Human prolactin receptors are insensitive to mouse prolactin: implications for xenotransplant modeling of human breast cancer in mice.

Experimental testing of growth, metastatic progression and drug responsiveness of human breast cancer in vivo is performed in immunodeficient mice. Drug candidates need to show promise against human breast cancer in mice before being allowed into clinical trials. Breast cancer growth is under endocrine control by ovarian steroids and the pituitary peptide hormone prolactin. While it is recognized that the most relevant biologic effects of prolactin are achieved with prolactin from the matching species, the biologic efficacy of mouse prolactin for human prolactin receptors has not been recorded. Thus, it is unclear whether the mouse endocrine environment adequately reflects the hormonal environment in breast cancer patients with regard to prolactin. We now show both recombinant and natural pituitary-derived mouse prolactin to be a poor agonist for human prolactin receptors. Mouse prolactin failed to induce human prolactin receptor-mediated biologic responses of cell clustering, proliferation, gene induction and signal transduction, including activation of Stat5, Stat3, Erk1/2 and Akt pathways. Consistent data were derived from human breast cancer lines T-47D, MCF-7 and ZR-75.1, as well as human prolactin receptor-transfected COS-7 and 32D cells. Failure of mouse prolactin to activate human prolactin receptors uncovers a key deficiency of the mouse endocrine environment for human xenotransplant studies. Since most human breast cancers express prolactin receptors, human breast cancer transferred into mice is unnaturally selected for growth in the absence of circulating prolactin. The new insight raises concerns about the validity of analyzing biology and drug responsiveness of human breast cancer in existing mouse xenotransplant models.

The Adrenomedullin gene is a target for negative regulation by the Myc transcription complex.

The Myc family of transcription factors plays a central role in vertebrate growth and development although relatively few genetic targets of the Myc transcription complex have been identified. In this study, we used mRNA differential display to investigate gene expression changes induced by the overexpression of the MC29 v-Myc oncoprotein in C3H10T1/2 mouse fibroblasts. We identified the transcript of the adrenomedullin gene (AM) as an mRNA that is specifically down-regulated in v-Myc overexpressing C3H10T1/2 cell lines as well as in a Rat 1a cell line inducible for c-Myc. Nucleotide sequence analysis of the mouse AM promoter reveals the presence of consensus CAAT and TATA boxes as well as an initiator element (INR) with significant sequence similarity to the INR responsible for Myc-mediated repression of the adenovirus major late promoter (AdMLP). Reporter gene assays confirm that the region of the AM promoter containing the INR is the target of Myc-mediated repression. Exogenous application of AM peptide to quiescent C3H10T1/2 cultures does not stimulate growth, and constitutive expression of AM mRNA in C3H10T1/2 cells correlates with a reduced potential of the cells to be cotransformed by v-Myc and oncogenic Ras p21. Additional studies showing that AM mRNA is underrepresented in C3H10T1/2 cell lines stably transformed by Ras p21 or adenovirus E1A suggest that AM gene expression is incompatible with deregulated growth in this cell line. We propose a model in which the repression of AM gene expression by Myc is important to the role of this oncoprotein as a potentiator of cellular transformation in C3H10T1/2 and perhaps other cell lines.