Pyrene acts as a cocarcinogen with the carcinogens benzo[a]pyrene, beta-propiolactone and radiation in the induction of malignant transformation in cultured mouse fibroblasts; soybean extract containing the Bowman-Birk inhibitor acts as an anticarcinogen

Development of the Bowman-Birk inhibitor for oral cancer chemoprevention and analysis of Neu immunohistochemical staining intensity with Bowman-Birk inhibitor concentrate treatment

OBJECTIVES/HYPOTHESIS

Cancer chemoprevention is a rapidly evolving approach to reverse or inhibit carcinogenesis, and there is active interest in development of effective chemopreventive agents against head and neck cancers. The retinoids are archetypal chemopreventive agents for oral premalignant lesions. They have significant clinical effect, but widespread use is limited by significant clinical toxicity. The Bowman-Birk Inhibitor is one of several nontoxic compounds exhibiting both potent anticarcinogenic activity and minimal toxicity. The purposes of the study were to summarize the preclinical and clinical development of Bowman-Birk Inhibitor and a Bowman-Birk Inhibitor concentrate against oral premalignant lesions and to evaluate Neu immunohistochemical staining intensity for lesions and simultaneously obtained biopsy specimens of normal-appearing mucosa from the Phase IIa Bowman-Birk Inhibitor concentrate oral leukoplakia chemoprevention trial.

STUDY DESIGN

Part I is a selected literature review. Part II is a retrospective analysis of pathological specimens prospectively obtained from the Phase IIa clinical trial of Bowman-Birk Inhibitor concentrate.

METHODS

Thirty-two sets of biopsy specimens from lesions and uninvolved oral mucosa before and after treatment with Bowman-Birk Inhibitor concentrate in doses ranging from 200 to 1066 chymotrypsin inhibitory units were examined in blinded fashion for Neu immunohistochemical staining intensity using the 3B-5 monoclonal antibody. Staining intensity scores among the lesion and control biopsy specimens before and after Bowman-Birk Inhibitor concentrate treatment were analyzed and compared with previously obtained values for serum Neu, oral mucosal cell Neu, protease activity, and clinical response to treatment.

RESULTS

Mean Neu staining score was significantly higher in lesions compared with uninvolved mucosa (P <.001). Pretreatment staining scores for biopsy specimens of lesions and control biopsy specimens of normal-appearing tissues were correlated (Spearman correlation coefficient [r] = 0.375, P =.045), but no correlation between lesion and control biopsy specimen scores was evident after treatment. The change in Neu staining score with Bowman-Birk Inhibitor concentrate treatment in control site biopsy specimens demonstrated an inverse relationship of change in lesion area with Bowman-Birk Inhibitor concentrate treatment (Spearman r = -0.493, P <.007).

CONCLUSION

Bowman-Birk Inhibitor concentrate shows promise to become an effective nontoxic chemopreventive agent based on results of extensive preclinical studies, and Phase I and Phase IIa clinical trials. Bowman-Birk Inhibitor concentrate has dose-related clinical activity against oral leukoplakia and modulates levels of Neu and protease activity. The current investigation identified increased Neu staining intensity in hyperplastic lesions compared with simultaneously obtained biopsy specimens of normal-appearing mucosa both before and after Bowman-Birk Inhibitor concentrate treatment. This finding supports prior observations that increased Neu expression is present in a subset of oral premalignant lesions and head and neck cancers. The trend of increased Neu staining score in control biopsy tissues of subjects exhibiting decreased lesion area following Bowman-Birk Inhibitor concentrate treatment raises questions about the mechanisms of Bowman-Birk Inhibitor concentrate action. One possible explanation is that Bowman-Birk Inhibitor stabilizes the extracellular domain of Neu, thereby preventing receptor truncation and internalization. Further study of modulation of Neu and protease activity by Bowman-Birk Inhibitor concentrate treatment may provide insights into the role of proteases and protease inhibitors in oral premalignant lesions and the mechanisms underlying Bowman-Birk Inhibitor concentrate effects. A Phase IIb randomized, placebo-controlled clinical trial to determine the clinical effectiveness of Bowman-Birk Inhibitor concentrate and further evaluate these candidate biomarkers is under way.

Treatment with field bean protease inhibitor can effectively repress ethylnitrosourea (ENU)-induced neoplasms of the nervous system in Sprague-Dawley rats

The ability of field bean protease inhibitor (FBPI) to inhibit ethylnitrosourea (ENU)-induced tumours of the nervous system of Sprague-Dawley rats was investigated. Groups of 1-day-old rats were injected intraperitoneally (i.p.) with neurocarcinogenic amounts of ENU and a few hours later, one group was treated i.p. with 80 mg of FBPI per kg body weight. This treatment was carried out three times a week for the first month and five times a week for the next month. Animals were killed when they were neurologically ill and their neural tissues were assessed for lesions. Those FBPI-treated rats which showed no illness were also killed to terminate the experiment about 8 weeks after the last rat of the control group was affected with paralysis. The neural tumours induced in all groups were predominantly large tumours found in the cerebrum of the rats. ENU-treated rats showed a 100% incidence of nervous system tumours with a mean time of manifestation of neurological symptoms of 282 days, which was significantly shorter in comparison to that noted in the FBPI-treated group. The latter group showed an incidence of 58.3%, i.e. a significant reduction of 41% in the incidence of neural tumours, as well as a lower mean value for the number of tumours per rat. All these aspects indicated that FBPI is a potential neurooncopreventive agent. A neural tumour incidence of 100% in the rats treated with heat-inactivated FBPI confirmed that the tumour suppressive activity of FBPI is related to its protease inhibitory activity.

Chemopreventive agents: protease inhibitors

Certain protease inhibitors, called the anticarcinogenic protease inhibitors in this review, are capable of preventing carcinogenesis in a wide variety of in vivo and in vitro model systems. The anticarcinogenic protease inhibitors are extremely potent agents with the ability to prevent cancer, with some unique characteristics as anticarcinogenic agents. The anticarcinogenic protease inhibitors have the ability to irreversibly suppress the carcinogenic process. They do not have to be continuously present to suppress carcinogenesis. They can be effective when applied in both in vivo and in vitro carcinogenesis assay systems at long time periods after carcinogen exposure, and are effective as anticarcinogenic agents at extremely low molar concentrations. While several different types of protease inhibitors can prevent the carcinogenic process, the most potent of the anticarcinogenic protease inhibitors on a molar basis are those with the ability to inhibit chymotrypsin or chymotrypsin-like proteases. The soybean-derived protease inhibitor, Bowman-Birk inhibitor (BBI), is a potent chymotrypsin inhibitor that has been extensively studied for its ability to prevent carcinogenesis in many different model systems. Much of this review is focused on the characteristics of BBI as the anticarcinogenic protease inhibitor, as this is the protease inhibitor that has risen to the human trial stage as a human cancer chemopreventive agent. Part of this review hypothesizes that the Bowman-Birk family of protease inhibitors plays a role in plants similar to that of alpha1-antichymotrypsin in people. Both BBI and alpha1-antichymotrypsin are potent inhibitors of chymotrypsin and chymotrypsin-like enzymes, are highly anti-inflammatory, and are thought to play important roles in the defense of their respective organisms. It is believed that BBI will be shown to play a major role in the prevention and/or treatment of several different diseases, in addition to cancer.

Insulin-like growth factor binding protein (IGFBP) proteases: functional regulators of cell growth

The IGFBP proteases were first described in pregnancy serum as a proteolytic activity against IGFBP-3. Since then, IGFBP proteases have been described in many other clinical situations, in various body fluids, and have been shown to cleave IGFBP-2 to -6 with varying specificity. The molecular nature of some of these proteases is being unraveled and three classes of IGFBP proteases have been recognized. These include kallikreins, cathepsins and matrix metalloproteinases (MMPs). We utilized two cellular systems to demonstrate the significance of IGFBP proteases in cellular growth regulation. In primary cultures of prostatic cells, we have shown that prostate-specific antigen (PSA) has the ability to enhance IGF mitogenic action by reducing the effects of IGFBPs. Similar kallikreins such as gamma nerve growth factor (NGF) share this activity. Within the prostatic milieu, we have also demonstrated epithelial production of the acid-activated IGFBP protease, cathepsin D, and its secretion into seminal plasma, as well as the serum of patients with prostate malignancy. We have also identified MMPs in prostatic cells and fluids. Using cultured airway smooth muscle (ASM) cells, we have demonstrated the synergism between IGFs and inflammatory agents in mediating ASM cell proliferation. Examination of this phenomenon revealed that these agents (e.g. leukotriene D4 and interleukin1-beta) induce the secretion of an IGFBP protease which cleaves the IGFBPs secreted by ASM cells, allowing IGFs to stimulate proliferation. Using several methods, including immunoblotting and immunodepletion techniques, we have identified this protease as MMP-1. These two pathophysiological systems demonstrate the importance of IGFBP proteases as autocrine paracrine growth regulators. Furthermore, IGFBP proteases may be critical elements in malignant and benign proliferative diseases, including prostate cancer and the ASM hyperplasia of long-standing asthma.

Mechanistic considerations in chemopreventive drug development

This overview of the potential mechanisms of chemopreventive activity will provide the conceptual groundwork for chemopreventive drug discovery, leading to structure-activity and mechanistic studies that identify and evaluate new agents. Possible mechanisms of chemopreventive activity with examples of promising agents include carcinogen blocking activities such as inhibition of carcinogen uptake (calcium), inhibition of formation or activation of carcinogen (arylalkyl isothiocyanates, DHEA, NSAIDs, polyphenols), deactivation or detoxification of carcinogen (oltipraz, other GSH-enhancing agents), preventing carcinogen binding to DNA (oltipraz, polyphenols), and enhancing the level or fidelity of DNA repair (NAC, protease inhibitors). Chemopreventive antioxidant activities include scavenging reactive electrophiles (GSH-enhancing agents), scavenging oxygen radicals (polyphenols, vitamin E), and inhibiting arachidonic acid metabolism (glycyrrhetinic acid, NAC, NSAIDs, polyphenols, tamoxifen). Antiproliferation/antiprogression activities include modulation of signal transduction (glycyrrhetinic acid, NSAIDs, polyphenols, retinoids, tamoxifen), modulation of hormonal and growth factor activity (NSAIDs, retinoids, tamoxifen), inhibition of aberrant oncogene activity (genistein, NSAIDs, monoterpenes), inhibition of polyamine metabolism (DFMO, retinoids, tamoxifen), induction of terminal differentiation (calcium, retinoids, vitamin D3), restoration of immune response (NSAIDs, selenium, vitamin E), enhancing intercellular communication (carotenoids, retinoids), restoration of tumor suppressor function, induction of programmed cell death (apoptosis) (butyric acid, genistein, retinoids, tamoxifen), correction of DNA methylation imbalances (folic acid), inhibition of angiogenesis (genistein, retinoids, tamoxifen), inhibition of basement membrane degradation (protease inhibitors), and activation of antimetastasis genes. A systematic drug development program for chemopreventive agents is only possible with continuing research into mechanisms of action and thoughtful application of the mechanisms to new drug design and discovery. One approach is to construct pharmacological activity profiles for promising agents. These profiles are compared among the promising agents and with untested compounds to identify similarities. Classical structure-activity studies are used to find optimal agents (high efficacy with low toxicity) based on good lead agents. Studies evaluating tissue-specific and pharmacokinetic parameters are very important. A final approach is design of mechanism-based assays and identification of mechanism-based intermediate biomarkers for evaluation of chemopreventive efficacy.

Protease inhibitors in chemoprevention of cancer. An overview

In the search for chemopreventive agents for cancer many natural products have been identified. Among them extensive experimental studies have been made on protease inhibitors which not only occur naturally in many plant products, but have also been synthesized in the laboratory. Many of these studies revealed the fact that protease inhibitors are able to prevent carcinogenesis and tumour promotion. A review of the reports available to date suggests that protease inhibitors are potential chemopreventive agents although their biological role and mechanism of action are not very clear.

Potential mechanisms of antitumorigenesis by protease inhibitors

The mechanisms by which protease inhibitors suppress carcinogenesis are unknown. From our studies, we believe the first event in carcinogenesis is a high frequency epigenetic event and that a later event (presumably genetic) leads to the malignant state. Protease inhibitors appear to be capable of reversing the "initiating" event in carcinogenesis, even at long times after carcinogen exposure. Protease inhibitors are thought to stop an ongoing process begun by carcinogen exposure. Effects of protease inhibitors on the following phenomena are thought to be related to their anticarcinogenic activity: 1) ability to effect the expression of certain oncogenes, and 2) ability to return carcinogen-increased levels of certain proteolytic activities (e.g., Boc-val-pro-arg-MCA hydrolyzing activity) to normal levels. Other effects of anticarcinogenic protease inhibitors have also been observed; for example, we have reported that they can bring carcinogen-induced, elevated levels of gene amplification to nearly normal levels. The mechanism(s) by which protease inhibitors suppress carcinogenesis will not be determined with certainty until the mechanisms involved in cancer induction are known.

Preparation and production of a cancer chemopreventive agent, Bowman-Birk inhibitor concentrate

We describe our studies to produce an extract of soybeans with anticarcinogenic activity that we believe will be useful as a human cancer chemopreventive agent for several different organs. The anticarcinogenic activity of the extract is thought to be due to chymotrypsin inhibitor activity, which is due to the Bowman-Birk protease inhibitor (BBI) present in the extract, termed BBI concentrate (BBIC). We describe the contents of BBIC, the ability of BBIC to inhibit malignant transformation in vitro in terms of its chymotrypsin inhibitor activity, and the results of long-term toxicity studies in which mice and rats were exposed to high levels of BBIC for long periods of time.

Effect of vitamin A dietary intake on in vitro and in vivo activation of aflatoxin B1

The mechanism by which vitamin A prevents or delays in chemical carcinogenesis remains unclear. In the present study, we assess the suggestive role of vitamin A in the initiation phase of carcinogenesis. We have conducted a dose-effect relationship between vitamin A dietary intake and aflatoxin B1 (AFB1) genotoxicity measured both in vitro and in vivo. Thus AFB1-induced mutagenesis in Salmonella typhimurium TA98 was investigated and compared to AFB1-induced single-strand breaks (SSBs) in DNA of rat hepatocytes. Rats were fed ad libitum with diet containing 0, 5, 50 or 500 IU of retinyl palmitate for 8 weeks. The AFB1-treated rats were injected i.p. with 1 mg/kg body weight. In the Ames test conditions TA98 back-reversion was negatively correlated with the log of vitamin A concentration in liver S9 fractions from experimental groups. However, the activities of metabolizing enzymes which specifically activate or deactivate AFB1 were found to be significantly decreased in vitamin A-deficient animals and weakly modified in vitamin A-supplemented animals. For in vivo experiments, the DNA elution rate of both AFB1-treated and untreated rats was increased in vitamin A deficiency condition (+79% and +17% respectively) and was reduced with the higher vitamin A dietary level (-44% and -53% respectively). DNA damage measured in vivo showed a significant positive correlation with mutagenic activity measured in the Ames test. These results confirm that the vitamin A status of animals can influence AFB1 genotoxic activity in vitro and indicate that this phenomenon also occurs in vivo. Thus a similar mechanism may be considered for the protective action of vitamin A both in vitro and in vivo. However, this mechanism is unlikely to involve modulation of the microsomal enzyme system responsible for AFB1 metabolism. Therefore a protective mechanism at the cytosolic or nuclear levels may be suggested.

Chemoprevention of cancer: protease inhibitors

1. The defense of the organism against cancer by inhibitors of proteolytic enzymes which are able to block the metastasizing stage of the disease is reviewed. 2. The contemporary views on the possible mechanisms of the process of prevention on both molecular and cellular levels are presented.

A growth-regulated protease activity that is inhibited by the anticarcinogenic Bowman-Birk protease inhibitor

The Bowman-Birk protease inhibitor (BBI) has been shown to be an effective suppressor of carcinogenesis in vivo and in vitro. To elucidate the mechanism(s) by which BBI suppresses carcinogenesis, we believe it will be necessary to identify and characterize the target enzymes that specifically interact with the BBI. We have shown previously that several cellular proteins in C3H/10T1/2 mouse embryo fibroblast cells specifically bind to a BBI affinity resin. In the current report, we demonstrate that one of these proteins has proteolytic activity as judged by its ability to degrade gelatin. The enzyme has a mass of 45 kDa and subcellular fractionation experiments demonstrate that this enzyme is located in the cytosol. Furthermore, the proteolytic activity was inhibited by diisopropylfluorophosphate but was not affected by EDTA, indicating that this enzyme is a serine protease. Higher levels of protease activity were found in logarithmic-phase C3H/10T1/2 cells compared with nondividing (confluent) cells, suggesting that this protease activity is growth regulated. Similar levels of this activity were present in nontransformed and in radiation-transformed C3H/10T1/2 cells. Treatment of nontransformed C3H/10T1/2 cells with phorbol 12-myristate 13-acetate increased the specific activity of this protease 5- to 10-fold. Our results suggest that this protease is a target enzyme of the BBI in these cells.

In vitro reduction of peroxidation in UVC-irradiated cell cultures by concurrent exposure with Bowman-Birk protease inhibitor

Bowman-Birk protease inhibitor (BBI), extracted from soybean flour, has been shown to protect cells from damage after UVC irradiation. BBI also reduces the number of transformed foci following exposure to the same doses of UVC irradiation. Proteins have been reported to act as antioxidants by their sheer intracellular bulk. In this study we assessed BBI's ability to attenuate UVC-irradiation-induced peroxidation. Balb c/3T3 cells, clone A31, subcloned by this laboratory were used. Treatment groups consisted of cells suspended in Hanks' Balanced Salt Solution with and without 100 micrograms/ml of a crude soybean extract containing the Bowman-Birk protease inhibitor. Cells were exposed to irradiation in suspension by using GE (G8T5) germicidal lamps which deliver predominantly 254 nm light at 0.385 J/m2/s as measured at the cell surface. Aliquots of UVC-exposed cells were homogenized in HBSS in the presence and absence of 100 micrograms/ml BBI with a motor-driven Teflon pestle and assayed for peroxidation. The results showed significant (P less than 0.0001) protection afforded by BBI against peroxidation at 1.16 (65%), 1.93 (60%, and 2.70 (48%) J/m2 of UVC irradiation. Transformation was reduced to 0.33 foci per dish at all levels of radiation exposure.

Mechanisms of inhibitors of mutagenesis and carcinogenesis. Classification and overview

The mechanisms of action of inhibitors of mutagenesis and carcinogenesis are reviewed in the light of our present knowledge. The identified mechanisms are classified into several categories and sub-categories, depending on the stage of intervention in the mutagenesis and carcinogenesis processes, and on the patterns of modulation of the host defense devices. Although a number of the known mechanisms fit into the proposed scheme, the available information on these problems is still fragmentary, and often inhibitors act through multiple mechanisms or can interact with other inhibitors. Moreover, due to the double-edged nature of many protective factors of the organism, and to the wide array of biological properties displayed by several inhibitors, the beneficial effects are in many instances counter-balanced by adverse reactions. Nevertheless, the present data-base on mechanisms of inhibitors, which is expected to grow rapidly in the near future, provides an extremely useful scientific premise for the primary prevention of mutation-related diseases. In this prospect, the elucidation of the underlying mechanisms complements the results emerging from the monitoring of protective end-points in mutagenicity and carcinogenicity test systems.

Dietary inhibitors of mutagenesis and carcinogenesis

Dietary inhibitors of mutagenesis and carcinogenesis are of particular interest because they may be useful for human cancer prevention. Several mutagenesis inhibitors have been demonstrated to be carcinogenesis inhibitors also, e.g., ellagic acid, palmitoleic acid, and N-acetylcysteine. This means that the search for mutagenesis inhibitors may be useful for discovering anticarcinogenic agents. Many mutagenesis inhibitors have been discovered by the use of short-term assays, particularly the Ames Salmonella test. This simple in vitro system has provided opportunities to elucidate the mechanisms of inhibition. The elucidation of the mechanism may allow us to infer the possible anticarcinogenic activity of the reagent. In this chapter, inhibitors of mutagenesis and carcinogenesis that can arise as components of diet have been reviewed. Most of the inhibitors have been demonstrated to be effective against a specific class of mutagens or carcinogens. Therefore, it may be argued that these inhibitors are antagonistic only to those particular agents. Here again, understanding of the mechanisms of these inhibitions is necessary for the assessment. Dietary inhibitors reviewed in this article include: (1) as inhibitors of mutagenesis: porphyllins, fatty acids, vitamins, polyphenols, and sulfhydryl compounds, (2) as inhibitors of carcinogenesis: vitamins A, E and C, ellagic acid, sulfhydryl compounds, fats, selenium, calcium, and fiber. Further studies in this area of science appear to help establish the recipe of a healthy diet.

Cocarcinogenic and tumor-promoting capabilities of anthralin

Numerous chemicals to which humans are exposed either therapeutically or as a result of living in an industrial environment constitute a potential threat as carcinogens, mutagens, and/or tumor promoters and cocarcinogens. Anthralin, and antipsoriatic agent, acts as a tumor promoter for Balb/c-3T3 mouse embryo cell cultures that were previously exposed to a low dose of either benzo-a-pyrene (BaP), an indirect-acting carcinogen needing metabolic conversion for its carcinogenic action, or beta-propiolactone (BPL), a direct-acting carcinogen which needs no metabolic conversion. As a cocarcinogen, i.e., when exposure of cells to anthralin was simultaneous with exposure to the carcinogen, anthralin enhanced neoplastic transformation only when the carcinogen was BaP. Several explanations are explored. The possibility that cocarcinogens and tumor promotion occur by separate mechanisms is suggested.

The influence of fluoranthene on the metabolism and DNA binding of benzo[a]pyrene in vivo in mouse skin

The effect of the cocarcinogen fluoranthene on the DNA binding and metabolism of [3H]benzo[a]pyrene (B[a]P) in vivo in mouse skin has been investigated. In the presence of fluoranthene the level of B[a]]P-DNA binding was increased at each of the time intervals examined (4, 8, 24 and 48 h) with enhancements ranging from 76% at 4 h to 36% at 48 h. The ratio of anti-7,8,-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE)-DNA adducts/syn-BPDE-DNA adducts was also increased in the presence of fluoranthene. This increase was greatest at 8 h (44%) but by 48 h the ratio was identical in the presence and absence of fluoranthene. The observed increase in anti-BPDE-DNA adducts/syn-BPDE-DNA adducts did not parallel increases in B[a]P-DNA binding suggesting that alteration of the anti-BPDE/syn-BPDE ratio is not a major contributing factor to the cocarcinogenic activity of fluoranthene. The influence of fluoranthene on the metabolism of B[a]P in vivo in mouse skin was also investigated. Fluoranthene was found to have little or no effect on the formation of ethyl acetate extractable metabolites of B[a]P in mouse skin. Specifically, there was no increase in the amount of B[a]P-7,8-diol in the presence of fluoranthene. Fluoranthene also had little or no effect on the levels of beta-glucuronide or sulfate conjugates of B[a]P metabolites formed in vivo in mouse skin. These studies suggest that the effect of fluoranthene is being expressed at some point after B[a]P has been activated to an ultimate carcinogen.