Inhibition of ras p21 membrane localization and modulation of protein kinase C isozyme expression during regression of chemical carcinogen-induced murine skin tumors by lovastatin

How to Target Activated Ras Proteins: Direct Inhibition vs. Induced Mislocalization

Oncogenic Ras proteins are a driving force in a significant set of human cancers and wildtype, unmutated Ras proteins likely contribute to the malignant phenotype of many more. The overall challenge of targeting activated Ras proteins has great promise to treat cancer, but this goal has yet to be achieved. Significant efforts and resources have been committed to inhibiting Ras, but these energies have so far made little impact in the clinic. Direct attempts to target activated Ras proteins have faced many obstacles, including the fundamental nature of the gain-of-function oncogenic activity being produced by a loss-of-function at the biochemical level. Nevertheless, there has been very promising recent pre-clinical progress. The major strategy that has so far reached the clinic aimed to inhibit activated Ras indirectly through blocking its post-translational modification and inducing its mislocalization. While these efforts to indirectly target Ras through inhibition of farnesyl transferase (FTase) were rationally designed, this strategy suffered from insufficient attention to the distinctions between the isoforms of Ras. This led to subsequent failures in large-scale clinical trials targeting K-Ras driven lung, colon, and pancreatic cancers. Despite these setbacks, efforts to indirectly target activated Ras through inducing its mislocalization have persisted. It is plausible that FTase inhibitors may still have some utility in the clinic, perhaps in combination with statins or other agents. Alternative approaches for inducing mislocalization of Ras through disruption of its palmitoylation cycle or interaction with chaperone proteins are in early stages of development.

Primarily screening and analyzing ESTs differentially expressed in rats' primary liver cancer

OBJECTIVE

To screen and analyze key express sequence tags (ESTs) which were differentially displayed in every period of SD rats' primary hepatic carcinoma and reveal the molecular mechanism of carcinogenesis.

METHODS

Using diethylnitrosamine (DENA) as a cancerigenic agent, animal models with different phases of primary hepatic cancer were constructed in SD rats. Rats were respectively sacrificed at d 14, d 28, d 56, d 77, d 105 and d 112 after the rats received DENA by gavage, then the livers were harvested. One part of the livers was classified according to their pathological changes, while the other was reserved for molecular mechanism studies on hepatocarcinogenesis. The differentially expressed genes were isolated from both normal and morbid tissues by mRNA differential display technique (DDRT-PCR). After the fragments were sequenced, bioinformatics were used to analyze the results.

RESULTS

Twelve differentially expressed cDNA fragments were obtained. Nine fragments had the homology with known cDNA clones, especially EST-7 was similar to BN/SsNHsdMCW mitochondrion gene and the identity was 100% which suggested EST-7 may be the part of BN/SsNHsdMCW mitochondrion gene. In contrast, other three fragments (EST-1, EST-3 and EST-5) had extremely low identity to any genes registered in GENBANK databases.

CONCLUSIONS

BN/SsNHsdMCW mitochondrion gene was expressed in different periods of hepatocarcinogenesis. Moreover, EST-1, EST-3 and EST-5 were suggested to contribute to the development of rat hepatocarcinogenesis, and thus may be candidates of new targets of oncogenes or cancer suppressor genes.

Statin use and risk of basal cell carcinoma

OBJECTIVE

We examined the association between statin use and basal cell carcinoma (BCC) risk.

METHODS

We identified all members of a large integrated health care delivery system with a diagnosis of a histologically proven BCC in 1997. Subsequent BCCs were identified through 2006 from health plan electronic pathology records. Longitudinal exposure to statins and other lipid-lowering agents was determined from automated pharmacy records. We used extended Cox regression to examine the independent association between receipt of statin therapy (ever vs never, cumulative duration) and risk of subsequent BCC. To minimize confounding by indication, we conducted sensitivity analyses in the subset of individuals considered eligible for lipid-lowering therapy based on national guidelines.

RESULTS

Among 12,123 members given a diagnosis of BCC who had no prior statin exposure, 6381 developed a subsequent BCC during follow-up. Neither "ever use of statins" (adjusted hazard ratio 1.02, 95% confidence interval: 0.92-1.12) or cumulative duration of statin (adjusted hazard ratio 1.02/year, 95% confidence interval: 0.99-1.11) was associated with subsequent BCC after adjustment for age, sex, and health care use. Risk estimates did not change appreciably when the analysis was limited to the subset of individuals who met eligibility criteria for initiating statin therapy. There was also no significant association between use of non-statin antilipemics and subsequent BCC (adjusted hazard ratio 1.10, 95% confidence interval: 0.76-1.58).

LIMITATIONS

No information was available for BCC risk factors, such as sun sensitivity and sun exposure.

CONCLUSIONS

Among a large cohort of individuals with BCC, statin therapy was not significantly associated with risk of subsequent BCC.

The effect of statins in colorectal cancer is mediated through the bone morphogenetic protein pathway

BACKGROUND & AIMS

Epidemiological evidence suggests that statins prevent colorectal cancer (CRC), but the biological mechanism remains obscure. Statins induce bone morphogenetic protein (BMP) expression in bone cells. We have previously shown that BMPs act as tumor suppressors in CRC. We hypothesized that the action of statins in CRC involves the induction of BMPs.

METHODS

We investigated the effects of statins on CRC cell lines using immunoblotting, measurements of apoptosis and cell proliferation, and luciferase reporter assays. The effect of statins was confirmed in a xenograft mouse model.

RESULTS

CRC cell lines show widely differing sensitivities to statin treatment. Sensitive cell lines show induction of BMP2 protein levels and a BMP2 reporter construct, activation of the BMP pathway, and induction of the BMP target gene ID-2, whereas resistant cell lines do not. The addition of the specific inhibitor of BMPs, noggin, completely prevents lovastatin-induced apoptosis in sensitive cells. Sensitive cell lines express the central BMP pathway element SMAD4, whereas the resistant cell lines do not. Targeted knockout of SMAD4 leads to the loss of statin sensitivity and reconstitution with SMAD4, to the restoration of statin sensitivity. In a xenograft mouse model, tumors from sensitive and insensitive cell lines were treated with oral simvastatin. Significant inhibition of tumor growth using sensitive cells but increased tumor growth when using insensitive cells was observed.

CONCLUSIONS

Statins induce apoptosis in CRC cells through induction of BMP2. Statin therapy may only be effective in SMAD4-expressing CRCs and may have adverse effects in SMAD4-negative tumors.

Elevated K-ras activity with cholestyramine and lovastatin, but not konjac mannan or niacin in lung--importance of mouse strain

Our previous work established that hypocholesterolemic agents altered K-ras intracellular localization in lung. Here, we examined K-ras activity to define further its potential importance in lung carcinogenesis. K-ras activity in lungs from male A/J, Swiss and C57BL/6 mice was examined. For 3 weeks, mice consumed either 2 or 4% cholestyramine (CS), 1% niacin, 5% konjac mannan (KM), or were injected with lovastatin 25mg/kg three or five times weekly (Lov-3X and Lov-5X). A pair-fed (PF) group was fed the same quantity of diet consumed by the Lov-5X mice to control for lower body weights in Lov-5X mice. After 3 weeks, serum cholesterol was assayed with a commercial kit. Activated K-ras protein from lung was affinity precipitated with a Raf-1 ras binding domain-glutathione-S-transferase fusion protein bound to glutathione-agarose beads, followed by Western blotting, K-ras antibody treatment, and chemiluminescent detection. Only KM reduced serum cholesterol (in two of three mouse strains). In C56BL/6 mice treated with Lov-3X, lung K-ras activity increased 1.8-fold versus control (p=0.009). In normal lung with wild-type K-ras, this would be expected to be associated with maintenance of differentiation. In A/J mice fed 4% CS, K-ras activity increased 2.1-fold (p=0.02), which might be responsible for the reported enhancement of carcinogenesis in carcinogen-treated rats fed CS. KM feeding and PF treatment had no significant effects on K-ras activity. These data are consistent with the concept that K-ras in lung has an oncogenic function when mutated, but may act as a tumor suppressor when wild-type.

Pharmacodynamic effects of high dose lovastatin in subjects with advanced malignancies

Lovastatin, an inhibitor of the rate-limiting enzyme in the cholesterol biosynthetic pathway, hydroxymethylglutaryl coenzyme A reductase, has shown interesting antiproliferative activities in cell culture and in animal models of cancer. The goal of the current study is to determine whether lovastatin bioactivity levels, in a range equivalent to those used in in vitro and preclinical studies, can be safely achieved in human subjects. Here we present the findings from a dose-escalating trial of lovastatin in subjects with advanced malignancies. Lovastatin was administered every 6 h for 96 h in 4-week cycles in doses ranging from 10 mg/m2 to 415 mg/m2. Peak plasma lovastatin bioactivity levels of 0.06-12.3 microM were achieved in a dose-independent manner. Cholesterol levels decreased during treatment and normalized during the rest period. A dose-limiting toxicity was not reached and there were no clinically significant increases in creatine phosphokinase or serum hepatic aminotransferases levels. No antitumor responses were observed. These results demonstrate that high doses of lovastatin, given every 4 h for 96 h, are well-tolerated and in select cases, bioactivity levels in the range necessary for antiproliferative activity were achieved.

Treatment with lovastatin, cholestyramine or niacin alters K-ras membrane association in mouse lung in a strain-dependent manner: results in females

Hypocholesterolemic drugs may themselves increase (cholestyramine, CS) or decrease (lovastatin, Lov) peripheral tissue de novo cholesterol biosynthesis. This will alter the abundance of prenyl groups and potentially increase (CS) or decrease (Lov) K-ras membrane localization, with possible pro- or anti-carcinogenic effects (K-ras is a proto-oncogene frequently mutated in lung cancer). Female A/J, Swiss, and C57BL/6 mice were fed 2 or 4% CS, 1% niacin, or injected with Lov three (Lov-3x) or five (Lov-5x) times per week. After three weeks, serum cholesterol and triglycerides were determined enzymatically. Total, membrane, and cytoplasmic K-ras proteins were determined in lung homogenates by immunoprecipitation followed by Western blotting with a K-ras specific antibody. CS feeding increased membrane K-ras as hypothesized in A/J and C57BL/6 mice, but had no effect in Swiss mice. Lov failed in all three strains to reduce membrane K-ras, and resulted in an increase in total K-ras in A/J and C57BL/6 mice, while again lacking effect in Swiss mice. Niacin had no effect on K-ras protein in any mouse strain. These results differ from our published results for male mice of the same strains, particularly for A/J mice. Increased amounts of K-ras protein in the membrane fraction of A/J females (but not males) treated with either Lov or CS imply that if K-ras were to become mutated, CS could result in increased lung tumorigenesis and Lov would be less likely to be protective in females. In the light of these data, both sexes should be included in future animal and human chemoprevention trials.

Alterations in membrane-bound and cytoplasmic K-ras protein levels in mouse lung induced by treatment with lovastatin, cholestyramine, or niacin: effects are highly mouse strain dependent

Agents that either increase (cholestyramine, CS) or decrease (lovastatin, Lov) de novo peripheral cholesterol synthesis may increase (CS) or decrease (Lov) ras protein membrane localization by altering protein prenylation, and potentially have pro- or anti-carcinogenic effects. Male A/J, Swiss, and C57/BL6 mice were treated with 2 or 4% CS, 1% dietary niacin, or 25mg/kg of Lov three times per week (Lov-3X) or five times per week (Lov-5X). After 3 weeks, serum cholesterol and triglycerides were determined enzymatically. Membrane and cytoplasmic K-ras proteins in lung were determined by immunoprecipitation followed by western blotting with a K-ras specific antibody. Results confirmed the hypothesis only in isolated instances. A/J mice had a significant 30% increase in cytoplasmic K-ras and a 40% decrease in membrane K-ras from Lov treatment, as predicted. C57/BL6 mice had a significant 77% increase in membrane K-ras, as expected from CS feeding. At variance with the hypothesis, Swiss mice had increased levels (3-28%) of membrane K-ras with all treatments (including Lov), and C57/BL6 mice treated with Lov had a 58-78% increase in cytoplasmic K-ras without any reduction in the levels of membrane K-ras. Niacin, predicted to have no effect on ras membrane localization, decreased cytoplasmic K-ras in A/J mice, increased both membrane and cytoplasmic K-ras in Swiss mice, and had no effect in C57/BL6 mice. Results may have differed from those predicted because of strain-dependent differences in response to the cholesterol-lowering agents. A difference in response among the mouse strains suggests that individual genetic differences may alter the effect of hypocholesterolemic agents on K-ras membrane localization, and potentially the risk of ras-dependent cancer.

Silymarin inhibits growth and causes regression of established skin tumors in SENCAR mice via modulation of mitogen-activated protein kinases and induction of apoptosis

This study reports in vivo therapeutic efficacy of silymarin against skin tumors with mechanistic rationale. 7,12-Dimethylbenz[a]anthracene-12-O-tetradecanoyl-phorbol-13-acetate (DMBA-TPA)-induced established skin papilloma (tumor)-bearing SENCAR mice were fed with 0.5% silymarin in AIN-93M-purified diet (w/w), and both tumor growth and regression were monitored during 5 weeks of feeding regimen. Silymarin feeding significantly inhibited (74%, P < 0.01) tumor growth and also caused regression (43%, P < 0.01) of established tumors. Proliferating cell nuclear antigen and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling immunohistochemical staining of tumors showed that silymarin decreases proliferation index by 48% (P < 0.001) and increases apoptotic index by 2.5-fold (P < 0.001), respectively. Skin tumor growth inhibition and regression by silymarin were also accompanied by a strong decrease (P < 0.001) in phospho-ERK1/2 levels in tumors from silymarin-fed mice compared with controls. In the studies evaluating bioavailability and physiologically achievable level of silymarin (as silibinin) in plasma, skin tumor, skin, liver, lung, mammary gland and spleen, we found 10, 6.5, 3.1, 13.7, 7.7, 5.9 and 4.4 microg silibinin/ml plasma or per gram tissue, respectively. In an attempt to translate these findings to human skin cancer and to establish biological significance of physiologically achievable level, effect of plasma concentration of silibinin was next examined in human epidermoid carcinoma A431 cells. Silibinin treatment of cells in culture at 12.5, 25 (plasma level) and 50 microM doses resulted in 30-74% (P < 0.01-0.001) growth inhibition and 7-42% death of A431 cells in a dose- and time-dependent manner; apoptosis was identified as a cell death response by silibinin. Similar silibinin treatments also resulted in a significant decrease in phospho-mitogen-activated protein kinase/extracellular signal-regulated protein kinase 1/2 (MAPK/ERK1/2) levels, but an up-regulation of stress-activated protein kinase/jun NH(2)-terminal kinase (SAPK/JNK1/2) and p38 mitogen-activated protein kinase (p38 MAPK) activation in A431 cells. The use of MEK1 inhibitor, PD98059, showed that inhibition of ERK1/2 signaling, in part, contributes to silibinin-caused cell growth inhibition. Together, the data suggest that an inhibition of ERK1/2 activation and an increased activation of JNK1/2 and p38 by silibinin could be possible underlying molecular events involved in inhibition of proliferation and induction of apoptosis in A431 cells. These data suggest that silymarin and/or its major active constituent silibinin could be an effective agent for both prevention and intervention of human skin cancer.

Prevention of radiation-induced mammary tumors

The radiation-induced rat mammary tumor model is useful for studying tumor prevention by treatment in the initiation or promotion stage. In anti-initiation experiments, the administration of radical scavengers or spin-trapping agents before or immediately after irradiation reduced the incidence of mammary tumors, suggesting that free radicals produced by exposure are a potent initiator. To evaluate the role of nitric oxide (NO) in the initiation, NO-specific scavengers or NO synthase inhibitors were administered during the initiation. These agents partially prevented the tumorigenesis, suggesting that radiation-induced NO contributes to tumor initiation. The administration of curcumin during irradiation reduced the incidence of the tumors in the presence of tumor promotor. In anti-promotion experiments on preventing diethylstilbestrol (DES)-dependent tumor development from mammary primodial cells exposed to radiation, tamoxifen decreased the tumor incidence. From the results, estrogen itself or prolactin induced by estrogen may be a promoter for the tumorigenesis. Bezafibrate and simvastatin, agents inducing hypolipidemia and hypocholesterolemia respectively, cause a decrease in the DES-dependent promotion of radiation-induced tumorigenesis. The simultaneous administration of curcumin and DES significantly reduces the development of mammary tumors in irradiated rats. In this review, the endocrinologic and pharmacologic significance of the anti-initiation and anti-promotion is discussed.

Impact of garlic organosulfides on p21(H-ras) processing

This study describes the novel anticarcinogenic activity of diallyl disulfide, a naturally occurring organosulfide from garlic. Oral administration of diallyl disulfide resulted in a dose-dependent and significant inhibition of the growth of H-ras oncogene transformed NIH 3T3 cells implanted in nude mice. The effect of diallyl disulfide was apparent in terms of delay in the appearance of measurable tumors, tumor volume and tumor weight. On the other hand, the growth of H-ras oncogene transformed tumors was not inhibited by dipropyl disulfide, a naturally occurring saturated analog of diallyl disulfide. The diallyl disulfide-mediated inhibition of H-ras oncogene transformed tumor growth correlated with the inhibition of p21(H-ras) membrane association. The levels of membrane-associated p21(H-ras) were markedly lower in the tumors of diallyl disulfide-treated mice than in those of controls. An opposite trend, however, was evident for the cytosolic p21(H-ras). The results of this study indicate that diallyl disulfide inhibits the growth of H-ras oncogene transformed tumors in vivo by inhibiting the membrane association of p21(H-ras) and that the allyl group may be an important determinant in the inhibitory effect of this organosulfide on tumor growth.

Lovastatin decreases mortality and improves liver functions in fulminant hepatic failure from 90% partial hepatectomy in rats

BACKGROUND/AIMS

Liver insufficiency occurs when the liver cannot perform critical functions such as ammonia metabolism, gluconeogenesis, or production of coagulation factors The hypothesis of this study was that decreased function of existing hepatocytes may contribute to hepatic failure, and that the function of these cells might be increased pharmacologically. Lovastatin is a 3-hydroxy-3-methylglutaryl CoA reductase inhibitor that inhibits cholesterol biosynthesis and affects the activity of some signal transduction pathways and liver transcription factors. Changes in hepatic transcription factors during liver regeneration might result in decreased liver functions, and lovastatin might prevent these changes

METHODS

Rats received 90% partial hepatectomy (90% PH), and either lovastatin or vehicle alone daily. Survival and liver functions were assessed.

RESULTS

Lovastatin increased survival to 58% (vs. 6% in controls that received 90% PH without drug), decreased the peak ammonia level to 427 microM (vs. 846 microM in controls), increased the nadir of glucose to 88 mg/dl (vs. 57 mg/dl in controls), decreased the peak prothrombin time to 23 s (vs 29 s in controls), and decreased the peak activated partial thromboplastin time to 29 s (vs. 39 s in controls). The full survival and metabolic benefits were observed when lovastatin was started at 30 min after 90% PH, but lovastatin was less efficacious when started at later times.

CONCLUSIONS

Lovastatin increases the function of existing hepatocytes and might be used to improve liver function after extensive hepatic resection.

Modulation of mitogen-activated protein kinase activation and cell cycle regulators by the potent skin cancer preventive agent silymarin

Recently we showed that the skin cancer preventive effect of silymarin involves inhibition of erbB1 activation. Here we assessed the effect of silymarin on cytoplasmic and nuclear signals employing human epidermoid carcinoma A431 cells. Silymarin treatment of cells resulted in a significant inhibition of mitogen-activated protein kinase (MAPK)/ERK1/2 activation only at lower doses, whereas higher doses activated MAPK/JNK1. These differential responses of silymarin were accompanied by its growth inhibitory and apoptotic cell death effects at low and high doses, respectively. Silymarin-caused growth inhibition was via both G2-M and G1 arrests due to a significant decrease in the kinase activity and protein levels of CDKs and cyclins. In other studies, only low doses of silymarin also showed an induction of Cip1/p21 and Kip1/p27. Together, these results identify distinct signaling pathways for the antiproliferative and apoptotic effects of silymarin and form a basis for developing strategies targeted to ERK and JNK pathways for the prevention and intervention of malignancies by silymarin.

Protein-kinase-Cmu expression correlates with enhanced keratinocyte proliferation in normal and neoplastic mouse epidermis and in cell culture

In order to gain insight into the biological function of a PKC iso-enzyme, the protein kinase Cmu, we analyzed the expression pattern of this protein in mouse epidermis and keratinocytes in culture. Daily analysis of neonatal mouse epidermis immediately after birth showed a time-dependent reduction in the PKCmu content. Expression of the proliferating-cell nuclear antigen (PCNA), indicative of the proliferative state of cells, was reduced synchronously with PKCmu as the hyperplastic state of the neonatal tissue declined. In epidermal mouse keratinocytes, fractionated according to their maturation state, PKCmu expression was restricted to PCNA-positive basal-cell fractions. In primary cultures of those cells, growth arrest and induction of terminal differentiation by Ca2+ resulted in strongly reduced PKCmu expression, concomitantly with the loss of PCNA expression. Treatment of PMK-R1 keratinocytes with 100 nM of the mitogen 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in activation of PKCmu, reflected by translocation from the cytosolic to the particulate fraction and by shifts in electrophoretic mobility. DNA synthesis was significantly inhibited by the PKCmu inhibitor Goedecke 6976, while Goedecke 6983 did not inhibit PKCmu. Carcinomas generated according to the 2-stage carcinogenesis protocol in mouse skin consistently exhibited high levels of PKCmu. These data correlate PKCmu expression with the proliferative state of murine keratinocytes and point to a role of PKCmu in growth stimulation. A correlation between PKCmu expression and enhanced cell proliferation was also observed for NIH3T3 fibroblasts transfected with and overexpressing human PKCmu.

Differential induction of Cyp1a1, Cyp1b1, Ahd4, and Nmo1 in murine skin tumors and adjacent normal epidermis by ligands of the aryl hydrocarbon receptor

Products of several phase I and II genes transcriptionally activated by ligands of the aryl hydrocarbon receptor (AHR) were quantitated in cutaneous samples isolated from non-tumor-bearing SENCAR or SSIN mice, and animals bearing skin tumors generated in initiation-promotion protocols. The constitutive 7-ethoxyresorufin O-deethylase (EROD) activities in papillomas and squamous cell carcinomas were less than or equal to 37% of the values measured in the adjacent normal cutaneoustissue. Dermal and epidermal EROD specific activities in microsomal samples prepared from both tumor-bearing and non-tumor-bearing mice were elevated 9- to 14- and 43- to 77-fold, respectively, above constitutive levels 16-20 h after a single topical application of 100 nmol of dibenz[a,c]anthracene (DB[a,c]A). EROD specific activities in tumors were maximally elevated two-fold after topical application of DB[a,c]A. Western blot, northern blot, and reverse transcription (RT)-polymerase chain reaction (PCR) analyses confirmed that the EROD measurements reflected cutaneous cytochrome P450 (CYP) 1A1 protein, mature mRNA, and heterogeneous nuclear RNA contents, respectively. Analyses of CYP1A1, CYP1B1, cytosolic aldehyde dehydrogenase class 3, and NAD(P)H:menadione oxidoreductase (NMO1) mRNA content by RT-PCR revealed significant increases in all four mRNAs in the normal tissue adjacent to papillomas after exposure to 4 nmol of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) but no increases in the tumors. NMO1 mRNA content in acetone-treated papillomas approached the levels detected in TCDD-treated normal skin. RT-PCR analyses also demonstrated elevated constitutive aryl hydrocarbon receptor nuclear translocator mRNA content (an approximately two-fold increase) in skin tumors. In contrast, AHR mRNA content in the tumors was about 20% of that measured in adjacent normal tissue. Collectively, these studies demonstrated that ligand-induced, AHR-mediated processes are absent in murine skin tumors that develop in initiation-promotion protocols.

Protection against malignant conversion in SENCAR mouse skin by all trans retinoic acid: inhibition of the ras p21-processing enzyme farnesyltransferase and Ha-ras p21 membrane localization

Many studies have shown that all trans retinoic acid (RA) exhibits significant protective effects against mouse skin tumor promotion and spontaneous as well as enhanced malignant conversion. In a recently completed study, we showed that under treatments in which papillomas on SENCAR mouse skin are induced at low and high probabilities to convert to malignant carcinomas, RA affords significant protection against both tumor promotion and subsequent malignant conversion. More than 95% of these mouse skin papillomas and carcinomas have been shown to contain point mutation at the 61 codon of Ha-ras oncogene. The ras oncogene encodes a p21 protein that, in its mutated form, transforms mammalian cells only when p21 is at the inner surface of the plasma membrane, by a series of enzymatic reactions in which the initial step is catalyzed by farnesyltransferase (FTase). In this study, we assessed whether the protective effect of RA against malignant conversion involves the inhibition of ras p21 processing in those tumors that contain the activated ras oncogene. The FTase activity and the levels of cytosolic and membrane-bound Ha-ras p21 were determined in all papillomas and carcinomas obtained from acetone- or RA-treated animals. No matter how the data were analyzed and what comparisons were considered, in all the protocols used, compared with controls, papillomas and carcinomas obtained from RA-treated groups showed significantly decreased (P < 0.01-0.001) FTase activity. Furthermore, the tissue samples from RA-treated groups in different protocols also showed significantly diminished membrane localization of Ha-ras p21, with a concomitant increase in cytosolic Ha-ras p21 levels. The analysis of these data also showed that in all the protocols used, the increased FTase activity and membrane localization of Ha-ras p21 were associated with the induction of papillomas and their subsequent malignant conversion to squamous cell carcinomas. Taken together, these results indicate a strong correlation between the inhibition of ras p21 farnesylation because of a decrease in FTase activity by RA and its protective effect against malignant conversion of papillomas to carcinomas. Based on the results of this study, it is tempting to suggest that clinical trials evaluating the preventive or therapeutic potential of retinoids may be directed more toward those clinical malignancies that are known to contain the activated ras oncogene.

Chemoprevention by pravastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, of N-methyl-N-nitrosourea-induced colon carcinogenesis in F344 rats

A potential chemopreventive action of pravastatin (Pr), a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, on colon carcinogenesis was evaluated in F344 rats. All rats at 7 weeks of age received an intrarectal dose of 2 mg of N-methyl-N-nitrosourea 3 times weekly for 2 weeks in experiment I (2 groups of 16 rats each), and for 3 weeks in experiment II (4 groups of 30 rats each). They were given drinking water containing 0 ppm (control) or 200 ppm Pr during weeks 1 to 40 in experiment I, and containing 0 ppm (control), 25 ppm, 5 ppm and 1 ppm Pr during weeks 4 to 40 in experiment II. The body weight gains, and food and water intakes were similar in all the groups. The incidence of colon carcinomas at termination of the experiment at week 40 was not different in the 200 ppm Pr and control groups in experiment I (63% vs. 69%), while it was significantly lower in the 25 ppm and 5 ppm groups, but not in the 1 ppm Pr group, compared with the control group in experiment II (50%, 48%, and 77% vs. 80%). This inhibitory effect of Pr against colon carcinogenesis was not related to the cholesterol-lowering effect of this agent. We postulate that Pr inhibits the promotion stage of colon carcinogenesis, perhaps through modulation of cholesterol synthesis in situ in the colonic mucosa, thereby suppressing farnesyl isoprenylation of growth-regulating proteins such as p21 ras.