An in vitro model of human pancreas carcinogenesis: effects of nitroso compounds

Does the Use of Proton Pump Inhibitors Increase the Risk of Pancreatic Cancer? A Systematic Review and Meta-Analysis of Epidemiologic Studies

Background: One of the most frequently used medications for treating gastrointestinal disorders is proton pump inhibitor (PPI), which reportedly has potential adverse effects. Although the relationship between the use of PPIs and the risk of pancreatic cancer has been extensively investigated, the results remain inconsistent. Hence, this meta-analysis aimed to evaluate such relationship. Methods: We searched for literature and subsequently included 10 studies (seven case–control and three cohort studies; 948,782 individuals). The pooled odds ratio (OR) and 95% confidence intervals (CI) for pancreatic cancer were estimated using a random-effects model. We also conducted sensitivity analysis and subgroup analysis. Results: The pooled OR of the meta-analysis was 1.698 (95% CI: 1.200–2.402, p = 0.003), with a substantial heterogeneity (I² = 98.75%, p < 0.001). Even when studies were excluded one by one, the pooled OR remained statistically significant. According to the stratified subgroup analyses, PPI use, and pancreatic cancer incidence were positively associated, regardless of the study design, quality of study, country, and PPI type. Conclusion: PPI use may be associated with the increased risk of pancreatic cancer. Hence, caution is needed when using PPIs among patients with a high risk of pancreatic cancer.

A Prospective Analysis of Intake of Red and Processed Meat in Relation to Pancreatic Cancer among African American Women

BACKGROUND

African Americans have the highest incidence of pancreatic cancer of any racial/ethnic group in the United States. There is evidence that consumption of red or processed meat and foods containing saturated fats may increase the risk of pancreatic cancer, but there is limited evidence in African Americans.

METHODS

Utilizing the Black Women's Health Study (1995-2018), we prospectively investigated the associations of red and processed meat and saturated fats with incidence of pancreatic adenocarcinoma (n = 168). A food frequency questionnaire was completed by 52,706 participants in 1995 and 2001. Multivariable-adjusted HRs and 95% confidence intervals (CI) were estimated using Cox proportional hazards regression. We observed interactions with age (P _interaction = 0.01). Thus, results were stratified at age 50 (<50, ≥50).

RESULTS

Based on 148 cases among women aged ≥50 years, total red meat intake was associated with a 65% increased pancreatic cancer risk (HR_{Q4 vs. Q1} = 1.65; 95% CI, 0.98-2.78; P _trend = 0.05), primarily due to unprocessed red meat. There was also a nonsignificant association between total saturated fat and pancreatic cancer (HR_{Q4 vs. Q1} = 1.85; 95% CI, 0.92-3.72; P _trend = 0.08). Red meat and saturated fat intakes were not associated with pancreatic cancer risk in younger women, and there was no association with processed meat in either age group.

CONCLUSIONS

Red meat-specifically, unprocessed red meat-and saturated fat intakes were associated with an increased risk of pancreatic cancer in African-American women aged 50 and older, but not among younger women.

IMPACT

The accumulating evidence-including now in African-American women-suggests that diet, a modifiable factor, plays a role in the etiology of pancreatic cancer, suggesting opportunities for prevention.

Exposure to proton pump inhibitors and risk of pancreatic cancer: a meta-analysis

Objectives: To estimate the pancreatic cancer risk among subjects exposed versus not exposed to proton pump inhibitors.Methods: The authors searched PubMed, EMBASE, Scopus, Cochrane Library, and clinicaltrials.gov to identify relevant studies. The authors quantified pancreatic cancer risk among subjects exposed versus not exposed to PPIs, expressed as the pooled (adjusted) odds ratio (OR/aOR) and 95% confidence interval (95%CI) in overall and sensitivity analyses.Results: One randomized trial, two cohort, four case-control, and five nested case-control studies with 700,178 subjects (73,985 cases; 626,193 controls) were retained. PPI exposure was associated with pancreatic cancer risk (OR = 1.75, 95%CI = 1.12-2.72, I2 = 99%); confirmed in sensitivity analyses for high-quality studies, observational studies, case-control studies, studies with pancreatic cancer as the primary outcome, and in sensitivity analyses for diabetes and obesity but not for pancreatitis and smoking. This association was independent of the duration and Defined Daily Dose of PPI exposure. Rabeprazole had a singular significant association with pancreatic cancer (OR = 5.40, 95%CI = 1.98-14.703, I2 = 87.9%).Conclusion: The class of PPIs is associated with a 1.75-fold increase in pancreatic cancer risk, confirmed in sensitivity analyses.

Proton pump inhibitors and histamine-2 receptor antagonists on the risk of pancreatic cancer: a systematic review and meta-analysis

BACKGROUND

Proton pump inhibitors (PPI) and histamine-2 receptor antagonists (H2RA) have been widely used for multiple purposes. Recent studies have suggested an association between these medications and the risk of pancreatic cancer. However, the results have been inconclusive.

AIM

We, therefore, conducted a study to assess the risk of developing pancreatic cancer in patients who used PPI and H2RA.

DESIGN

A systematic review and meta-analysis.

METHODS

A literature search was performed using MEDLINE and EMBASE databases from inception through February 2019. Studies that reported risk ratio comparing the risk of pancreatic cancer in patients who received PPI or H2RA versus those who did not receive treatments were included. Pooled risk ratios (RR) and 95% confidence intervals (CI) were calculated using a random-effect generic inverse variance method. Sensitivity analysis, excluding one study at a time, was performed.

RESULTS

After screening abstracts from the searching methods, seven studies (six case-control studies and one cohort study) were included in the analysis with total 546 199 participants. Compared to patients who did not take medications, the pooled RR of developing pancreatic cancer in patients receiving PPI and H2RA were 1.73 (95% CI: 1.16-2.57) and 1.26 (95% CI: 1.02-1.57), respectively. However, the sensitivity analysis of PPI changed the pooled RR to 1.87 (95% CI: 1.00-3.51) after a study was dropped out. Likewise, H2RA sensitivity analysis also resulted in non-significant pooled RR.

CONCLUSIONS

This meta-analysis did not find the strong evidence for the associations between the use of PPI and H2RA and pancreatic cancer.

The phenotypic characterization of A13/BACii, a novel bovine chondrocytic cell line with differentiation potential

In cartilage research bovine articular cartilage is used as an alternative to human tissue. However, animal material is subject to availability and primary cultures undergo senescence, limiting their use. Here we report the immortalization of primary bovine chondrocytes, which could be used as a surrogate for freshly isolated chondrocytes. Chondrocytes were isolated from cartilage explants and immortalized using 1.0 µg/ml benzo[alpha]pyrene. For 3-dimensional culture, chondrocytes were resuspended in 0.5% low-melt agarose at high density (HD) and cultured for 24 h prior to determining changes in expression profile and morphology. A13/BACii chondrocytes acquired a 'flat' irregular morphology and a foetal-like cell volume (1,509.59 ± 182.04 µm(3)). The human cell line C-20/A4 showed a statistically similar volume and length to A13/BACii. Two-dimensional-cultured A13/BACii expressed elevated levels of type I collagen (col1), reduced levels of type II collagen (col2) compared to freshly isolated chondrocytes and an overall col2 to col1 expression ratio (col2:col1) of 0.11 ± 0.01. Upon 3-dimensional encapsulation, there was a significant rise in col2 expression in both A13/BACii and C-20/A4, suggesting a capacity for redifferentiation in both cell lines with a return of col2:col1 values of A13/BACii to values previously observed in primary chondrocytes. A13/BACii chondrocytes expressed aggrecan, matrix metalloproteinase (MMP)-3, MMP-9 and MMP-13, further supporting indications of the differentiated phenotype. Here we report the creation of a novel chondrocytic cell line and demonstrate its strong potential for redifferentiation upon HD 3-dimensional encapsulation, providing an alternative to conventional dedifferentiated cell lines and primary culture.

Proton pump inhibitors and histamine-2-receptor antagonists and pancreatic cancer risk: a nested case-control study

BACKGROUND

The relationship between use of proton pump inhibitors (PPIs) and histamine-2-receptor antagonists (H(2)RAs) and pancreatic cancer risk has yet to be examined. Data from a range of studies suggest biologically plausible mechanisms, whereby these drugs (or the conditions for which they are prescribed) may affect pancreatic cancer risk. The objective of this study was to investigate the relationship between use of PPIs/H(2)RAs and pancreatic cancer risk.

METHODS

A nested case-control study was conducted within the UK general practice research database (GPRD). Cases had a diagnosis of exocrine pancreatic cancer and controls were matched to cases on general practice site, sex and year of birth. Exposure to PPIs and to H(2)RAs since entry into GPRD until 2 years before the diagnosis date (corresponding date in controls) and in the 5 years before the diagnosis date were separately assessed. Conditional logistic regression analyses were used to generate odds ratios (ORs) and 95% confidence intervals (CIs) associated with PPI or H(2)RA use compared with nonuse.

RESULTS

Ever use of PPIs since entry into the GPRD (excluding the 2 years prior to diagnosis) was not associated with risk of pancreatic cancer; OR (95% CI) 1.02 (0.85-1.22). Neither the dose nor the duration of PPI or H(2)RA use was associated with pancreatic cancer risk. No consistent patterns of association were seen when cumulative exposure (dose and duration) to these drugs was examined separately or together.

CONCLUSION

PPI/H(2)RA use, in a UK population, was not associated with pancreatic cancer risk.

DNA damage and cytotoxicity in pancreatic β-cells expressing human CYP2E1

Epidemiological studies have identified nitrosamines as a risk factor for Type I (insulin dependent) diabetes mellitus. These compounds require bioactivation by cytochrome P450 2E1 (CYP2E1) for exertion of their toxic effects. Two mammalian insulin secreting pancreatic beta-cell lines BRIN BD11h2E1 and INS-1h2E1, which express human full length CYP2E1 cDNA, were used to elucidate the role of CYP2E1-mediated nitrosamine bioactivation in pancreatic beta-cell dysfunction and destruction. These cell lines were shown to metabolise dimethylnitrosamine to produce formaldehyde at rates of 3.41 +/- 0.24 and 3.65 +/- 0.26 nmol/minmg microsomal protein, respectively. Following incubation with various concentrations of the nitrosamines dimethylnitrosamine, N-nitrosopyrrolidine and 1-nitrospiperidine, all of which are bioactivated by CYP2E1, cytotoxicity and DNA damage were assessed using either the neutral red assay or comet assay respectively. Exposure of CYP2E1 expressing cells to nitrosamines resulted in significant dose-dependent decreases in cell viability, which were not seen in cells which did not express CYP2E1. Following culture with nitrosamine concentrations as low as 2.5mM 1-nitrosopiperidine, cell viability was significantly lower in BRIN BD11h2E1 and INS-1h2E1 cell lines in comparison to the BRIN BD11 and INS-1 parental cell lines (72.5 +/- 4.96 and 66.4 +/- 3.09% in BRIN BD11h2E1 and INS-1h2E1 versus 109.0 +/- 3.40 and 100.0 +/- 3.25% in BRIN BD11 and INS-1 respectively, P < 0.001). The highest dose of any of the nitrosamines tested failed to significantly reduce cell viability in the cells which lacked CYP2E1. Expression of CYP2E1 did not cause any change in the basal level of DNA damage in any of the cell lines. However, 16 h exposure to various nitrosamines resulted in significant dose-dependent DNA damage in the BRIN BD11h2E1 and INS-1h2E1 cells compared to their respective non CYP2E1-expressing parental controls, e.g. DNA damage increased from 34.38 +/- 1.25 to 44.01 +/- 1.56% DNA in comet tail in BRIN BD11h2E1 cells incubated with 10 or 40 mM N-nitrosopyrrolidine, respectively (P < 0.001). Similar treatment of the BRIN BD11 and INS-1 cell lines did not result in a significant increase in DNA damage (20.33 +/- 1.0 and 22.4 +/- 0.98% DNA in comet tail). The pancreatic beta-cell is richly vascularised and expresses CYP2E1. This study suggests that expression of human CYP2E1 in pancreatic beta-cells make them highly susceptible to cytotoxicity and DNA damage by nitrosamines and other agents bioactivated by CYP2E1.

Etiology of pancreatic cancer, with a hypothesis concerning the role of N-nitroso compounds and excess gastric acidity

In the United States, pancreatic cancer is the fourth most frequent cause of cancer death in males as well as females, after lung, prostate or breast, and colorectal cancer. Each year, approximately 30 000 Americans are diagnosed with pancreatic cancer and about the same number die of it. Germline mutations in a few genes including p16 and BRCA2 have been implicated in a small fraction of cases, as has chronic pancreatitis. The one established risk factor for pancreatic cancer is cigarette smoking: current smokers have two to three times the risk of nonsmokers. Studies of dietary factors have not been entirely consistent but do suggest associations of higher risk with consumption of smoked or processed meats or with animal foods in general and lower risk with consumption of fruits and vegetables. Colonization by Helicobacter pylori appears to increase risk, and a history of diabetes mellitus may also increase risk. The purpose of this epidemiologic review is to consider the possibility that risk of pancreatic cancer is increased by factors associated with pancreatic N-nitrosamine or N-nitrosamide exposures and with chronic excess gastric or duodenal acidity. Host genetic variation in inflammatory cytokine mechanisms may also be involved in this process. Many features of the evidence bearing on the pathophysiology of pancreatic cancer appear to support connections with N-nitroso compounds and with gastric acidity.

Role of N-nitroso compounds (NOC) and N-nitrosation in etiology of gastric, esophageal, nasopharyngeal and bladder cancer and contribution to cancer of known exposures to NOC

The questions of whether and how N-nitroso compounds (NOC) may be inducing cancer in humans are discussed. The principal subjects covered include nitrite-derived alkylating agents that are not NOC, reasons for the wide tissue specificity of carcinogenesis by NOC, the acute toxicity of nitrosamines in humans, mechanisms of in vivo formation of NOC by chemical and bacterial nitrosation in the stomach and via nitric oxide (NO) formation during inflammation, studies on nitrite esters, use of the nitrosoproline test to follow human gastric nitrosation, correlations of nitrate in food and water with in vivo nitrosation and the inhibition of gastric nitrosation by vitamin C and polyphenols. Evidence that specific cancers are caused by NOC is reviewed for cancer of the stomach, esophagus, nasopharynx, urinary bladder in bilharzia and colon. I review the occurrence of nitrosamines in tobacco products, nitrite-cured meat (which might be linked with childhood leukemia and brain cancer) and other foods, and in drugs and industrial situations. Finally, I discuss clues from mutations in ras and p53 genes in human tumors about whether NOC are etiologic agents and draw some general conclusions.

Initiating activity of diethylnitrosamine in a rapid production model for pancreatic carcinomas in Syrian hamsters

The potential initiating activity of diethylnitrosamine (DEN) was studied in a rapid production model for pancreatic carcinogenesis in hamsters developed in our laboratory incorporating the principle of selection based on resistance of cytotoxicity, originally demonstrated for liver carcinogenesis in rats. Female Syrian golden hamsters were given DEN at a dose of 100 mg/kg body weight or N-nitrosobis-(2-oxopropyl)amine (BOP) at a dose of 70 mg/kg body weight as initiators followed by 3 cycles of augmentation pressure (choline-deficient diet combined with DL-ethionine, L-methionine upon return to basal diet and then administration of 20 mg/kg body weight BOP), and killed 10 weeks after the beginning of the experiment. DEN followed by the augmentation pressure induced a 65% incidence of total pancreatic lesions including 15% carcinomas, while BOP followed by the augmentation pressure induced 100% incidence of total pancreatic lesions and 84.2% for carcinomas. These yields were significantly greater than those observed for augmentation pressure alone. The results thus indicate that DEN possesses weak initiating activity for pancreatic carcinogenesis under the present experimental conditions.

Multicentric focal acinar cell hyperplasia and hepatocyte-like cell metaplasia are induced by nitrosomethylurea in rat pancreas

The present report is a study of the effect of the carcinogen nitrosomethylurea (NMU) on pancreas of rats receiving during lifetime a lipid-poor diet, that is essential fatty acid deficient or control diets. Rats fed a commercial stock chow were mated. At day 10 of pregnancy, dams were divided into three groups, that were respectively supplied with the commercial chow, the essential fatty acid deficient or the sufficient diet. Each litter was separated at random in two groups that received at day one of life one intraperitoneal injection of NMU (50 mg/kg b.w.) or saline. After weaning, they were maintained for life with the diet that was supplied to their mothers. The pancreas of NMU-treated rats presented diffuse proliferative changes, focal acinar cell hyperplasias (FACH), and focal hepatocyte-like metaplasia (FHLCM). FACH were expansive presumably preneoplastic growths, showing abnormal differentiation. The number of NMU-treated rats bearing FACH and FHLCM did not significantly differ in the three nutritional conditions.

L-azaserine induced preneoplasia in the rat pancreas. A morphometric study of dietary manipulation (lipotrope deficiency) and ultrastructural differentiation

Putatively preneoplastic, pancreatic atypical acinar cell foci (AACF) and nodules (AACN), collectively termed atypical acinar cell lesions (AACL), were induced in male Lewis rats by L-azaserine (300 mg/kg body weight [bw] in divided doses). Rats given carcinogen and then fed a lipotrope deficient (LD) diet developed a significantly greater number of larger lesions than animals fed complete diet throughout the experiment. It is suggested that lipotrope deficiency plays a promoting role in this model of pancreatocarcinogenesis. Ultrastructural morphometric studies of AACF, when compared to control tissues, revealed the following significant results: 1) a decrease in surface area of cell cytoplasm with no change in nuclear area, and hence increased nucleus/cytoplasm (N/C) ratio; 2) a reduction in size and uniformity of zymogen granules; and 3) an increase in number of granules per microns 2 of cell. The results suggest that arrested development of the AACF cells is associated with reduced cytoplasm and zymogen production per cell. AACL may be eosinophilic due to an overall increased concentration of zymogen in these hyperplastic lesions and not because individual acinar cells in the AACL contain an increased amount of zymogen or are "zymogen-rich," as has been reported.

Comparative stages of expression of human squamous carcinoma cells and carcinogen transformed keratinocytes

The mouse monoclonal antibody OSU 22-3 was prepared using cells from a squamous cell carcinoma (SCC) as an immunogen. This antibody reacts with an antigen found on squamous cell carcinomas but does not react with normal keratinocytes. This antibody and two antibodies that react with normal keratinocytes were used as markers of malignant and normal phenotypes. These markers were used to evaluate several spontaneous and carcinogen initiated SCC tumors and to identify the expression of an antigen associated with a malignant phenotype. A variety of subpopulations in carcinogen initiated tumors and spontaneous SCC tumors were noted. The subpopulations that reacted only with MoAb OSU 22-3 exhibited features of anchorage independent growth and cellular invasiveness, and formed progressively growing tumors in nude mice. Other SCC spontaneous tumor cell subpopulations reacted with the antibodies associated with normal keratinocytes. These cells did not proliferate in vitro and did not form tumors in the nude mouse. There were other carcinogen transformed cells which reacted with MoAb OSU 22-3 but not with the antibodies associated with normal keratinocytes. These cells exhibited anchorage independent growth and cellular invasiveness but did not form tumors in nude mice. We conclude from this work that human SCC tumors contain multiple cell populations. These cell populations have varied growth properties and express surface antigens that may indicate their malignant vigor. Carcinogen transformed keratinocytes do exhibit some of the characteristics of SCC tumor phenotypes but not the property of malignant progressively growing cells on a routine and consistent basis. This feature is transiently and inconsistently expressed in a surrogate host by populations prepared from spontaneous SSC tumors.

Pancreatic ductal mucinous hyperplasia: distribution within the pancreas, and effect of variation in ampullary and pancreatic duct anatomy

The prevalence and amount (hyperplasia score) of ductal mucinous hyperplasia (DMH) were estimated in 12 predetermined areas taken from 102 pancreases obtained at necropsy from patients dying of non-pancreatic diseases. The mean prevalence of DMH was 18% (213/1206) per section and 63% (63/102) per pancreas. Different areas of the pancreas could be stratified by amount of DMH into 'high' (hyperplasia score greater than 0.09), 'intermediate' (0.06-0.09), and 'low' (less than 0.06). There was no significant difference in either prevalence or amount of DMH with variation in ampullary or pancreatic ductal anatomy. There was significantly (p less than 0.05) less DMH adjacent to the accessory papilla when it was patent, compared with when it was not patent and the accessory duct communicated with the main pancreatic duct. The findings suggest that DMH is a proliferative response to exogenous agents which injure the pancreas, and that some areas of the pancreas are more vulnerable than others to this damage. There was no evidence that this injury was associated either with reflux, or with any particular variation in pancreatic duct anatomy. The association between DMH and occlusion of the accessory papilla may explain the susceptibility of pancreas divisum to pancreatitis.

Multi-step neoplastic transformation of normal human fibroblasts by Co-60 gamma rays and Ha-ras oncogenes

As reported previously (Namba et al., 1985; Namba, 1985), normal human fibroblasts were transformed into immortal cells with abnormal karyotypes by Co-60 gamma-ray irradiation. These immortally transformed cells (KMST-6) showed no clonability in soft agar and were not tumorigenic. However, by treatment with Ha-ras oncogenes derived from a human lung carcinoma or Harvey murine sarcoma virus, the KMST-6 cells acquired elevated clonability in soft agar and transplantability in nude mice. All the tumors produced grew progressively without showing regression and killed the mice. The tumors were also serially transplantable into other mice. The Ha-ras oncogene alone did not convert normal human fibroblasts into either immortal or tumorigenic cells. Our current data suggest that gamma rays worked as an initiator of carcinogenesis in normal human cells, giving rise to chromosome aberrations and immortality, and the Ha-ras oncogene played a role in the progression of the immortally transformed cell population to a neoplastic one showing enhanced colony formation in soft agar and tumorigenicity in nude mice.

Progression in a chemically induced transplantable human pancreas carcinoma

A methylnitrosourea (MNU)-induced transplantable human pancreas carcinoma was examined, at 3, 12, 18 and 36 months after its development, for growth and invasiveness in nude mice, karyotypic alteration and the evolution of marker chromosomes. Progression in tumorigenicity and invasiveness of cells were evident by a significant increase in tumor diameters produced within 8 weeks by the cells at 36 months as compared to those developed by cells from 3-month-old cell lines. Chromosome analysis at 3 months showed normal 46 XX karyotype in about 80% and minor anomalies in 20% of the cells. At 12, 18 and 36 months, all cells were hyperdiploid with 53-61 chromosomes and several abnormal marker chromosomes. Marker chromosomes showed non-reciprocal translocations, deletions, inversion and isochromosomes. The absence of chromosome 13 from the earlier stage onward may have resulted in the loss of genes which suppress tumorigenicity. The increase in homogeneously staining regions of marker chromosome 3 at later stages appears to parallel the augmentation in tumor growth and mitotic indices.

What is the significance of pancreatic ductal mucinous hyperplasia?

The prevalence, distribution, and clinical associations of pancreatic ductal mucinous hyperplasia were studied in 102 non-malignant pancreases. Ductal mucinous hyperplasia was found in over 60% of specimens and was frequently associated with increased fibrosis--occasionally resembling pancreatitis. Significantly more ductal mucinous hyperplasia was found in pancreas from patients who were receiving corticosteroid treatment. Neither a history of hypercalcaemia in the three months before death, diabetes mellitus, alcoholism, tobacco smoking, nor the presence of gall stones was associated with an increase in ductal mucinous hyperplasia. The age of maximum prevalence, and the distribution of ductal mucinous hyperplasia in the pancreas were similar to those of pancreatic carcinoma. These similarities may be because both ductal mucinous hyperplasia and pancreatic carcinoma are proliferative responses, rather than because ductal mucinous hyperplasia is a precursor of pancreatic carcinoma.

Histogenesis of pancreatic carcinomas: a study based on 248 cases

Primary pancreatic carcinomas were studied histologically and histochemically, to assess the frequency of ductal hyperplasia in tissue adjacent to malignant neoplasms. Hyperplasia was divided into four types: simple, papillary, atypical and ductular, affecting large, medium and small ducts (ductules). All types of hyperplasia were frequently seen in areas adjacent to carcinomas, including ductal, pleomorphic, mucinous, adenosquamous, small and spindle cell and cystadenocarcinomas. In contrast, acinar cell carcinoma and microadenocarcinoma were less frequently associated with ductal hyperplasia. Mucin histochemistry revealed differences in types of mucin between the normal ducts and hyperplastic pancreatic ducts and carcinomas. The former group contained small amounts of sulphated mucin while the latter showed a marked increase in neutral and sialomucins. Our study also suggests that both papillary and atypical hyperplasia are precancerous lesions, supporting an hypothesis of ductal origin of pancreatic carcinomas.

Induction of transformation and continuous cell lines from normal human mammary epithelial cells after exposure to benzo[a]pyrene

Rapidly growing primary cultures of normal human mammary epithelial cells (HMEC) were exposed to 1 microgram of benzo[a]pyrene (B[a]P) per ml for two or three 24-hr periods. The B[a]P-treated populations consistently contained cells displaying a longer period of active growth in culture compared to the untreated control cells. Widespread heterogeneity in morphology and growth patterns was evidenced in these "extended life" (EL) cultures, with multiple sequential changes in these parameters occurring during the course of their life in culture. Two apparently immortal continuous cell lines have thus far emerged from these EL cultures. These lines have been characterized to be of human mammary epithelial origin and derived from the originally treated HMEC specimen. The continuous lines do not appear to be malignantly transformed as they do not cause tumor formation in nude mice and show little or no anchorage-independent growth. Nonetheless, they have acquired several properties characteristic of tumor-derived HMEC, which distinguish them from their normal progenitors. These cell lines, as well as the EL strains, may provide useful substrates for studies to determine what agents can induce further transforming events. Additionally, analysis of the multiple steps occurring in the El cultures, as well as in the emergence of the continuous cell lines, could potentially elucidate the processes occurring during human epithelial cell carcinogenesis and escape from senescence.

DNA adduct formation in rat, human and hamster pancreas treated with methylnitrosourea

The methylation of cellular macromolecules with dimethylnitrosamine (DMN) and methylnitrosourea (MNU) was studied in organ cultured rat, hamster and human pancreatic explants. At concentrations of DMN and MNU that caused similar methylation of protein in human explants DMN caused only 2.6% and 0.3% of the methylation of DNA and RNA that was produced by MNU. The DNA of explants treated with MNU was analyzed. The O6-methylguanine (O6-MeG)/7-methylguanine (7-MeG) ratio was greater in the hamster DNA than in DNA isolated from either rat or human. The time course of removal of methyl adducts from DNA was followed for 6 h after treatment with MNU. No decline in O6-MeG occurred during this period in hamster explants, although there was a decline in the content of 7-MeA and 3-MeA, whereas there was removal of O6-MeG in the DNA from human pancreas explants.