Heat shock protein Hsp72 plays an essential role in Her2-induced mammary tumorigenesis

The major heat shock protein Hsp72 is expressed at elevated levels in many human cancers and its expression correlates with tumor progression. Here we investigated the role of Hsp72 in Her2 oncogene-induced neoplastic transformation and tumorigenesis. Expression of Her2 in untransformed MCF10A mammary epithelial cells caused transformation, as judged by foci formation in culture and tumorigenesis in xenografts. However, expression of Her2 in Hsp72-depleted cells failed to induce transformation. The anti-tumorigenic effects of Hsp72 downregulation were associated with cellular senescence due to accumulation of p21 and depletion of survivin. Accordingly, either knockdown of p21 or expression of survivin reversed this senescence process. Further, we developed an animal model of Hsp72-dependent breast cancer associated with expression of Her2. Knockout of Hsp72 almost completely suppressed tumorigenesis in the MMTVneu breast cancer mouse model. In young Hsp72 KO mice, expression of Her2 instead of mammary tissue hyperplasia led to suppression of duct development and blocked alveolar budding. These effects were due to massive cell senescence in mammary tissue, which was associated with upregulation of p21 and downregulation of survivin. Therefore Hsp72 plays an essential role in Her2-induced tumorigenesis by regulating oncogene-induced senescence pathways.