The expression of the human cyclin B1 gene was investigated with Western blot analysis in human colorectal carcinomas and in adjacent non-neoplastic colorectal mucosas. Out of 41 cancers, 36 (88% of patients) showed much higher expression of cyclin B1 than did the non-neoplastic mucosa. Proliferating-cell nuclear antigen (PCNA) immunohistochemistry revealed that the labeling indexes of these cancer tissues were 47.3 +/- 11.3% while those of the mucosa were 15.6 +/- 5.5%. Only 5 cancers (12% patients) demonstrated the same expression level of cyclin B1 as the mucosa; however, the PCNA labeling indexes were 42.3 +/- 11% for the cancer tissue, compared to 12.6 +/- 2.4% for the mucosas. Southern blot analysis showed that there was no change of the cyclin B1 gene at the somatic DNA level in spite of its high expression at the protein level. These results proved that majority of colorectal cancers express high levels of cyclin B1, consistent with a high rate of cell proliferation, whereas a small fraction of these cancers lose control of cyclin B1 expression, diverging from their fast cell proliferation.

Recent studies have suggested that apoptosis is a key phenomenon in the chemopreventive action of nonsteroidal antiinflammatory drugs (NSAIDs), which exhibit cancer‐preventive and tumor‐regressive effects in the human colon. The effect of NS‐398, N‐(2‐cyclohexyloxy‐4‐nitrophenyl)methanesul‐fonamide, which is a selective inhibitor of cyclooxygenase‐2 (COX‐2), on the induction of apoptosis in two human colorectal cancer cell lines (Colo320 and THRC) was determined. The apoptotic ratios (‐fold vs. control value) of Colo320 in the presence of 100 μM indomethacin and NS‐398 were 3.3 ± 1.5 and 9.0±0.94, and those of THRC were 2.3±0.46 and 7.4±0.87, respectively. The ability of NS‐398 to induce apoptosis is greater than that of indomethacin. Both indomethacin and NS‐398 reduced the cell proliferation in a concentration‐dependent manner. The IC50 values of NS‐398 (54.8+3.6 and 77.2±4.9μM) were significantly lower than those of indomethacin (206.3±43.0 and 180.3±22.6/μM) at P<0.01 in Colo320 and THRC cell lines, respectively. These findings suggest that NS‐398, a selective inhibitor of COX‐2, is a possible candidate for a chemopreventive agent with a potent apoptosis‐inducing effect and low nlcerogenic activity.

The modifying effects of dietary exposure to two natural products, protocatechuic acid (PCA) and Costunolide during the development of neoplasms in oral carcinogenesis initiated with 7,12‐dimethyl‐benz[a]anthracene (DMBA) were investigated in male Syrian golden hamsters. All hamsters except those in the test chemical alone and control groups received DMBA (0.5%) in mineral oil to the right buccal pouch 3 times per week for 4 or 6 weeks. At 13 weeks of age, the groups exposed to DMBA were fed diet containing PCA or Costunolide at a dose of 0.2 g/kg diet (200 ppm) for 17 weeks. The other groups consisted of hamsters given mineral oil alone for 6 weeks, or given 200 ppm PCA or Costunolide alone, or untreated. All animals were necropsied at the termination of the experiment (week 24). PCA or costunolide significantly decreased the tumor burden (P<0.001‐P<0.05) and the extent of dysplastic areas (%) (P<0.001‐P<0.05). PCA significantly decreased the mean number of AgNORs/nucleus (P<0.05). The BrdUrd‐labeling index was reduced by dietary administration of test compounds, though not significantly. These results suggest that PCA and costunolide inhibited hamster buccal pouch carcinogenesis and such inhibition may be related to suppression of cell proliferation in the buccal mucosa. It was also found that telomerase activity expressed in neoplastic and preneoplastic lesions of hamster buccal pouch epithelium after DMBA treatment correlated with the histopathological degree of malignancy.

No abstract

Telomerase activity in tissues may be related to tumor development, especially malignant conversion, in humans. However, there are few reports about telomeres and telomerase activity in animals. In this study, we examined telomerase activity in rat colon carcinogenesis and in normal rat liver tissue and compared it with that of human colon cancer tissues. This is the first report concerning telomerase activity in rats. F344 rats were used, and colon neoplasms were induced with methylazoxymethanol acetate. There was telomerase activity in not only the induced colon neoplasms but also the colon mucosa and livers of untreated rats, in contrast with the results from normal human somatic tissues in previous reports. Indeed, we also observed negative results in normal human mucosa, despite the positive results in colon-cancer tissues. These findings suggest that there is a difference in the telomerase activities in humans and rats. Because rat telomeres are very long (20-100 kp, average 50 kp) compared with human telomeres (5-15 kp, average 12 kp), the difference in the telomere lengths of rats and humans might be related to their enzyme activities, although this is still unclear. Furthermore, because the inhibition of telomerase has been proposed as a novel cancer therapy for humans, the rat model presented here, in which telomerase is expressed in somatic tissues, may be useful for studies of telomerase inhibition, including inhibition by chemopreventive agents.