Comparative genomics of susceptibility to mammary carcinogenesis among inbred rat strains: role of reduced prolactin signaling in resistance of the Copenhagen strain

To elucidate the molecular basis for differential susceptibilities to mammary carcinogenesis, we compared the transcriptomes of normal mammary glands from pubescent female rats of the resistant Copenhagen (Cop) strain with those of the susceptible Fischer 344 (F344), August x Copenhagen Irish (ACI), Buffalo/N (Buf/N), Wistar-Furth (WF) strains and F1 (Cop x F344) progeny (Fl). Gene expression profiles in mammary tissue within each rat strain were remarkably similar, indicating that gene expression was determined by genetic background. We next identified the subset of genes that were differentially expressed in all susceptible strains relative to the resistant Cop strain. Among these, the messenger RNAs encoding prolactin (Pr1) and its cell surface receptor were significantly elevated in all susceptible strains. The expression levels of several Pr1-regulated genes were also significantly elevated, indicating the presence of increased Pr1 signaling in mammary glands of all susceptible strains. Pathway analysis of gene expression profiles further identified the Pr1-activated Jak/ STAT-signaling pathway among the pathways that most distinguished sensitive rat strains from the resistant Cop rat. To test the hypothesis that reduced levels of the Pr1 signaling in mammary tissue partially contributed to the genetic resistance to mammary carcinogenesis, we used the neuroleptic drug, perphenazine, to transiently elevate serum Pr1 levels in the Cop strain. Whereas Cop rats are resistant to N-nitroso-N-methylurea (NMU)-induced mammary carcinogenesis, ∼5% of pubescent Cop females treated with perphenazine and NMU exposure developed mammary adenocarcinomas with latencies comparable with those of sensitive strains. Together, these finding indicated that in the rat, the molecular mechanisms underlying genetic susceptibility to mammary carcinogenesis include de-regulation of Pr1 signaling.