disease. Interestingly, in all 3 cases, upregulation of Pgp by cytotoxics was demonstrated ex vivo in blasts harvested at the time of presentation. These data suggest that upregulation of the MDR1 gene may represent a normal response of leukemic cells to cytotoxic stress and may contribute to clinical drug resistance.

Absolute quantitation of MDR1 transcripts using heterologous DNA standards--validation of the competitive RT-PCR (CRT-PCR) approach

The multidrug resistance (MDR1) gene product, P-glycoprotein (Pgp), is a 170-kDa ATP-dependent pump that expels a variety of anticancer drugs out of malignant cells, reducing drug accumulation and thus antitumor activity. In recent years, considerable data has been presented that indicates the need to standardize detection methods for Pgp and MDR1. Reverse transcription (RT)-PCR is one of the most sensitive and specific techniques used to detect MDR1. Nevertheless, there is the need to address working criteria for quantitation by RT-PCR. In this study, we describe a flexible assay used to quantify MDR1 gene expression using heterologous (nonhomologous) standards for use in competitive RT-PCR (CRT-PCR). Our guidelines were to use a RT-PCR quantitation method that was independent of exponential phase kinetics, sensitive (detect low levels of gene measurement in clinical samples) and did not require radiolabel. Furthermore, the method would need to be flexible enough for the user to express quantitation as either the number of cells or amount of cDNA used in CRT-PCR. Using low-stringency amplification, heterologous DNA competitors were constructed for MDR1 and as an internal reference, the ubiquitously expressed human histone variant 3.3 (H3.3). The benefits of this approach are threefold: (i) amplification kinetics of target and competitor molecules are identical, (ii) low-stringency PCR is a simple way of constructing heterologous DNA competitors that do not require special storage conditions and (iii) heterologous competitors avoid the formation of heteroduplex molecules. We conclude that CRT-PCR is an extremely flexible and sensitive assay that can quantify MDR1 based on competitive amplification of a heterologous competitor. This might complement future efforts to standardize MDR1 detection methods using RT-PCR.

Physical mapping of a tandem duplication on the long arm of chromosome 7 associated with a multidrug resistant phenotype

Both the expression of the multidrug transporter, P-glycoprotein (Pgp), and abnormalities of the long arm of chromosome 7 have been shown to be adverse prognostic indicators in acute leukemias. In this study, a clonal duplication, dup(7)(q11.1q31.1), inherited with the classical multidrug resistant phenotype in a drug-resistant derivative of a human T-cell leukemia cell line was characterized. The position of the duplication was of interest as the gene which encodes Pgp, MDR1, is located on the long arm of chromosome 7 at position 7q21.1. Fluorescence in situ hybridization (FISH) analysis with a chromosome 7-specific painting probe confirmed the composition of the abnormal chromosome. A YAC clone hybridizing to the MDR1 locus confirmed that this gene was located within the duplicated region of the derivative chromosome. With a panel of well-characterized YAC clones, the duplicated segment was found to be a direct tandem duplication, somewhat larger than estimated by conventional cytogenetics. The proximal and distal breakpoints of the abnormality were located and a YAC clone spanning the distal breakpoint was identified. This clone is of particular interest, as it harbors the markers D7S523 and D7S471, close to which a putative tumor suppressor gene is thought to lie. Further examination of the breakpoint region may therefore illuminate the mechanism of Pgp upregulation as well as providing information about a tumor suppressor gene.