Cyclin D1 expression during rat mammary tumor development and its potential role in the resistance of the Copenhagen rat

Abstract

Background: Resistance to mammary tumorigenesis in Copenhagen rats is associated with loss of early preneoplastic lesions known as intraductal proliferations. The cause of this disappearance, however, is unknown.

Results: There were no differences in the numbers of lesions in mammary whole-mounts prepared from Copenhagen or Wistar-Furth rats at 20 or 30 days after N-methyl-N-nitrosourea treatment, but at 37 days there were significantly fewer lesions in Copenhagen glands. Furthermore, lesions in Copenhagen glands were exclusively intraductal proliferations, whereas in Wistar-Furth glands more advanced lesions were also present. Immunohistochemical staining showed frequent cyclin D1 overexpression in Wistar-Furth lesions at 37 days, but not in Copenhagen lesions. There were, however, no differences in p16INK4a protein expression, bromodeoxyuridine labeling and apoptotic indices, or mast cell infiltration between Copenhagen and Wistar-Furth lesions at any time.

Conclusions: Overexpression of cyclin D1 in preneoplastic lesions may be important in the development of mammary tumors in susceptible rats, although this overexpression does not appear to cause significant changes in cell kinetics. Furthermore, the low levels of cyclin D1 expression in Copenhagen intraductal proliferations may play a role in the resistance of these rats to mammary tumorigenesis.

Figure 1

Number of preneoplastic lesions in the mammary glands of treated Copenhagen (Cop) and Wistar-Furth (WF) rats at 20, 30, and 37 days after N-methyl-N-nitrosourea treatment. ^*P < 0.05, versus Wistar-Furth rats.

Figure 2

Examples of mammary whole-mounts from (a) a Wistar-Furth rat and (b) a Copenhagen rat, both 37 days after N-methyl-N-nitrosourea treatment. Note the striking differences between the glands, with multiple preneoplastic lesions present in the Wistar-Furth gland but only one putative lesion present in the Copenhagen gland, as indicated by the arrows. Bar = 1 mm.

Figure 3

Immunohistochemical staining for (a-c) cyclin D¹, (d-f) p16^INK4a(g-I) bromodeoxyuridine. Positive staining cells appear brown, counter-stained negative cells appear purple. (a), (d), and (g) are serial sections of an intraductal proliferation (IDP) from a Copenhagen rat; (b), (e), and (h) are serial sections of a large IDP from a Wistar-Furth rat; and (c), (f), and (I) are serial sections of a tumor from a Wistar-Furth rat. Note the overexpression of cyclin D¹ in Wistar-Furth lesions (b and c) but not in the Copenhagen IDP (a). All lesions are from mammary glands of rats 37 days after MNU treatment. (a-f) 1000× magnification and (g-I) 400× magnification.

Figure 4

Bromodeoxyuridine (BrdU) labeling indices in Copenhagen (Cop) and Wistar-Furth (WF) rats at 20, 30, and 37 days after MNU treatment.

Figure 5

Apoptotic indices in Copenhagen (Cop) and Wistar-Furth (WF) lesions at 20, 30, and 37 days after MNU.