More like this The Role of Amphiregulin in Mammary Gland Development and Breast Cancer

Abstract

The purpose of our studies was to elucidate the role of amphiregulin (AREG) in breast cancer pathophysiology. Based on previous work, AREG has been proposed to promote tumor growth by stimulating proliferation of tumor cells. Some evidence also suggests that AREG expression in tumor cells induces production of proteases that contribute to tumor invasiveness. However, the specific mechanisms of AREG-mediated effects on tumor proliferation has not yet been well characterized.

In our studies, we utilized the MMTV-PyMT mouse model of breast cancer. The PyMT model has been shown to generate spontaneous mammary tumors that progress in multiple-steps, much like human breast cancer. We generated AREG -/- PyMT tumors by crossing mice with a global knockout of AREG with PyMT mice. Using these mice, we investigated the effects of AREG loss on tumor growth and progression in PyMT mice. Surprisingly, loss of AREG resulted in accelerated growth and progression. In addition, we characterized the AREG -/- tumor in terms of cell death, tumor architecture, and the tumor microenvironment. In these studies, we showed that the loss of AREG in early-stage tumors leads to protection from apoptosis and enrichment of ER-negative epithelial cells. In late-stage tumors, AREG-/- tumors are significantly less necrotic, but more cystic. The tumor growth pattern also shifts towards more papillary growth in the absence of AREG. In luminal B breast cancer, we found that papillary growth is associated with lower AREG expression. Along with changes in the tumor architecture, the loss of AREG is also associated with increased number of intratumoral vessels and macrophages.

When we examined the spatial expression of AREG in early PyMT lesions, we found little to no AREG expression in the lesions. However, in the normal ducts that surround the lesion, AREG expression was present. Therefore, we hypothesized that AREG is a negative regulator of PyMT expression. By culturing PyMT tumor cells in the presence of AREG, we found that AREG suppresses PyMT expression. In AREG-/-. ducts without the PyMT transgene, we also discovered reduced number of basal cells and a thinner basal layer. We co-cultured basal cells with PyMT tumor cells and found that basal cells can also inhibit PyMT expression. Therefore, AREG can directly and indirectly suppress PyMT expression.

Finally, we conducted a series of transplantation experiments to determine the importance of epithelial and stromal AREG in tumor growth. Strikingly, tumors with epithelial AREG, regardless of the presence or absence of AREG in the stroma, grew at a comparable level and rate. However, when AREG is absent from the tumor cells, most tumors failed to grow in the host mammary gland, regardless of the genotype of the host.

Overall, our results have provided novel insight into the role of AREG in breast cancer and regulation of PyMT expression.