Methylselenocysteine modulates proliferation and apoptosis biomarkers in premalignant lesions of the rat mammary gland

In the rat mammary carcinogenesis model, premalignant lesions known as intr aductal proliferations (IDPs) are detectable within a few weeks after carci nogen treatment. These early transformed colonies are the precursors for th e eventual formation of carcinomas. Our past research indicated that methyl selenocysteine added to the diet of vats reduced the development of IDPs of all sizes (the size of each IDP was estimated operationally by the number of 3-micron serial sections showing the same pathology). The appearance of an IDP lesion represents a balance between cell proliferation and cell deat h. The modulation of these two cellular events by methylselenocysteine was investigated. The abdominal-inguinal mammary gland was excised 6 weeks afte r MNU administration. Proliferation and apoptosis were evaluated by BrdU la beling and the TUNEL assay, respectively: The expression levels of several cell cycle and apoptosis regulatory proteins, including cyclin DI, cyclin A , p27, p16, bcl-2, bar and bak, were also assessed. AN of the above endpoin ts were quantified by immunohistochemistry in paraffin-embedded sections. T he results showed that the magnitude of the response to methylselenocystein e intervention seemed to depend on the size of the IDP lesion. For the purp ose of this study, the small and large lesions were classified as those con taining less than or equal to 30 or >30 serial sections, respectively. With the small lesions, methylselenocysteine significantly inhibited BrdU label ing and the expression of cyclin D1 and cyclin A, but increased the express ion of p27. Interesting, only p27 was upregulated in the larger IDP lesions , while BrdU labeling and the cyclins were not affected it is possible that the transformed phenotype becomes less sensitive to selenium-mediated arre st of proliferation once it progresses to a more advanced pathological stag e. In contrast, methylselenocysteine stimulated apoptosis (TUNEL assay) by 3 to 4 fold, and this increase was evident in both the small and large IDP lesions. Consistent with the induction of apoptosis, a reduced expression o f bcl-2 was also observed in the methylselenocysteine group. In summary, ou r data suggest that exposure to methylselenocysteine blocks clonal expansio n of premalignant lesions at an early stage. This is achieved by simultaneo usly modulating certain molecular pathways that are responsible for inhibit ing cell proliferation and enhancing apoptosis.