H-ras induced transformation of mammary epithelium is favoured by increased oncogene expression or by inhibition of mammary regression.

The promoter of the mammary specific murine whey acidic protein gene was used to direct Ha-ras expression in different lines of transgenic mice. We found that this promoter contains a tissue specific enhancer which directed expression in both orientations albeit to different levels. We used this feature to generate low and high ras expressing transgenic lines. The reversed orientation led to a weak expression in lines 3 and 58 and to a tumor frequency of 2%. In contrast, 72% of mice from line 25 showing high ras expression developed mammary tumors. Nulliparity is one risk factor for human breast cancer, suggesting a protective effect of post-lactational mammary regression. In order to investigate the effect of post-lactational regression, the low tumor frequency lines were crossed with mice expressing ubiquitously the human growth hormone gene, which induces permanent development of the mammary epithelium. Indeed, mammary tumors were observed in 76% of double transgenic females. Thus, the tumorigenic potential of the ras oncogene in mammary cells in vivo correlates with the level of its expression and with the developmental history of the mammary gland. Transformation coincides with the escape of oncogene expression from the regulation of the Wap promoter and the extinction of endogenous Wap gene expression.