Allosteric activation via kinetic control: potassium accelerates a conformational change in IMP dehydrogenase

Allosteric activators are generally believed to shift the equilibrium distribution of enzyme conformations to favor a catalytically productive structure; the kinetics of conformational exchange is seldom addressed. Several observations suggested that the usual allosteric mechanism might not apply to the activation of IMP dehydrogenase (IMPDH) by monovalent cations. Therefore, we investigated the mechanism of K(+) activation in IMPDH by delineating the kinetic mechanism in the absence of monovalent cations. Surprisingly, the K(+) dependence of k(cat) derives from the rate of flap closure, which increases by ≥65-fold in the presence of K(+). We performed both alchemical free energy simulations and potential of mean force calculations using the orthogonal space random walk strategy to computationally analyze how K(+) accelerates this conformational change. The simulations recapitulate the preference of IMPDH for K(+), validating the computational models. When K(+) is replaced with a dummy ion, the residues of the K(+) binding site relax into ordered secondary structure, creating a barrier to conformational exchange. K(+) mobilizes these residues by providing alternate interactions for the main chain carbonyls. Potential of mean force calculations indicate that K(+) changes the shape of the energy well, shrinking the reaction coordinate by shifting the closed conformation toward the open state. This work suggests that allosteric regulation can be under kinetic as well as thermodynamic control.

Mycophenolate induced diarrhoea

Mycophenolate (MMF) has arisen as an important addition in the immunosuppression armamentarium. GI disturbances (diarrhea) and bone marrow suppression are its main side effects requiring dose reduction or even withdrawal. The mechanism of diarrhoea is unknown, although some theories have been postulated. We evaluated three of our patients on MMF who came to us with chronic diarhoea. Their evaluation consisted of CBC, stool routine examination, stool culture, endoscopy and biopsy. Histopathologic examination in all three cases showed villous atrophy. All of them improved with discontinuation of MMF and addition of folic acid suggesting that diarrhoea was related to MMF. Since this complication is seen in only a few cases, we can hypothesize that it may be due to lower levels of the enzyme inosine monophosphate dehydrogenase (IMPDH)--the site of action of MMF.

Recent development of IMP dehydrogenase inhibitors for the treatment of cancer

Inosine 5'-monophosphate dehydrogenase (IMPDH) represents an attractive target for the development of anticancer agents; however, there are no drugs aimed at this target for the treatment of cancer currently available on the market. Tiazofurin, a potent IMPDH inhibitor, reached clinical trials with Orphan Drug status for the treatment of patients in blast crisis of chronic myelogenous leukemia (CML); however, it was considered too toxic for application against other malignancies and no development has been reported for this drug since 2002. Formulations of mycophenolic acid, another potent inhibitor of IMPDH, are currently used for the prevention of rejection following transplantation, and against autoimmune diseases. More recently, numerous studies have demonstrated the potential of mycophenolic acid as an anticancer agent, with a phase I clinical trial in patients with advanced multiple myeloma ongoing. Furthermore, synergy between imantinib and mycophenolic acid in CML treatments has also been reported. Related compounds such as mycophenolic adenine dinucleotides, along with second-generation analogs, are undergoing preclinical evaluation, while another inhibitor of IMPDH, AVN-944, is currently in phase I clinical trials to investigate the treatment of hematological malignancies. This article reviews recent applications of IMPDH inhibitors as anticancer agents, and highlights the progress that has been made in this field.

Effects of Mycophenolic Acid on Inosine Monophosphate Dehydrogenase I and II mRNA Expression in White Blood Cells and Various Tissues in Sheep

Mycophenolic acid (MPA) is a mycotoxin commonly found as Penicillium genus secondary metabolite in feedstuffs and silages. Feeding with MPA contaminated silages may modulate the immune system in the farm animals and can cause appetite lost, ketosis, paralysis and abortion. The aim of the present study was to characterize the long-term MPA effect on both the inosine monophosphate dehydrogenase (IMPDH) isoforms I and II mRNA expression in white blood cells (WBC) and various tissue of healthy sheep. In treated animals 300 mg MPA/day/sheep was applied. In all investigated tissues the IMPDH I and II mRNA was abundant: WBC, spleen, thymus, ileum, jejunum, kidney, liver, pharyngeal and mesenterial lymph node. An efficiency-corrected relative quantification of the IMPDH types I and II isoforms mRNA were performed by normalizing with the constant reference gene expression of beta-actin. High IMPDH I mRNA expression levels were seen in kidney > mesenterial lymph node > jejunum > spleen > pharyngeal lymph node. Medium and low abundance was found in ileum > WBC > liver > thymus. Type II mRNA was highly expressed in liver > thymus > jejunum. In pharyngeal lymph node > spleen > ileum > mesenterial lymph node > kidney > WBC medium to low IMPDH II mRNA concentrations were detected. Under MPA treatment the IMPDH I mRNA expression was not significantly regulated in WBC, only trends of down- and upregulation were observed. Surprisingly in jejunum an upregulation could be observed (P < 0.05). In pharyngeal lymph node a tendency to downregulation was shown. This may be due to frequent ruminant activities and frequent exposition of MPA to the pharyngeal lymph nodes. In contrast to type I mRNA expression, IMPDH II mRNA was significantly downregulated in ileum (3.4-fold, P < 0.01) and tendencies in downregulation could be seen in jejunum (5.1-fold, P = 0.14). In addition, significant downregulation of IMPDH II gene expression over the entire feeding experiment could be shown in WBC of MPA-treated animals compared with untreated animals (P < 0.05). In conclusion, the recent study demonstrates that feeding sheep with MPA-contaminated silage did not induce IMPDH I mRNA expression in various tissues and blood, except in jejunum, but has suppressive effects on IMPDH II mRNA expression in WBC and ileum.

Nucleic acid synthesis in cancerous cells under the effect of gnidilatimonoein from Daphne mucronata

Cytotoxicity evaluation of gnidilatimonoein, the most active isolated diterpene ester from Daphne mucronata [Sadeghi H, Mianabadi M, Yazdanparast R, (2002) Journal of Tropical. Medicinal Plant1 3: 169-173], revealed the strong antiproliferative activity among several different human cancer cell lines (K562, CCRF-CEM, HL-60 and MOLT-4 leukemia cell lines, LNCaP-FGC-10 a prostate cancer cell line) and a mouse BALB/C fibrosarcoma cell line (WEHI-164). Using flow cytometry technique, it was found that treatment of the most responsive cells (K562) with gnidilatimonoein inhibited the progression of cells through G1 phase by almost 15% compared to the untreated cells. The population of the treated cells in the S and G2 phases also reduced by 8.3% and 5.4%, respectively. Based on the extent of [3H]-thymidine and [3H]-uridine incorporation into DNA and RNA, respectively, the major metabolic effects of gnidilatimonoein were found to be mainly on DNA and to a less extent on RNA synthesis. Additionally, the activity of inosine-5'-monophosphate dehydrogenase (IMPDH), under the effects of genidilatimonoein, was reduced in the treated cells by 44%. These data strongly suggest that the purine biosynthetic pathway is significantly affected by gnidilatimonoein.

Anti-inflammatory effects of inosine in human monocytes, neutrophils and epithelial cells in vitro

Inosine is an endogenous purine, which has been recently shown to exert immunomodulatory, anti-inflammatory and anti-shock effects in rodent experimental systems. Some of these actions may be related to partial adenosine receptor agonistic effects. It has not been investigated previously whether inosine exerts similar immunomodulatory or anti-inflammatory effects in human cells or enzymes. Here we investigated the effects of inosine on the activation of human monocytes, neutrophils and epithelial cells in vitro. Furthermore, using a human inosine-5'-monophosphate dehydrogenase (IMPDH) enzyme, we examined the potential effects of inosine on the activity of IMPDH, an enzyme involved in the regulation of certain inflammatory/immune processes. Tumor necrosis factor alpha (TNF-alpha) production of bacterial lipopolysaccharide (LPS) stimulated whole blood was used as an indicator of human monocyte activation. The response was dose-dependently, partially suppressed in the presence of inosine. Inosine exerted a dose-dependent and, at the highest dose (3 mM), complete inhibition of the ability of human neutrophils activated with N-formyl-methionyl-leucyl-phenylalanine (fMLP) to induce cytochrome C reduction in vitro. In the human colon cancer cell line HT-29, inosine dose-dependently attenuated the production of IL-8. Inosine failed to affect the activity of IMPDH. Taken together, we conclude that inosine exerts anti-inflammatory effects in many human cell types. Further studies need to establish whether inosine supplementation exerts anti-inflammatory effects in human beings.

Specific inhibition of type II inosine monophosphate dehydrogenase activity of Tmolt4 T cell human leukaemia cells by 3-methoxy and di-benzohydroxamic acids, maleic hydrazide and malonic acids

Small-molecular-weight benzohydroxamic and malonic acids and maleic hydrazide proved to be potent inhibitors of the activity of human Tmolt4 leukaemia Type II IMP (inosine monophosphate) dehydrogenase (IMPDH) activity. They were competitive inhibitors with respect to IMPDH demonstrating Ki values in the range 2.57-41.3 microM, less than half the values of the IC50 (microM) for the inhibition of Type II IMPDH. The IC50 microM values positively correlated with the ability of each compound to inhibit crude IMPDH activity, de-novo purine and DNA syntheses and growth of the T leukaemia cell line. Compounds were not inhibitors of Type I IMPDH. Type I IMPDH predominates in normal resting cells compared with Type II which is found in rapidly proliferating cells. Discovery of agents which would selectivity target IMPDH found in proliferating cells should eliminate any antineoplastic therapeutic toxic effects in normal cells of the body.

Pharmacodynamics of the inhibition of GTP synthesis in vivo by mycophenolic acid

Mycophenolic acid is effective against a wide range of experimental tumors in rodents when given orally, despite rapid metabolism to the inactive glucuronide derivative and rapid clearance from plasma. In the light of this, the pharmacodynamic action of mycophenolic acid on the radiolabelling of GTP and ATP by [14C]hypoxanthine in spleen and heart has been investigated in vivo in the rat as a preliminary to studies in tumor tissue. The data indicate that inhibition of GTP, and more surprisingly, ATP synthesis in spleen was sustained for at least 24.25 hr after single oral doses of the disodium salt of mycophenolic acid, indicating that the inhibitor is retained in sensitive cells for considerably longer than might be expected from the pharmacokinetic profile in the plasma in this species. GTP and ATP levels became depressed in rat spleen subsequent to the inhibition of nucleotide radiolabelling. The persistence of mycophenolic acid in proliferating cells may account for the effectiveness of once daily dosing against rapidly growing experimental tumors. In contrast with spleen, there was no incorporation of radiolabel from [14C]hypoxanthine into either GTP or ATP in rat heart and mycophenolic acid had no effect on the cardiac content of either nucleotide. The lack of effect of mycophenolic acid on cardiac GTP levels is consistent with the absence of any pharmacological action on cardiac functions associated with receptor-G-protein-GTP interactions. The ability of the morpholinoethyl ester of mycophenolic acid (a clinically effective immunosuppressive agent) to inhibit GTP synthesis and depress GTP levels in rat spleen in vivo was compared with that of mycophenolic free acid and its disodium salt. The ester derivative was clearly more effective than the poorly water-soluble free acid but showed comparable activity with the freely soluble disodium salt.

GM-CSF: modulation of biochemical and cytotoxic effects of tiazofurin in HL-60 cells

Cytokines, such as granulocyte macrophage colony stimulating factor (GM-CSF) or interleukin-3 (IL-3) recruit quiescent cells into the cell cycle and sensitize these cells towards cell cycle specific chemotherapeutic agents. We examined the in vitro effects of GM-CSF on HL-60 cells and tested its modulatory influence on biochemical and cytotoxic effects seen with tiazofurin, a potent and specific inhibitor of IMP dehydrogenase. Incubation of HL-60 cells with 500 U/ml GM-CSF for 4 d enhanced cell proliferation, which was accompanied by a significant increase in IMP dehydrogenase activity (from 2.22 in control cells to 3.70 nmol/mg/h in cells pretreated with GM-CSF). When HL-60 cells were incubated with 100 microM tiazofurin for 2 h, intracellular GTP decreased to 46% of untreated control cells. In HL-60 cells pretreated with GM-CSF, GTP pools decreased to 38% of control after incubation with tiazofurin which is 69% of the predicted value for additive effect. The MTT chemosensitivity assay yielded significantly decreased IC50 values for tiazofurin in HL-60 cells, preincubated with GM-CSF (IC50 decreased from 13 microM to 10 microM). Therefore our results suggest that combination therapy with GM-CSF and tiazofurin may be beneficial for the treatment of refractory leukaemia patients.

Cell cycle dependent regulation of IMP dehydrogenase activity and effect of tiazofurin

The activity of IMP dehydrogenase (IMP DH), the rate-limiting enzyme of de novo GTP biosynthesis, was shown to be increased in cancer cells. Tiazofurin, an inhibitor of IMP dehydrogenase, proved to be an effective agent in the treatment of refractory granulocytic leukemia. To examine the cell cycle dependent alterations of GTP synthesis and sensitivities to tiazofurin, we measured IMP DH activities and GTP pools, as well as the effects of tiazofurin on cell cycle phase enriched HL-60 cells. We now show that IMP DH activities and GTP concentrations are increased in S-phase enriched fractions of HL-60 cells. Moreover, the depletion of GTP concentrations by tiazofurin is most effective in S-phase enriched HL-60 cells. These results may be utilized in cancer chemotherapy to combine tiazofurin with biologic response modifiers which recruit quiescent leukemic cells into the cell cycle.

Synergistic action of tiazofurin and difluorodeoxycytidine on differentiation and cytotoxicity

Tiazofurin (TR), an inhibitor of IMP dehydrogenase, causes remissions and induced differentiation in human leukemia through lowering the concentrations of GTP and dGTP. A deoxycytidine analog, difluorodeoxycytidine (DFDC), is an anti-tumor agent phosphorylated by deoxycytidine kinase, resulting in decreased concentration of dCTP, leading to inhibition of DNA synthesis. In HL-60 cells DFDC induced differentiation and inhibited proliferation in a dose-dependent manner (IC50 = 4 nM); TR provided synergism with DFDC. DFDC inhibited proliferation in OVCAR-5 human ovarian carcinoma cells (IC50 = 25 nM) and colony formation in PANC-1 human pancreatic carcinoma cells (IC50 = 2 nM) and rat hepatoma 3924A cells (IC50 = 22 nM). TR and DFDC are synergistically cytotoxic in hepatoma cells and additive in PANC-1 cells. The two drugs together should be helpful in treating leukemias and solid tumors in humans.