Anticancer activity of rViscumin (recombinant mistletoe lectin) in tumor colonization models with immunocompetent mice.

The antitumoral and immunostimulating properties of rViscumin (recombinant mistletoe lectin) were investigated in two mouse tumor models. After intravenous inoculation with RAW-117-P or L-1 sarcoma cells in Balb/c mice, rViscumin was given s.c. at non-toxic doses ranging from 0.3 to 150 ng rViscumin/kg. One set of experiments was performed to investigate the survival of rViscumin-treated animals. Another set was carried out to analyze the effect of rViscumin treatment on the number of tumor colonies in infiltrated lungs (RAW-117P) or liver (L-1) and the activation of immune cell subsets, respectively. An overall prolonged survival time after treatment with rViscumin and a reduction in the number of tumor colonies after administration of certain rViscumin doses was observed. Immunophenotyping of the peripheral leukocytes of treated mice revealed increased numbers of T-lymphocytes, pan-NK cells and activated monocytes. The results indicate that rViscumin has antineoplastic properties and might therefore be a promising candidate in cancer therapy.

Cytotoxic activity of recombinant bFGF-rViscumin fusion proteins.

A fusion protein (bFGF-rMLA), containing the mitogen basic fibroblast growth factor (bFGF) and the cytotoxic component of rViscumin (recombinant mistletoe lectin), the enzymatic A-chain (rMLA), was expressed in Escherichia coli, purified, and functionally characterized. bFGF-rMLA is cytotoxic for mouse B16 melanoma cells expressing the FGF receptor with an IC(50) value of approximately 1 nM. rMLA shows no significant effect on the viability of the B16 cells up to a concentration of 141 nM. Additionally, bFGF-rMLA was associated with the rViscumin B-chain (rMLB) in an in vitro folding procedure. The IC(50) value of bFGF-rMLA/rMLB to B16 cells in the presence of lactose-to block rMLB lectin activity-was 134 pM. Thus, it was possible to enhance the efficacy of a rViscumin A-chain mitotoxin through addition of rMLB. We conclude that rViscumin fusion proteins may be generally applicable for the receptor-specific inactivation of target cells and point out their potential in drug development.

Site-specific mutagenesis of mistletoe lectin: the role of RIP activity in apoptosis.

Recombinant mistletoe lectin (rML) belongs to the class of type II ribosome-inactivating proteins (RIP) composed of a catalytically active A-chain with rRNA N-glycosidase activity and a B-chain with carbohydrate binding properties. To investigate the contribution of the enzymatic activity of the rML A-chain to the observed cytotoxic and apoptotic effects, an rMLA E166Q R169Q molecule was developed by means of site-specific mutagenesis. Following heterologous expression, the activity of mutant rMLA was measured in a cell-free assay for rRNA-N-glycosidase activity. Moreover, after generation of heterodimer, the activities of mutant rML E166Q R169Q and rML wild type were determined in a cytotoxicity and apoptosis assay. Although the reduction of activity as measured in the cell-free RIP assay was more pronounced (factor 237) than in both cellular assays (factors 20-22), the data clearly indicate a close correlation between cytotoxicity, apoptosis, and the enzymatic activity of the rML A-chain. Thus, RIP activity is an essential feature of rML and therefore a prerequisite for its biological function as an anticancer agent.

Characterization of recombinant and plant-derived mistletoe lectin and their B-chains.

Mistletoe lectin I (pML) and its isoforms ML II and III constitute the active principle in extract preparations from mistletoe, commonly used as immunomodulator in adjuvant tumour therapy. The heterodimeric disulfide-linked cytotoxic protein is classified as type II ribosome inactivating protein (RIP). Recently, the sequence coding for the mistletoe lectin prepro-protein was identified and the existence of a single intron-free gene was shown [Eck, J., Langer, M., Möckel, B., Baur, A., Rothe, M., Zinke, H. & Lentzen, H. (1999) Eur. J. Biochem. 264, 775-784]. The aim of this study was to prepare pure and homogeneous rMLB-chain as well as rML heterodimer for studying the carbohydrate binding specificity of recombinant versus natural protein and its contribution to the observed cytotoxic effect. Expression in E. coli resulted in the production of insoluble protein (inclusion bodies). A procedure for generating correctly folded, biochemically and biologically active rMLB was established starting from the insoluble single chain. Carbohydrate binding and specificity of pMLB and rMLB were analysed by a competitive enzyme linked lectin assay (ELLA). Asialofetuin was able to compete with binding of both chains (50% at 0.8 microM). The specificity of the B-chains to lactose was more distinct with halfmaximal competition at 4.9 mM (pMLB) and > 90 mM (rMLB), respectively. Furthermore, in a coassociation process rMLA- and rMLB inclusion bodies were associated in one step by defined dilution yielding active rML-heterodimer. The activities of recombinant (rML) and plant derived mistletoe lectin (pML) were compared. Cytotoxicity was determined using MOLT-4 cells and enzymatic rRNA N-glycosidase activity was measured in a coupled transcription/translation assay. The IC50 values of the two heterodimers were similar in both assays; rMLB-chain did not show any cytotoxic effect. In the ELLA with lactose as a competitor 50% competition of binding to asialofetuin was achieved at 1.6 mM (rML) and 1.8 mM (pML). Hence, using three different assays we found no significant differences between the recombinant protein and the glycosylated form of ML. Comparing the biological activities of the single chains with those of the heterodimer we conclude, that both, lectin activity and the rRNA N-glycosidase activity, are prerequisites for the cytotoxic effects on target cells.

Cloning of the mistletoe lectin gene and characterization of the recombinant A-chain.

Mistletoe lectin I (MLI) is the major active constituent of mistletoe extracts, which are widely used for adjuvant tumour therapy. The 66-kDa heterodimeric disulphide-linked glycoprotein is classified as type II ribosome-inactivating protein (RIP) due to the rRNA-cleaving enzyme activity of the A-subunit, also referred to as toxic entity. MLI and the close relative ricin both belong to the family of the two-chain plant type II RIP proteins. Isolation of the glycosylated proteins from plant material yield inhomogeneous material probably due to post-translational modifications. The aim of this study was to prepare pure and homogeneous protein as a prerequisite for structural and mechanistic studies in order to gain insight into the mode of action of this cytotoxic plant protein on tumour and immune cells. Of particular interest was to explain whether the differences in toxicity of ML and ricin are the result of variations of their enzymatic activities. By investigating the sequence homologies between the active sites of different RIPs we were able to deduce a set of primers which were suitable for specific amplification of the mistletoe lectin gene. Applying this PCR strategy the full-length 1923 nucleotide DNA sequence coding for the prepro-protein was obtained showing the existence of a single intron-free gene. In order to elucidate the molecular basis for the observed differences in cytotoxicity within the family of RIP the enzymatic A-subunit was expressed in a heterologous system. Expression of the A-chain in E. coli BL21/pT7 resulted in production of insoluble inclusion bodies constituting 20-30% of total protein. Refolding led to a pure and homogeneous protein species with an apparent molecular mass of 27 kDa and a pI value of 6.4. The ribosome-inactivating activity of the unglycosylated recombinant A-chain (IC50 20.5 pM) protein was in the same range as that of the glycosylated plant-derived ML A-chain (IC50 3.7 pM), which was very similar to that of ricin A-chain (IC50 4.9 pM). Thus, the higher cytotoxicity of ricin cannot be accountable for differences in the enzymatic activities of the type II RIP A-chains.

Effects of mistletoe lectin I on human blood cell lines and peripheral blood cells. Cytotoxicity, apoptosis and induction of cytokines.

The effects of mistletoe lectin I (ML I) on the human T-cell leukemia line MOLT-4, the monocytic line THP-1 and on human peripheral blood mononuclear cells (PBMC) were investigated with regard to general cell viability and induction of apoptosis. Using a sensitive serum-free cytotoxicity assay, the time- and concentration-dependent direct toxicity towards MOLT-4 cells was determined with IC50-values ranging from 20-40 pg/ml (300-600 fmol/l). Investigations on the time course of the toxic effect using selected concentrations of ML I revealed distinct response curves for concentrations of high, low and intermediate toxicity, respectively. The ratio of apoptotic to viable MOLT-4 cells was determined after treatment with ML I for 24 h. Apoptosis and cytotoxicity were correlated at low and intermediate concentrations, whereas at long intervals and high concentrations of ML I mostly necrotic effects were observed. The data showed that in the concentration range of low cytotoxicity ML I-induced cell death is quantitatively due to apoptotic processes. The immunomodulatory activity of ML I was investigated in vitro by measuring cytokine release. At concentrations of low cytotoxicity ML I showed immunostimulatory activity on PBMC and THP-1. RT-PCR with THP-1 cells confirmed that cytokine induction by ML I is regulated on the transcriptional level. These findings suggest that in the blood cells investigated both apoptosis and cellular signalling are induced by the same concentration range of ML I.

A nonradioactive assay for ribosome-inactivating proteins.

A sensitive nonradioactive method to determine the activity of ribosome-inactivating proteins (RIPs) based on a combined transcription/translation in vitro assay was established. Using this assay we investigated the RIP activities of the heterodimeric toxic plant lectins ricin and mistletoe lectin I (ML-I). The enzymatic activities of the holoproteins were compared to that of the RIP-active chain of ML-I (ML-I A-chain) and recombinant ML-I A-chain expressed in Escherichia coli. The IC50 values determined for the plant toxins showed that the translation-inactivating activity of ML-I (39.8 pM) is about four times higher than that of ricin (176.0 pM). The plant-derived ML-I A-chain is more toxic (3.4 pM) in the cell-free translation system than the respective holoprotein. The recombinant ML-I A-chain was found to be about three times less active (IC50 10.6 pM) than the A-chain from plant. The in vitro assay described here is a convenient method for the fast determination of RIP activity with a 1000-fold lower detection limit than that of commonly used RIP assays.

Expression of apolipoprotein A-I in porcine brain endothelium in vitro.

Apolipoprotein (apo) A-I is the major protein component of high-density lipoproteins (HDLs), which are responsible for reverse cholesterol transport from peripheral tissues to the liver. A low level of plasma HDL is correlated with susceptibility to atherosclerosis and coronary heart disease. Mammalian apo A-I synthesis has been attributed mainly to liver and intestine. Recently, apo A-I expression has been shown in porcine brain capillaries, suggesting an independent lipid metabolism within the brain. In this study, protein synthesis and secretion were investigated in primary cultures of porcine brain microvascular endothelial cells and compared with those in large vessel endothelium. Active protein synthesis in vitro was demonstrated by metabolic labeling. Cerebral endothelial cells were shown to secrete apo A-I into the culture supernatant, whereas aortic endothelial cells were negative for apo A-I expression. Further studies of transcriptional regulation showed that cerebral endothelium was responsive to apo A-I-inducing agents, such as cholesterol, insulin, and retinoic acid, as previously shown in human hepatoma HepG2 cells. Thus, cultures of porcine cerebral endothelial cells may represent a suitable model for physiological studies of apo A-I-regulation with regard to brain lipid metabolism and blood-brain barrier function. To investigate the interspecies conservation of regulatory elements, 178 bp of the 5' flanking region of the porcine apo A-I gene was cloned using PCR techniques. Alignments of the cDNA, of the deduced apo A-I protein sequence, and of the 5' promoter region with the corresponding genomic sequences of different species show a high degree of similarity between the porcine and the primate apo A-I genes, thus indicating a similar function and possibly common regulatory mechanisms in those species. In contrast, the rodent and avian apolipoprotein A-I promoter sequences differed significantly.