Stochastic development of invasive potential in rat mammary tumors induced by N-methyl-N-nitrosourea.

Abstract

We have studied the progression of breast tumors induced by N-methyl-N-nitrosourea in Wistar/Furst rats during serial passages through isogeneic animals, using characteristic markers previously defined. The tumors contained various types of structure, which evolve from intraductal nodes to duct-like structures and then to invasive strands or cells. Most primary tumors were not invasive but became so in subsequent transfers. The distribution of invasive and noninvasive tumors in the various lineages was not random, implying that the tumors differed from their inception in the propensity toward invasion. In different lineages invasive structures appeared repeatedly and independently but with different frequencies, suggesting that they were produced by a stochastic mechanism with different probability of occurrence. The possible nature of this mechanism is discussed. This experimental system is useful for further studies of tumor invasion; the results obtained are relevant for understanding and evaluating the evolution of human breast cancer.The genetic mechanisms by which cancer initiates are known (1), but those that drive the progression of cancer through a series of stages to full malignancy (2), mainly through local invasion and metastasis, are unknown. The conditions allowing cells to metastasize have been studied extensively (for a review, see ref.3) by taking advantage of experimental systems based on cell cultures. In contrast, for lack of a suitable model system, the study of invasion has been limited to basic histological observations.An experimental system widely used as a model for human breast cancer is mammary cancer induced in rats by Nmethyl-N-nitrosourea (4). However, these tumors are not regularly invasive (5) or metastatic (6). This difference between rat and human tumors might be due to the vast difference in developmental time, from a few months in the rat to decades in humans. To increase the time available for the expression ofthe invasive potential, we have transplanted the tumors serially to isogeneic rats, an approach already used for detecting the oncogenic potential of mouse hyperplastic alveolar nodules induced in the mammary gland by the mouse mammary tumor virus (7). The observations made show the usefulness of the experimental system, clarify several aspects of progression and invasion, and establish a similarity between the rat tumors and human mammary cancers. We show that tumors are heterogeneous in their ability to invade neighboring tissues, which behaves like a stochastic process of varying probability. This result is in accord with the finding that heterogeneity is the hallmark of tumors, whether one considers the presence of specific products (5) or the ability of premalignant lesions to develop into neoplasias (8). MATERIALS AND METHODSFifteen Wistar/Furst rats received a subcutaneous injection of 5 mg of N-methyl-N-nitrosourea per 100 g of body weight at the age of 5 weeks and a second identical injection 4 weeks later. The animals were maintained with standard die...

Serial transplantation of NMU‐induced rat mammary tumors: A model of human breast cancer progression

“…Additionally the tumor growth kinetics differed considerably; the primary and first transplant tumors grew more slowly taking 239 ± 80 and 131 ± 115 days, respectively, to reach a tumor volume of 2 cm 3 , whereas the later passaged tumors reached the same size in only 34 ± 16 days (average of the final 3 transplants in all series, p < 0.05). This accelerated tumor growth rate during transplantation is consistent with previous reports 29, 34. Two tumor series (Fig.…”

“…By cross species analysis of gene expression profiles, we previously reported that N ‐methyl‐ N ‐nitrosourea (NMU) induced primary rat tumors are similar to estrogen receptor (ER)‐positive, low‐grade human breast cancer 24. Though NMU‐induced primary rat mammary tumors are typically low‐grade in situ carcinomas, following serial transplantation they develop the capacity for invasion 29. Aldez et al .…”

Reversion toward an earlier stage of differentiation and loss of polarity during progression of N-methyl-N-nitrosourea-induced rat mammary tumours.

“…The induction of the tumors in Wistar/Furth rats and passaging to isogeneic animals have been described (2). Several…”

“…This natural evolution of cancers has been reproduced experimentally in rat mammary cancers induced by N-methyl-N-nitrosourea by serially passaging them to isogeneic rats (2). In this way progression is made possible because either the time available is increased or the modality of passaging accelerates progression.…”

“…Singer, Massachusetts Medical School, Worcester, MA). Immunocytology was carried out on frozen sections by the sandwich technique, using a combination ofa monoclonal antibody and a polyclonal serum as first layer, and a mixture of fluorescein-labeled goat anti-mouse immunoglobulin and rhodamine-labeled goat anti-rabbit immunoglobulin as second layer, as described (2). For examining the same section with two different monoclonal antibodies, a destainingrestaining procedure (19) was used.…”