Characterization of multidrug-resistant pituitary tumor cells

Dephosphorylation of Tak1 at Ser412 greatly contributes to the spermatocyte-specific testis toxicity induced by (5R)-5-hydroxytriptolide in C57BL/6 mice

(5R)-5-Hydroxytriptolide (LLDT-8), a novel triptolide derivative, will proceed to phase II clinical trials for the treatment of rheumatoid arthritis and cancer. However, the selection of disease and patients is largely limited by the testis toxicity, yet toxicity mechanisms are still poorly understood. In this study, LLDT-8 dose and time-dependently decreased the testes weight, germinal cell layers and induced abnormal spermatid development. Analysis of the germ cell-specific marker showed that spermatocytes were more sensitive to LLDT-8, which was confirmed by the in vitro sensitivity assay with spermatocyte-like GC-2spd and sertoli-like TM4 cells. In GC-2spd, LLDT-8 induced G1/S arrest and apoptosis. MAPK activity screening identified that TGF-β activated kinase 1 (Tak1) is critical in LLDT-8 induced apoptosis. LLDT-8 reduced the Tak1 protein and dephosphorylated Tak1 at Ser412 in GC-2spd and the testes, but not in TM4. RNAi mediated depletion or pharmacologic inhibition of Tak1 induced apoptosis in GC-2spd. Meanwhile, activating Tak1 rescued up to 50% of the GC-2spd cells from the apoptosis induced by LLDT-8. Altogether, our study firstly revealed the important role of Tak1 in the survival of spermatocytes, and dephosphorylation of Tak1 at Ser412 may contribute to the spermatocyte-specific testis toxicity induced by LLDT-8.

MDR1a/1b gene silencing enhances drug sensitivity in rat fibroblast-like synoviocytes

BACKGROUND

Drug resistance mediated by P-glycoprotein (P-gp) is one of the major reasons for the failure of rheumatoid arthritis (RA) therapy with disease modifying anti-rheumatic drugs and glucocorticoids. In the present study, we aimed to investigate the in vitro effectiveness of small interfering RNA (siRNA) to render rat fibroblast-like synoviocytes (FLS) susceptible to drugs. We also attempted the electroporation-mediated transfer of siRNA against multidrug resistance (MDR) genes into rat knee joints.

METHODS

FLS were transfected with siRNAs corresponding to MDR1a and MDR1b genes. FLS were treated with dexamethasone (DEX) and lipopolysaccharide. The mRNA and protein levels of tumor necrosis factor-alpha, interleukin (IL)-6 and IL-1beta were measured. Both siRNAs were co-transduced into rat knee joints by an electroporation method and evaluated the target gene expressions in the synovium.

RESULTS

Each siRNA could sequence-specifically reduce the target gene expression by over 70% and effectively suppressed P-gp expression and function in the FLS. Both gene expression and protein production of the inflammatory cytokines in the cells transfected with siRNA were reduced by a greater amount compared to in control cells. The in vivo electroporation-mediated transduction of siRNA could significantly inhibit the target gene expressions.

CONCLUSIONS

MDR1a/1b gene silencing by siRNA could effectively inhibit P-gp in rat FLS, resulting in a significant enhancement of the anti-inflammatory effects of DEX. The in vivo siRNA transduction could successfully silence MDR gene expression in the rat synovium. These findings indicate that the siRNA targeting MDR gene could be a useful tool for treating refractory arthritis in RA.

Involvement of P-glycoprotein in the release of cytokines from peripheral blood mononuclear cells treated with methotrexate and dexamethasone

P-glycoprotein (P-gp), a product of the MDR1 gene, is an important factor in the turnover of many drugs and xenobiotics. Recent reports have suggested that P-gp can also be involved in the transport of cytokines. The aim of this study was to examine the role of P-gp in cytokine release from phytohaemagglutinin (PHA)-stimulated peripheral blood mononuclear cells (MNCs) as well as in the release of cytokines from MNCs treated with methotrexate (MTX) and dexamethasone (DEX). The study was carried out on PHA-stimulated MNC from 10 healthy subjects. Flow cytometry was applied to measure interleukin (IL)-2, IL-4, IL-6, IL-10, interferon (IFN)-gamma and tumour necrosis factor (TNF)-alpha levels in the culture supernatants. In the experiments verapamil (VER) and P-gp specific monoclonal antibodies (mAb) (clone 17F9) were used to inhibit P-gp function. P-gp inhibitors suppressed the release of IL-2, IL-4, IFN-gamma and TNF-alpha from PHA-stimulated MNC, whereas release of IL-6 and IL-10 remained unaffected. VER and mAb significantly decreased the release of IL-2, IL-4, TNF-alpha and INF-gamma in MNC cultures treated with MTX or DEX. The results of this study suggest that P-gp may be involved in the transmembrane transport of some cytokines. Moreover, it seems that blocking of P-gp function may influence the release of some cytokines from MNCs, displaying an additive inhibitory effect to DEX and MTX.

The effect of 3435C>T MDR1 gene polymorphism on rheumatoid arthritis treatment with disease-modifying antirheumatic drugs

OBJECTIVE

Rheumatoid arthritis (RA) is a multifactorial disease, with immunological, genetical as well as environmental factors being implicated in its pathogenesis. Treatment of RA is based mainly on drugs modulating the course of the disease, e.g. methotrexate (MTX) or sulfasalazine (SL). The MDR1 gene product, P-glycoprotein (P-gp), is probably one of the most important and best defined transporters for drug delivery in humans. P-gp transports a wide range of substrates with diverse chemical structures, among them anticancer agents, cardiac drugs, and immunosuppressants. The aim of this study was to examine the effect of the 3435C>T MDR1 gene polymorphism on the efficacy of RA treatment with disease-modifying antirheumatic drugs, i.e. MTX plus methylprednisolone (MP), and SL.

METHODS

The study was carried out on 255 patients with RA treated according to two regimes: (1) MTX (7.5-15.0 mg weekly) plus low doses of MP (n=174), (2) SL (1.5-3 g daily, n=81).

RESULTS

The probability of remission of RA symptoms after MTX plus MP therapy was 4.65-fold higher in carriers of the TT genotype compared to patients with CC genotype (P=0.003, OR 4.65, 95%CI 1.66-13.05), whereas the probability of remission of RA symptoms in patients treated with SL was 2-fold higher in carriers of TT genotype compared to patients with CC genotype, but did not reach statistical significance (P=0.358, OR=2.00 95% CI=0.58-6.87).

CONCLUSION

The results from the present study suggest that the 3435C>T MDR1 gene polymorphism may influence the efficacy of RA therapy with disease-modifying antirheumatic drugs.

Thyroid hormone export varies among primary cells and appears to differ from hormone uptake

We characterized T3 efflux in primary cultures of cells derived from human placenta, neonatal rat cardiac myocytes, and rat inner medullary collecting ducts (IMCD). The T3 efflux rate was highest in placenta cells, followed by ventriculocytes, atriocytes, and IMCD cells. Verapamil reversibly blocked [125I]T3 efflux in these cells in a manner that correlated with their T3 efflux rate. Thus, verapamil inhibition of [125I]T3 efflux in placenta cells led to a 432% increase in the [125I]T3 content compared with 33% increase in IMCD cells. Several unlabeled iodothyronines, but not TRIAC, differentially blocked [125I]T3 efflux such as (T4 > T3 > rT3 = D-T3 > D-T4) in placenta cells and (T4 > rT3 = D-T4 = T3 > D-T3) in ventriculocytes, suggesting tissue-specific differences in the carriers/transporters responsible for T3 efflux. This hypothesis draws further support from the fact that D-T3 inhibited [125I]T3 efflux in placenta cells, but not in ventriculocytes. TRIAC did not affect T3 efflux in ventriculocytes or placenta cells, but it greatly inhibited [125I]T3 uptake in these cells, suggesting that [125I]T3 uptake and efflux mechanisms are distinct and appear to be mediated by distinct carrier/transporter proteins. Collectively, these data suggest that differences in thyroid hormone transport in target cells may provide an important mechanism for regulating hormone action in a tissue-specific fashion.

Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.

Thyroid hormone export in rat FRTL-5 thyroid cells and mouse NIH-3T3 cells is carrier-mediated, verapamil-sensitive, and stereospecific

Export of L-T3 out of the cell is one factor governing the cellular T3 content and response. We previously observed in liver-derived cells that T3 export was inhibited by verapamil, suggesting that it is due to either ATP-binding cassette/multidrug resistance (MDR1/mdr1b) or multidrug resistance-related (MRP1/mrp1) proteins. To test this hypothesis we measured T3 export in FRTL-5, NIH-3T3, and rat hepatoma (HTC) cells that varied in expression of these proteins. FRTL-5 and NIH-3T3 cells were found to contain a T3 efflux mechanism that is verapamil inhibitable, saturable, and stereospecific. By contrast, T3 efflux in HTC cells was slow and unaffected by verapamil. Neither FRTL-5 nor NIH-3T3 cells express mdrlb, but all three cell types express mrpl, as assessed by immunoblotting. Overexpression of MDR1 in NIH-3T3 cells did not enhance verapamil-inhibitable T3 efflux. Photoaffinity labeling of FRTL-5 and NIH-3T3 cells with [125I]L-T3 revealed a labeled 90- to 100-kDa protein that was not present in HTC cells. Verapamil and excess nonradioactive L-T3, but not D-T3, inhibited labeling of this protein. The lack of correlation between T3 efflux and MDR1 and mrpl expression and the finding of a photoaffinity-labeled putative transport protein smaller than MDR1 or mrp1 protein (approximately 170 kDa) suggest that a novel protein is involved in the transport of T3 out of cells.

Thyroid hormone export regulates cellular hormone content and response

Actions of thyroid hormones (THs) are determined by intracellular free hormone concentration. Here we report that enhanced TH extrusion via a saturable, cold-sensitive mechanism lowers intracellular TH and causes TH resistance in hepatoma cells. Since these cells overexpress multidrug resistance P-glycoproteins and TH extrusion and resistance are blunted by verapamil, P-glycoproteins may mediate this resistance. Verapamil-inhibitable TH efflux was also found in primary hepatocytes, cardiocytes, and fibroblasts. These findings demonstrate that TH extrusion can modulate TH availability and action in mammalian cells.