Testing for induction of DNA synthesis in human hepatocyte primary cultures by rat liver tumor promoters.

Parenchymal liver cells were isolated from human liver pieces of surgical waste as well as from rat livers. DNA synthetic activity was measured after different times in primary culture by [3H]thymidine incorporation and autoradiography. Labeling of control cultures of human hepatocytes at densities between 8,000 and 15,000 cells/cm2 was very low (0.4 to 1.3%). Human recombinant epidermal growth factor increased labeling 2- to 4-fold (P less than 0.01). Treatment with known inducers of liver growth in rats, namely, cyproterone acetate, alpha-hexachlorocyclohexane, nafenopin, phenobarbital, and rifampicin did not increase the number of labeled human liver cells. In some of the experiments, a 24-h exposure to the chemicals of rat or human hepatocytes was followed by a 24-h treatment with epidermal growth factor (EGF). In rat hepatocytes, incorporation rates were significantly increased. Cyproterone acetate and EGF acted in an additive manner, alpha-hexachlorocyclohexane and EGF were clearly overadditive, and phenobarbital had little effect. In human hepatocytes, little alteration in labeling indices was found; in some cases labeling was, rather, found to be lower than in cultures treated with EGF alone. These results show that human hepatocytes cultured in vitro are sensitive to stimulation of DNA synthesis by EGF; they differ from rat hepatocytes in their response to some drugs which show liver growth-promoting activity in rodents.     

Blockade of SDF-1 after irradiation inhibits tumor recurrences of autochthonous brain tumors in rats

Background

Tumor irradiation blocks local angiogenesis, forcing any recurrent tumor to form new vessels from circulating cells. We have previously demonstrated that the post-irradiation recurrence of human glioblastomas in the brains of nude mice can be delayed or prevented by inhibiting circulating blood vessel–forming cells by blocking the interaction of CXCR4 with its ligand stromal cell-derived factor (SDF)–1 (CXCL12). In the present study we test this strategy by directly neutralizing SDF-1 in a clinically relevant model using autochthonous brain tumors in immune competent hosts.

Methods

We used NOX-A12, an l-enantiomeric RNA oligonucleotide that binds and inhibits SDF-1 with high affinity. We tested the effect of this inhibitor on the response to irradiation of brain tumors in rat induced by n-ethyl-N-nitrosourea.

Results

Rats treated in utero with N-ethyl-N-nitrosourea began to die of brain tumors from approximately 120 days of age. We delivered a single dose of whole brain irradiation (20 Gy) on day 115 of age, began treatment with NOX-A12 immediately following irradiation, and continued with either 5 or 20 mg/kg for 4 or 8 weeks, doses and times equivalent to well-tolerated human exposures. We found a marked prolongation of rat life span that was dependent on both drug dose and duration of treatment. In addition we treated tumors only when they were visible by MRI and demonstrated complete regression of the tumors that was not achieved by irradiation alone or with the addition of temozolomide.

Conclusions

Inhibition of SDF-1 following tumor irradiation is a powerful way of improving tumor response of glioblastoma multiforme.     

Arachidonic acid potentiates superoxide anion radical production by murine peritoneal macrophages stimulated with tumor promoters

The role of arachidonic acid (AA) metabolism in the stimulation of oxygen radical production by murine peritoneal macrophages treated with tumor promoters was assessed. In vivo administration of the phospholipase A2 inhibitor dibromacetophenone, the anti-inflammatory steroid fluocinolone acetonide or the lipoxygenase inhibitor nordihydroguiaretic acid just prior to i.p. injection of phorbol-12-myristate-13-acetate (PMA, 100 ng) into unmanipulated CD-1 female mice resulted in a dose-dependent decrease in the number of peritoneal exudate cells (PEC) producing superoxide anion radical (O2) as assessed by the reduction of nitroblue tetrazolium, i.e. the formation of formazan-positive PEC. The cycloxygenase inhibitor indomethacin had no effect on the number of formazan-positive PEC caused by PMA treatment. The ability of PMA, phorbol-12,13-dibutyrate mezerein, phorbol-12,13-diacetate and 4-O-Me-PMA to stimulate the production of oxygen radicals by murine peritoneal macrophages correlated with their ability to stimulate the release of [3H]AA equivalents from the macrophages. The calcium ionophore A23187 which stimulated significant [3H]AA equivalent release did not stimulate superoxide anion radical production by the macrophages. PMA administered i.p. to SENCAR mice increased the number of formazan-positive PEC 4-to 5-fold compared with similarly treated C57BL/6 mice. PMA also stimulated the release of twice the amount of [3H]AA equivalents from peritoneal macrophages from SENCAR mice compared with that released by macrophages from C57BL/6 mice. The addition of low concentrations of AA (1-10 microM) in vitro to casein-elicited murine peritoneal macrophages treated with low concentrations of PMA (1 ng/ml) resulted in a 2-fold potentiation of the amount of superoxide anion radical produced compared with PMA treatment alone as assessed by the reduction of cytochrome c. These results demonstrate that AA and/or a lipoxygenase product can potentiate the production of oxygen radicals by murine peritoneal macrophages treated with tumor promoters.     

Stimulation of DNA synthesis in primary rat hepatocyte cultures by liver tumor promoters: interactions with other growth factors.

Mechanism(s) of tumor promotion in liver by xenobiotics such as hexachlorocyclohexane (HCH), 1,1,1-trichloro-2,2-bis (4-chlorophenyl)ethane (DDT) and phenobarbital (PB) are not understood in detail although growth-stimulatory effects may be significant in their action. As a basis for studying mechanisms of growth control by liver tumor promoters, effects of xenobiotics on DNA synthesis have been examined in primary cultures of normal rat hepatocytes, maintained under fully-defined conditions. The xenobiotics alone were relatively ineffective but they exhibited synergism with epidermal growth factor (EGF), insulin and dexamethasone in stimulating DNA synthesis and were effective in moderate-to-low density cultures but not in confluent monolayers. Under conditions optimized for HCH or pregnenolone 16 alpha-carbonitrile (PCN) (i.e. subconfluent cultures exposed to insulin, EGF and dexamethasone), HCH, PCN, DDT or PB caused a transient stimulation of DNA synthesis, apparent after 2 days in culture. This probably reflected earlier entry of hepatocytes to S-phase. HCH was shown to increase total DNA, total numbers of nuclei and numbers of cells undergoing mitosis per culture. In optimized conditions, HCH or PCN were about additive with norepinephrine, dialyzed serum or pyruvate or with a small effect of tri-iodothyronine in stimulating DNA synthesis. Although conditions optimal for HCH or PCN were not necessarily optimal for detecting growth-stimulatory effects of other xenobiotics or steroids, these culture conditions were shown to support stimulation of DNA synthesis by a variety of known liver tumor promoters including barbiturates, estrogens, progestins, peroxisomal proliferators and bile acids. Several compounds known not to promote liver carcinogenesis failed to stimulate DNA synthesis in similar hepatocyte cultures.     

Blockade of the chemokine receptor CXCR2 inhibits pancreatic cancer cell-induced angiogenesis

A central feature of all solid tumor growth is the presence of neovascularization. The CXC chemokines GRO-gamma/CXCL3, ENA-78/CXCL5, and IL-8/CXCL8 have profound angiogenic potential mediated through the CXCR2 receptor. The aim of the present study was to evaluate the expression of the angiogenic chemokines in three human pancreatic cancer cell lines and to determine the role of these proteins in pancreatic cancer angiogenesis. Secreted CXC protein levels in the supernatant of the cell lines were analyzed by ELISA. A rat corneal micropocket model was used to determine the angiogenic potential of these secreted CXC chemokines in vivo. ELISA confirmed expression of all three tested CXC chemokines in the supernatant of two cell lines. In the corneal micropocket assay, neovascularization was induced using pelleted supernatant of all three-cell lines. Using an anti-CXCR2 antibody, neovascularization was significantly inhibited in the high expressing BxPC-3 cell line samples. In addition, the expression of ENA-78/CXCL5 and IL-8/CXCL8 has been evaluated in human pancreatic cancer tissue samples by using immunohistochemistry in order to further investigate the potential role of CXC chemokines in pancreatic cancer angiogenesis and tumorigenesis.     

Long-term treatment with hepatic tumor promoters inhibits mitogenic responses of hepatocytes to acidic fibroblast growth factor and hepatocyte growth factor.

Studies with hepatocyte cultures have defined four hepatocyte mitogens which can transmit a complete mitogenic signal in cultures kept in completely defined conditions. These four mitogens are epidermal growth factor (EGF), acidic fibroblast growth factor (aFGF), hepatopoietin A/hepatocyte growth factor (HPTA/HGF) and hepatopoietin B (HPTB). In this study, we investigated the effect of aFGF, HGF and the mito-inhibitor transforming growth factor beta (TGF-beta) on cultured hepatocytes isolated from livers of rats treated with the xenobiotic hepatic tumor promoters phenobarbital (PB) and alpha-hexachlorocyclohexane (alpha-HCH). Male F344 rats were treated with each of these two xenobiotics to stimulate hepatic DNA synthesis and augmentative hepatomegaly. At different times on the regimens with tumor promoters, hepatocytes were isolated and placed in primary culture. DNA synthesis of hepatocytes in culture stimulated by these two growth factors and the suppression of DNA synthesis affected by TGF-beta were examined as a function of time of treatment in vivo with these two promoters. Following day 10, hepatocytes from both promoter regimens became unresponsive to these two growth factors for the rest of the duration of the treatment (day 90). TGF-beta suppressed DNA synthesis stimulated by growth factors but did not affect the high background DNA synthesis stimulated by xenobiotics themselves.     

Inhibition of the glucocorticoid receptor expression by diverse tumor promoters in SENCAR mouse epidermis

Glucocorticoid hormones are very potent inhibitors of keratinocyte proliferation. Their function is mediated by the glucocorticoid receptor (GR) which is highly expressed in mouse epidermis. In the study reported here we compared the effect of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and non-phorbol ester tumor promoters such as okadaic acid, chrysarobin, and benzoyl peroxide on the levels of GR protein and mRNA in SENCAR mouse epidermis. Glucocorticoid binding assay and Northern blot analysis revealed that all four tumor promoters decreased both GR protein and mRNA levels in keratinocytes in vivo. We also found that TPA and okadaic acid inhibited GR expression in keratinocyte cell line. These results suggest that GR inhibition may play an important role in mouse skin hyperplasia and promotion of skin carcinogenesis.     

Regulation of cytokine production in carcinoembryonic antigen stimulated Kupffer cells by beta-2 adrenergic receptors: implications for hepatic metastasis

Elevated Carcinoembryonic antigen (CEA) levels in the serum indicate a poor prognosis for colorectal cancer patients. Induction of proinflammatory cytokines by CEA interaction with Kupffer cells has been proposed as a mechanism for hepatic metastasis formation. Studies show that the cytokine response in circulating and peritoneal macrophages is regulated by beta-adrenergic receptor signals, though little information is available regarding Kupffer cells. We investigated the relationship between beta-adrenergic receptor stimulation and the response of Kupffer cells to CEA. Comparisons between unstimulated and CEA stimulated rat Kupffer cells, using cDNA arrays, showed up-regulation (>4 fold) of the beta2-adrenergic receptor mRNA. Peak up-regulation occurred after 30 min with a decline at 1 h. We examined the effects of the specific beta2-adrenergic receptor agonist terbutaline on cytokine production by CEA stimulated rat Kupffer cells. Pre-treatment of Kupffer cells with terbutaline followed by CEA caused a significant increase in IL-6 and IL-10 production, but a significant reduction in TNF-alpha production (>3 fold). mRNA levels reflected those of the ELISA assays for IL-6 and IL-10 but not for TNF-alpha. For IL-6 and TNF-alpha, these changes were serum independent, while IL-10 was serum dependent. This response is different from LPS treated Kupffer cells where all three cytokines showed serum dependency. Overall, these data suggest that Kupffer cell stimulation by CEA is under beta-adrenergic receptor control and induction of the beta-receptor is an early event following CEA binding to its receptor. Control of TNF-alpha production is negatively affected by terbutaline, while that of IL-6 and IL-10 is positively controlled suggesting that very different beta-adrenergic receptor signaling pathways are involved.     

Requirements for protein synthesis and calcium for stimulation of prostaglandin synthesis in cultured rat liver cells by tumor promoters

The tumor promoters, 12-O-tetradecanoylphorbol-13-acetate (TPA), phorbol-12,13-didecanoate, teleocidin, and dihydroteleocidin, at nM levels, but not the non-tumor-promoting 4 alpha-phorbol-12,13-didecanoate even at microM concentrations, stimulated arachidonic acid metabolism in cultured rat liver cells. These liver cells synthesize primarily prostaglandin I2 [measured as its nonenzymatic hydrolytic product, 6-keto-prostaglandin F1 alpha (PGF1 alpha)]. The production of 6-keto-PGF1 alpha increased with time of incubation with TPA and was essentially complete in 4 hr. Cycloheximide, at nM levels, blocked the TPA-stimulated 6-keto-PGF1 alpha production in a dose-dependent manner; this inhibition was related to inhibition of protein synthesis. Chelation of Ca2+ by ethyleneglycol-bis(beta-aminoethyl ether)-N,N'-tetraacetic acid, treatment of the cells with the Ca2+ channel blocker, nifedipine, or inhibition of intracellular Ca2+ mobilization by 8-(diethylamine)octyl-3,4, 5-trimethoxybenzoate hydrochloride also inhibited TPA-stimulated 6-keto-PGF1 alpha production. The steroidal antiinflammatory drug, dexamethasone, a potent in vivo inhibitor of tumor promotion, was an inhibitor of 6-keto-PGF1 alpha stimulation by TPA.     

The role of prostaglandin E1 in ornithine decarboxylase induction by tumor promoters

The effect of topical application of PGE on induction of ODC in mouse epidermis was measured. When direct induction of ODC by TPA was blocked by also applying indomethacin, maximum ODC activity occurred only when PGE was applied simultaneously with TPA 4 1/2 hr before killing of the mice. If either TPA or PGE was applied at other times, ODC activity decreased substantially. Induction of ODC by mezerein was blocked by indomethacin but restored by PGE, as was observed with TPA, but induction by ethyl phenylpropiolate was not affected by indomethacin or PGE. DMBA did not cause a consistent increase in ODC activity, nor was its inductive action affected by indomethacin or PGE. However, another weak inducer, acetic acid, exhibited elevated ODC activity when PGE was also applied. Inhibition by topical retinoic acid of ODC induction by TPA was partially overcome in a dose-response fashion by PGE. The results indicate that at least 2 events, elevation of PGE and another independent event, are required for induction of ODC activity. It appears that TPA causes at least 4 independent events essential for tumor promotion. A model for the events in the 2-stage tumor promotion model is proposed.     

Interleukin 4 inhibits stimulation of hepatic lipogenesis by tumor necrosis factor, interleukin 1, and interleukin 6 but not by interferon-alpha

Multiple cytokines stimulate hepatic lipogenesis in rodents. We have previously shown that lipogenic cytokines can be divided into 2 classes by their mechanism of action and their synergistic interactions. We now report the effects of interleukin 4, a cytokine known to inhibit the synthesis and action of other cytokines. Interleukin 4 by itself did not alter hepatic lipogenesis. However, interleukin 4 inhibited the characteristic stimulation of hepatic lipogenesis that is seen with tumor necrosis factor, interleukin 1, and interleukin 6. These 3 cytokines stimulate hepatic lipogenesis by the same mechanism, increasing hepatic levels of citrate, a key allosteric activator of acetyl CoA carboxylase, the rate-limiting enzyme of fatty acid synthesis. Interleukin 4 blocks the ability of tumor necrosis factor to increase hepatic citrate. In contrast, interleukin 4 does not block the stimulation of hepatic lipogenesis by interferon-alpha, a cytokine that increases hepatic lipogenesis by a mechanism other than increasing hepatic citrate levels. These results demonstrate that interleukin 4 can inhibit the metabolic action of selected cytokines, which provides strong support for our proposal that lipogenic cytokines operate through 2 distinct mechanisms of action and can therefore be divided into 2 separate classes based on their interactions. These results also emphasize the multiple relationships between the immune response and lipid metabolism.     

黏蛋白1多肽通过small-MUC1蛋白抑制肿瘤细胞生长

目的：探讨黏蛋白1（mucin 1,MUC1）串联重复序列多肽(简称黏蛋白1多肽,MUC1多肽)对肿瘤细胞生长抑制的作用机制。方法：MUC1多肽与多种肿瘤细胞Jurkat、Raji、U937、MCF7、SMMC7721及活化的T细胞、小鼠巨噬细胞RAW264.7共同培养,观察MUC1多肽对上述细胞生长的影响;建立BABL/c小鼠Jurkat细胞皮下移植瘤动物模型,应用MUC1多肽进行治疗;采用GST免疫沉降实验鉴定与MUC1多肽结合的肿瘤细胞表面蛋白。结果：MUC1多肽对Jurkat、Raji、U937、MCF7和SMMC7721细胞的生长均有抑制作用,对活化的T细胞和小鼠RAW264.7细胞生长无明显抑制作用。MUC1多肽对BABL/c小鼠皮下Jurkat细胞移植瘤的生长均有明显抑制作用（P<0.05）。GST免疫沉降实验显示,Jurkat 和MCF7细胞裂解上清中与MUC1多肽结合的蛋白可与两种抗MUC1串联重复序列抗体（GP1.4和HMPV）及抗胞内段抗体（Ab5）发生反应,相对分子质量大约115 000,提示可能是MUC1新的同种型,命名为small MUC1（sMUC1）。结论：MUC1多肽可通过与肿瘤细胞表面small MUC1蛋白的相互作用向细胞传导生长抑制信号     

Estrogen receptor alpha mediates progestin-induced mammary tumor growth by interacting with progesterone receptors at the cyclin D1/MYC promoters

Synthetic progesterone used in contraception drugs (progestins) can promote breast cancer growth, but the mechanisms involved are unknown. Moreover, it remains unclear whether cytoplasmic interactions between the progesterone receptor (PR) and estrogen receptor alpha (ERα) are required for PR activation. In this study, we used a murine progestin-dependent tumor to investigate the role of ERα in progestin-induced tumor cell proliferation. We found that treatment with the progestin medroxyprogesterone acetate (MPA) induced the expression and activation of ERα, as well as rapid nuclear colocalization of activated ERα with PR. Treatment with the pure antiestrogen fulvestrant to block ERα disrupted the interaction of ERα and PR in vitro and induced the regression of MPA-dependent tumor growth in vivo. ERα blockade also prevented an MPA-induced increase in CYCLIN D1 (CCND1) and MYC expression. Chromatin immunoprecipitation studies showed that MPA triggered binding of ERα and PR to the CCND1 and MYC promoters. Interestingly, blockade or RNAi-mediated silencing of ERα inhibited ERα, but not PR binding to both regulatory sequences, indicating that an interaction between ERα and PR at these sites is necessary for MPA-induced gene expression and cell proliferation. We confirmed that nuclear colocalization of both receptors also occurred in human breast cancer samples. Together, our findings argued that ERα-PR association on target gene promoters is essential for progestin-induced cell proliferation.     

Stimulation of growth of human melanocytes by tumor promoters

In previous studies we found that, when added to primary normalhuman epidermal cultures, 12-O-tetradecanoyl phor-bol 13-acetate(IPA) (10 ng/ml) selectively suppresses the growth of the otherwisepredominant keratinocyte cell population and that this is associatedwith the outgrowth of normal melanocytes. The present studyindicates that these melanocytes can be subsequently grown forat least 30 passages if the medium contains TPA, but if thecompound is removed the cells cease to divide. The ability ofa series of phorbol esters to support the growth of normal humanmelanocytes correlates, in general, with their tumor promotingactivity on mouse skin. Two structurally unrelated types ofcompounds which have recently been shown to have tumor promotingactivity on mouse skin, teleocidin and aplysiatoxin, also supportmelanocyte growth. On the other hand, several polypeptide growthfactors could not substitute for TPA. Since human melanoma celllines grow vigorously in the absence of tumor promoters ourresults suggest that the malignant transformation of melanocytesis associated with the acquisition of autonomy from certainunidentified endogenus growth factors.     

Antiserum raised against an epitope of the cholecystokinin B/gastrin receptor inhibits hepatic invasion of a human colon tumor

Serum gastrin is known to be elevated in patients with liver-metastasizing colon cancer; thus, cholecystokinin (CCK) B/gastrin receptors may also be up-regulated. A liver-invasive model of colon cancer was established with the human colonic cell line C170HM2, which expresses the CCKB/gastrin receptor at both the gene and protein level. An antiserum has been derived that is directed against the NH2-terminal 17 amino acids of the human CCKB/gastrin receptor coupled to diphtheria toxoid. The peptide was denoted gastrin receptor protein (GRP) 1. The therapeutic effect of GRP1 antiserum was evaluated on the liver invasion of C170HM2 tumors. Biodistribution studies revealed that GRP1 antiserum localized preferentially within the liver tumors when compared with normal liver tissue (1.5-fold increase after 24 h; P < 0.05). Antiserum against GRP1 inhibited both tumor take rate and final liver tumor weight when compared with treatment with control serum in mice with an increasing tumor burden. Liver tumor weights were reduced from 0.37 to 0.10 gram (P = 0.0155), 1.25 grams to 0.76 gram (P = 0.003) and 1.89 grams to 0.76 gram (P = 0.0068, all Mann-Whitney nonparametric U test). Necrosis and apoptosis were increased in the GRP1 antiserum-treated tumors when compared with control serum-treated tumors. As shown by Western blotting, CCKB/gastrin receptor expression of C170HM2 xenografts after treatment with GRP1 antiserum shifted to a predominantly lower molecular weight form (Mr 45,000) that is known to be an internalized form of the receptor. In conclusion, targeting of the CCKB/gastrin receptor may yield a valuable therapeutic modality for the treatment of advanced colon cancer.     

Effects of promoters on DNA synthesis in C3H/10T1/2 mouse fibroblasts

The synthesis of DNA has been studied by autoradiography and by measurements of tritiated thymidine ([3H]TdR) incorporation in cultured C3H/10T1/2 mouse embryo fibroblasts. The cells were first treated with 3-methylcholanthrene as an initiator and then with promoters according to schedules that produce oncogenic transformation. The levels of 3-methylcholanthrene used did not affect the growth or [3H]TdR incorporation of the cells. Treatment during the log phase of growth with 12-O-tetradecanoyl-phorbol-13-acetate, phorbol didecanoate, or 4alpha-phorbol didecanoate produced a transient inhibition of [3H]TdR incorporation with the maximum at 12 hr after treatment. This resulted in a temporary delay of growth followed by recovery of the normal cell-doubling time. Phorbol did not produce these effects, suggesting that the inhibition of DNA synthesis is associated with the process of promotion. Although treatment of the cells with 12-O-tetradecanoyl-phorbol-13-acetate during stationary phase resulted in a 2- to 3-fold stimulation of [3H]TdR incorporation, multiple treatments spanning log and stationary phases were found to be necessary for promotion.     

Blockade of insulin-like growth factor I receptor function inhibits growth and angiogenesis of colon cancer.

Purpose and Experimental Design: Insulin-like growth factors (IGFs) I and II and their principle receptor, IGF-I receptor (IGF-IR), are frequently expressed in human colon cancers and play a role in preventing apoptosis, enhancing cell proliferation, and inducing expression of vascular endothelial growth factor (VEGF). To elucidate the in vitro and in vivo effects of IGF-IR in human colon cancer growth and angiogenesis, HT29 cells were transfected with a truncated dominant-negative (DN) form of IGF-IR or vector alone. RESULTS: IGF-I increased VEGF expression in parental and vector-transfected cells, whereas IGF-I induction of VEGF mRNA and protein was abrogated in IGF-IR DN cells. The IGF-IR DN cells demonstrated inhibited growth in both monolayer culture and soft agar (P < 0.05). s.c. injections of IGF-IR DN cells in nude mice led to significantly decreased tumor growth (P < 0.05). Immunohistochemical analyses revealed that IGF-I DN tumors demonstrated decreased tumor cell proliferation, VEGF expression, and vessel count and increased tumor cell apoptosis (P < 0.05 for all parameters compared with controls). Furthermore, IGF-IR DN-transfected cells yielded significantly decreased tumorigenicity and growth in the liver. CONCLUSIONS: These studies demonstrate that the IGF ligand-receptor system plays an important role in multiple mechanisms that mediate human colon cancer growth including regulation of VEGF and angiogenesis.     

Cell transformation induced by bovine papillomavirus DNA as an assay for tumor promoters and chemopreventive agents.

An in vitro assay was designed to examine and quantitate the action of chemical promoters and chemopreventive agents on papillomavirus DNA-carrying cells. Cultured C3H/10T1/2 cells transfected with bovine papillomavirus type 1 DNA (plasmid pdBPV-1) were used as targets, and the frequency of transformed foci was used as an endpoint. The development of foci with a transformed phenotype was greatly enhanced by tumor promoters (e.g., mezerein, 12-O-tetradecanoyl-phorbol-13-acetate, teleocidin, and okadaic acid) and complex mixtures such as extracts of the areca nut, which is an integral part of a betel quid and is linked to oral cancers among chewers. The degree of promotion depended on the length of exposure, the type of promoter, and the time of application after transfection with BPV DNA. The inhibitory effect of chemopreventive agents on transformation can be tested either directly on BPV DNA transfected cells (promoter-independent transformation), or on transfected cells that were exposed to tumor promoters (promoter-dependent transformation). Retinol, and to a lesser degree beta-carotene, exerted an inhibitory effect on promoter-dependent and promoter-independent transformation of BPV DNA transfected cells. The inhibitory effect was conveyed either by the addition of retinol simultaneously with promoters, or after exposure to the promoting agents was completed. The significance of this short-term in vitro assay for the design of chemopreventive trials is discussed.