Expression, purification, and characterization of active recombinant prostate-specific antigen in Pichia pastoris (yeast).

BACKGROUND: Prostate-specific antigen (PSA), a member of the kallikrein family of serine proteases, is a chymotrypsin-like glycoprotein produced by the prostate epithelium. Elevated serum PSA (> 4 ng/ml) is a tumor marker for prostatic cancer and benign prostatic hypertrophy; increasing serum PSA over time is indicative of metastatic disease. It has been suggested that PSA may contribute to tumor metastasis through degradation of extracellular matrix glycoproteins, as well as cleavage of IGF binding protein-3, a modulator of IGF-1. To elucidate the role of PSA in the development and progression of prostatic cancer, it is necessary to have a reliable, cost-effective source of enzymatically active protein. Previous efforts to express recombinant PSA (rPSA) produced inactive proPSA, or mixtures of active and inactive PSA requiring activation by removal of the propeptide. We describe the expression of active recombinant mature PSA in yeast. METHODS: Stable chromosomal integration of a construct consisting of the yeast alpha-factor signal sequence preceding the mature PSA sequence resulted in secretion of rPSA. The rPSA was purified from the yeast cell culture supernatant to homogeneity by strong cation-exchange chromatography, and characterized by SDS-PAGE, Western analysis, electrospray mass spectrometry, N-glycanase digestion, N-terminal amino acid sequencing, and inactivation by a PSA-specific inhibitor. RESULTS: We report the production of active, mature rPSA in Pichia pastoris. Two forms of rPSA varying slightly in glycosylation were identified. The specific activity of the rPSA was equal to that of human seminal plasma PSA (0.56 micromol/min mg) as determined using a chromogenic substrate. CONCLUSIONS: Large-scale production of active rPSA will be useful in the exploration of PSA effects on tumor cell proliferation, migration and metastasis. In addition, a large supply of enzyme should facilitate the discovery of novel inhibitors for in vitro and in vivo evaluation, and may provide a reproducible source of rPSA for use as a standard in diagnostic testing.

Enzyme inhibition, polyglutamation, and the effect of LY231514 (MTA) on purine biosynthesis.

The pyrrolopyrimidine-based antifolate, N-¿4-[2-(2-amino-3,4-dihydro-4-oxo-7H-pyrrolo[2,3-d]pyrimidin-5-yl)ethyl ]benzoyl¿glutamic acid, LY231514 (MTA) has demonstrated antitumor activity in a broad array of human tumors, including breast cancer, colon cancer, non-small cell lung cancer, head and neck cancer, pancreatic cancer, and other solid tumors. The biochemical basis of this activity was explored by measuring activation of MTA by polyglutamation and the activity of MTA to inhibit several folate-dependent enzymes: thymidylate synthase, dihydrofolate reductase, and glycinamide ribonucleotide formyltransferase (GARFT). The enzyme folylpolyglutamate synthase (FPGS) activated MTA very efficiently. Using FPGS from two different species, Km values below 2 micromol/L and high relative first order rate constants, k' (Vmax/Km) of 6.4 and 13.7 compared with another substrate, lometrexol, were obtained. The formation of polyglutamates of several antifolates were compared in vitro at high and low substrate concentrations. At low MTA concentrations, tetraglutamated and pentaglutamated MTA were the predominant forms identified after a 24-hour incubation period. In contrast, only diglutamyl methotrexate and a mixture triglutamylated, tetraglutamylated, and pentaglutamylated forms of the GARFT inhibitor lometrexol were formed under the same conditions. At higher substrate concentrations (20 micromol/L, 24 hours), greater amounts of each product were formed. The major metabolites, however, were triglutamated MTA or triglutamated lometrexol, while only diglutamyl methotrexate was recovered. Thus, MTA was an excellent substrate for FPGS and it was efficiently metabolized to highly polyglutamated species by this enzyme. The activity of MTA and its polyglutamated metabolites to inhibit several folate-dependent enzymes was measured. In vitro, MTA and its polyglutamates were potent, tight-binding inhibitors of several folate-dependent enzymes, including thymidylate synthase, dihydrofolate reductase, and GARFT. Preliminary cell-based assays (CCRF-CEM) demonstrated inhibition of the purine de novo pathway by MTA, consistent with its multitargeted mechanism of action against tumor cells. The combined effects of activation of MTA to highly polyglutamated metabolites and the potency of these polyglutamates to inhibit multiple folate-dependent enzymes provide a mechanistic basis for understanding the broad antitumor activity of this compound against many human tumor types.

The synthesis and biological activity of a series of 2,4-diaminopyrido[2,3-d]pyrimidine based antifolates as antineoplastic and antiarthritic agents.

A new series of 2,4-diaminopyrido[2,3-d]pyrimidine based antifolates 1-3 were synthesized through an efficient conversion of 2-pivaloyl-4-oxo-6-ethynylpyrido[2,3-d]pyrimidine 5 to the corresponding 4-amino analog 7 via the activated 1,2,4-triazole intermediate 6. Compound 7 was used as the key intermediate for the preparation of the final products. The detailed biological evaluation of these compounds both as antineoplastic and antiarthritic agents will be discussed.

Multiple folate enzyme inhibition: mechanism of a novel pyrrolopyrimidine-based antifolate LY231514 (MTA).

Extensive biochemical and pharmacological evidence indicates that LY231514 is a novel antifolate antimetabolite. LY231514 is transported into cells mainly through the reduced folate carrier system and extensively metabolized to polyglutamated forms. The polyglutamates of LY231514 inhibit at least three key folate enzymes: TS, DHFR, and GARFT, and to a lesser extent AICARFT and C1-tetrahydrofolate synthase. The combined effects of the inhibition exerted by LY231514 at each target give rise to an unusual end-product reversal pattern at the cellular level that is distinct from those of other inhibitors such as methotrexate and the quinazoline antifolates. The metabolic effects exerted by LY231514 on the folate and nucleotide pools are also quite distinct from those of MTX and LY309887. The efficient polyglutamation, longer cellular retention and the multiple folate enzyme inhibition mechanism may all have contributed directly to the exciting antitumor responses now observed in Phase I and II studies. The multitargeted inhibition mechanism of LY231514 is particularly intriguing. This new level of mechanistic insight, which has evolved from the study of LY231514, challenges the traditional concept and paradigm of antifolate drug discovery and development which focused on developing very potent and selective inhibitors of single folate enzyme targets, such as DHFR, TS or one of the enzymes along the de novo purine biosynthetic pathway. Given the complex nature of folate metabolism and the critical role of folates in maintaining the physiological functions of living systems, it is completely reasonable to suspect that agents which can interfere at multiple sites in the folate pathway may trigger and cause more biochemical imbalance in the cellular DNA and RNA synthesis of malignant cells than agents that act on a single point (Fig. 5). In conclusion, LY231514 (MTA) is a new generation antifolate antimetabolite demonstrating inhibitory activity against multiple folate enzymes including TS, DHFR and GARFT. In current phase II studies, MTA is broadly active as a single agent and is showing very encouraging antitumor activity in multiple solid tumors including colorectal, breast and non-small cell lung cancers (38-43). The every three week dosing schedule has proven to be convenient and easy to administer and the clinical toxicities of LY231514 seem to be well tolerated. More advanced and extensive clinical trials of LY231514 are currently in progress.

The antiproliferative and cell cycle effects of 5,6,7, 8-tetrahydro-N5,N10-carbonylfolic acid, an inhibitor of methylenetetrahydrofolate dehydrogenase, are potentiated by hypoxanthine.

5,6,7,8-Tetrahydro-N5,N10-carbonylfolic acid (LY354899) has been demonstrated to inhibit the dehydrogenase activity of C1-tetrahydrofolate synthase. This compound was only moderately antiproliferative toward CCRF-CEM lymphocytic leukemia cells in culture, but induced apoptosis after long incubation times. Slightly greater potency was observed in CEM cells adapted to grow in low folate media. Cell cycle alterations induced by LY354899 were unique relative to antifolates that inhibit either the purine or thymidine de novo biosynthetic pathways. Based on the observed changes in DNA content, we hypothesized that inhibition of the dehydrogenase resulted in two temporally distinct events: the first was a purineless-like effect and the second was a thymineless-like effect that resulted in apoptosis. To test this hypothesis, we combined LY354899 with the purine salvage metabolite, hypoxanthine. This combination resulted in an earlier and more dramatic apoptotic response, indicating that the thymineless effect had been potentiated. Biochemical analysis of ribo- and deoxyribonucleoside triphosphates confirmed that inhibition of the dehydrogenase activity initially resulted in decreased pools of deoxypurines and deoxypyrimidines, followed 16 hr later by an increase in deoxyadenosine triphosphate (dATP) and a further decrease in deoxythymidine triphosphate (dTTP). These studies demonstrate that the inhibition of the dehydrogenase activity of C1-tetrahydrofolate synthase may represent a viable target for the development of novel antifolates. The results are discussed in terms of deoxypurine and deoxypyrimidine biosynthesis.

Whole-body disposition and polyglutamate distribution of the GAR formyltransferase inhibitors LY309887 and lometrexol in mice: effect of low-folate diet.

PURPOSE: The whole-body autoradiographic distribution of two radiolabeled antifolate inhibitors of GAR formyltransferase, lometrexol and LY309887, were compared in tumor-bearing mice maintained on standard diet (SD) and a low-folate diet (LFD) in order to determine the total amounts of drug that accumulated in blood, tumor, liver and kidney. The time-dependent changes in tissue distribution were evaluated over a 7-day period in order to compare the pharmacokinetic properties of both inhibitors and to assess the influence of dietary folate on this distribution. In addition, the effect of dietary folate on polyglutamation of compound accumulating in the liver was measured. The results have bearing on the potential of these two clinical agents to produce delayed toxicity in cancer patients and the use of dietary folate to modulate or prevent the development of this toxicity. METHODS: Single equimolar i.v. doses of [14C]LY309887 and [14C]lometrexol were administered to C3H mammary tumor bearing mice on SD or LFD, and the disposition of these compounds was quantitated using whole-body autoradiography. Livers were also harvested and extracted for determination of polyglutamate distribution. Animals were sacrificed both early and late (7 days) after dosing to determine the long-term retention of these compounds. RESULTS: Whole-body autoradiography revealed that the highest concentrations of both compounds were in liver and kidney. Concentrations of both compounds were two-fold higher in livers from LFD mice than in livers from SD mice. Lometrexol concentrations in liver averaged 2.8- and 2.2-fold higher than LY309887 in SD and LFD livers, respectively. In SD livers, the polyglutamate profiles of both compounds were similar, with hexaglutamates being the longest chain species detected. In LFD livers, hexaglutamates of LY309887 were observed, while hepta- and octaglutamates of lometrexol were detected after 168 h. CONCLUSIONS: The reduced hepatic retention and biochemical profile of LY309887 compared to lometrexol suggest that it may be less likely to produce delayed cumulative toxicity while still retaining antitumor activity. However, the increased hepatic accumulation observed in LFD mice emphasizes the importance of assessing and supplementing folate in cancer patients treated with this class of compounds.

LY231514, a pyrrolo[2,3-d]pyrimidine-based antifolate that inhibits multiple folate-requiring enzymes.

N-[4-[2-(2-amino-3,4-dihydro-4-oxo-7H-pyrrolo[2,3-d]pyrimidin-5-yl)ethyl ]-benzoyl]-L-glutamic acid (LY231514) is a novel pyrrolo[2,3-d]pyrimidine-based antifolate currently undergoing extensive Phase II clinical trials. Previous studies have established that LY231514 and its synthetic gamma-polyglutamates (glu3 and glu5) exert potent inhibition against thymidylate synthase (TS). We now report that LY231514 and its polyglutamates also markedly inhibit other key folate-requiring enzymes, including dihydrofolate reductase (DHFR) and glycinamide ribonucleotide formyltransferase (GARFT). For example, the Ki values of the pentaglutamate of LY231514 are 1.3, 7.2, and 65 nM for inhibition against TS, DHFR, and GARFT, respectively. In contrast, although a similar high level of inhibitory potency was observed for the parent monoglutamate against DHFR (7.0 nM), the inhibition constants (Ki) for the parent monoglutamate are significantly weaker for TS (109 nM) and GARFT (9,300 nM). The effects of LY231514 and its polyglutamates on aminoimidazole carboxamide ribonucleotide formyltransferase, 5,10-methylenetetrahydrofolate dehydrogenase, and 10-formyltetrahydrofolate synthetase were also evaluated. The end product reversal studies conducted in human cell lines further support the concept that multiple enzyme-inhibitory mechanisms are involved in cytotoxicity. The reversal pattern of LY231514 suggests that although TS may be a major site of action for LY231514 at concentrations near the IC50, higher concentrations can lead to inhibition of DHFR and/or other enzymes along the purine de novo pathway. Studies with mutant cell lines demonstrated that LY231514 requires polyglutamation and transport via the reduced folate carrier for cytotoxic potency. Therefore, our data suggest that LY231514 is a novel classical antifolate, the antitumor activity of which may result from simultaneous and multiple inhibition of several key folate-requiring enzymes via its polyglutamated metabolites.

Characterization of folate receptor from normal and neoplastic murine tissue: influence of dietary folate on folate receptor expression.

Membrane-associated folate receptors (FRs) have been detected in many mammalian species, and multiple isoforms have been identified. The pharmacological properties of FRs from murine kidney, liver, and six murine tumors were characterized. Murine kidney expressed primarily folate-binding protein 1, analogous to human FR-alpha, whereas murine liver expressed predominantly folate-binding protein 2, analogous to human FR-beta. Five of six murine tumors expressed high-affinity FRs with pharmacological properties consistent with folate-binding protein 1 isoform expression. Restriction of dietary folate resulted in significant changes in the FR expression in most murine tissues. Kidney and tumor FRs showed a decreased affinity for folic acid, suggesting a change in isoform expression in response to a low folate diet. Density of the FR in the kidney decreased, and, in contrast, density of the FR in all tumors increased. The response of the liver to a low folate diet was unique in that there were no detectable changes in affinity or density of liver FR. Changes in dietary folate that modulate FR isoform expression may have relevance for cancer patients treated with antifolates.

The role of dietary folate in modulation of folate receptor expression, folylpolyglutamate synthetase activity and the efficacy and toxicity of lometrexol.

We have studied the molecular effects of a LFD in a murine model in order to better define the biochemical changes associated with folate deficiency. In addition, we have demonstrated the effect of a LFD on the pharmacokinetic profile and therapeutic activity and toxicity of lometrexol. These studies showed increased density of FR in tumors implanted in LFD mice and a decrease in the affinity of these receptors for folic acid. The results suggest that tumors can compensate for low folate bioavailability by up-regulation of a second FR with slightly lower affinity for folic acid. The higher density of this FR would provide greater capacity for garnering serum folate. FPGS activity increased in several tumors and liver and kidney of LFD mice. The increase in this enzyme activity would result in enhanced polyglutamation of folates and classical antifolates and thus increased cellular retention. Consistent with these changes in liver FPGS, mice injected i.v. with a single dose of lometrexol accumulated significantly more drug in liver and tumors of LFD animals compared to SD mice. Also, higher liver concentrations of lometrexol persisted longer in LFD mice. Polyglutamate analysis showed that longer polyglutamate forms appeared earlier in liver of LFD mice. After 7 days, longer polyglutamyl forms were recovered from liver of LFD mice (octa- and hepta-glutamyl lometrexol) compared to those on SD. A comparison of the efficacy and toxicity of lometrexol in C3H mammary tumor-bearing mice showed that in mice on LFD, lometrexol treatment produced a delayed toxicity with an LD50 of 0.1-0.3 mg/kg, a 3000-fold increase in lethality compared to SD mice. Supplementation of mice with folic acid restored anti-tumor activity and increased the therapeutic dose-range over which efficacy could be assessed. These studies support the use of folic acid supplementation for cancer patients treated with antifolate therapy in order to prevent the biochemical changes in FR and FPGS associated with folate deficiency, prevent delayed toxicity to GARFT inhibitors and enhance the therapeutic potential of this class of drugs.

Substrate specificity of mammalian folylpolyglutamate synthetase for 5,10-dideazatetrahydrofolate analogs.

The metabolism of 5,10-dideazatetrahydrofolate (DDATHF [lometrexol]) to polyglutamate derivatives by folylpoly-gamma-glutamate synthetase (FPGS) plays a central role in the activity of this compound as an antineoplastic agent. The availability of a series of DDATHF derivatives differing in structure throughout the molecule has allowed a study of the structural requirements for substrate activity with mouse liver and hog liver FPGS. Kinetics of the polyglutamation reaction in vitro have been related to the potency of these compounds as inhibitors of the growth of human CEM leukemic cells. The structure-activity relationships for enzyme from both sources were nearly identical. FPGS from both species showed a broad acceptance for structural changes in the pyridopyrimidine ring, in the phenyl group, and in the intermediate bridge region, with structural changes in these regions being reflected in changes in Km for FPGS but much more modest alterations in Vmax. The data suggested that the phenyl ring was not contributing to any pi-pi hydrophobic interactions. It appeared to function primarily in maintaining a favorable distance between the pyridopyrimidine ring and the glutamate side chain. The lowest Km values were found for DDATHF analogs in which there were small alterations at the 10 position, e.g., 5-deazatetrahydrofolate, 10-methyl-DDATHF, and 10-formyl-5-deazatetrahydrofolate; the first-order rate constants for these substrates were the highest in this series, an indication of the efficiency of polyglutamation at low substrate concentrations. After correction for the intrinsic inhibitory activity of the parent DDATHF analog as an inhibitor of the target enzyme, the first-order rate constants for FPGS were found to be predictive of the potency of tumor cell growth inhibition for most of the compounds in this structural series.

A novel class of monoglutamated antifolates exhibits tight-binding inhibition of human glycinamide ribonucleotide formyltransferase and potent activity against solid tumors.

Tight-binding inhibition of recombinant human monofunctional glycinamide ribonucleotide formyltransferase by Lometrexol (6R-5,10-dideazatetrahydrofolate) requires polyglutamation. LY254155 and LY222306 differ from 5,10-dideazatetrahydrofolate in the replacement of the 1',4'- phenylene moiety by a 2',5'-thiophene and a 2',5'-furan, respectively. Compared to Lometrexol, the thiophene and furan analogues had 25- and 75-fold greater inhibitory potencies against human monofunctional glycinamides ribonucleotide formyltransferase (Ki = 2.1 and 0.77 nM, respectively). The binding affinities of the thiophene and furan analogues for membrane folate-binding protein from human KB cells were 6- and 350-fold weaker than Lometrexol, respectively. Both the thiophene analogue and 5,10-dideazatetrahydrofolate inhibited the in vivo growth of murine 6C3HED lymphosarcoma, murine C3H mammary carcinoma, and human xenograft HXGC3, HC1, and VRC5 colon carcinomas by 95-100%. The thiophene analogue was efficacious against human xenograft PANC-1, a pancreatic carcinoma which was completely resistant to 5,10- dideazatetrahydrofolate. These novel antifolates represent the first monoglutamated tight-binding inhibitors of glycinamide ribonucleotide formyltransferase. By eliminating the need for polyglutamation, this class of antifolates may have clinical activity in the treatment of solid tumors expressing low levels of folylpolyglutamate synthetase or tumors resistant to antifolate therapy due to increased gamma-glutamyl hydrolase activity.

Subdomains of the octopine synthase upstream activating element direct cell-specific expression in transgenic tobacco plants.

Previous work has shown that the octopine synthase (ocs) gene encoded by the Agrobacterium tumefaciens Ti-plasmid contains an upstream activating sequence necessary for its expression in plant cells. This sequence is composed of an essential 16-bp palindrome and flanking sequences that modulate the level of expression of the ocs promoter in transgenic tobacco calli. In this study, we have used RNA gel blot analysis of RNA extracted from transgenic tobacco plants to show that the octopine synthase gene is not constitutively expressed in all plant tissues and organs. This tissue-specific pattern of expression is determined, to a large extent, by the 16-bp palindrome. Histochemical analysis, using an ocs-lacZ fusion gene, has indicated that the 16-bp palindrome directs the expression of the ocs promoter in specific cell types in the leaves, stems, and roots of transgenic tobacco plants. This expression is especially strong in the vascular tissue of the leaves, leaf mesophyll cells, leaf and stem guard cells, and the meristematic regions of the shoots and roots. Sequences surrounding the palindrome in the upstream activating sequence restrict the expression of the ocs promoter to fewer cell types, resulting in a reduced level of expression of beta-galactosidase activity in the central vascular tissue of leaves, certain types of leaf trichomes, and the leaf primordia.

vir genes influence conjugal transfer of the Ti plasmid of Agrobacterium tumefaciens.

Mutation of the genes virA, virB, virC, and virG of the Agrobacterium tumefaciens octopine-type Ti plasmid pTiR10 was found to cause a 100- to 10,000-fold decrease in the frequency of conjugal transfer of this plasmid between Agrobacterium cells. This effect was not absolute, however, in that it occurred only during early times (18 to 24 h) of induction of the conjugal transfer apparatus by octopine. Induction of these mutant Agrobacterium strains by octopine for longer periods (48 to 72 h) resulted in a normal conjugal transfer frequency. The effect of these vir gene mutations upon conjugation could be restored by the introduction of cosmids harboring wild-type copies of the corresponding disrupted vir genes into the mutant Agrobacterium strains. In addition, transfer of the self-mobilizable plasmid pPH1JI was not impaired in any of the mutant Agrobacterium strains tested. The effect of vir gene function on the conjugal transfer of the Ti plasmid suggests that a relationship may exist between the processes that control the transfer of the T-DNA from Agrobacterium to plant cells and the conjugal transfer of the Ti plasmid between bacterial cells.