PC cell derived growth factor (PCDGF/granulin precursor) expression, antiestrogen resistance and tumorigenesis in human breast cancer cells

Abstract

Breast cancer is one of the most common malignant diseases in women. Several mechanisms have been proposed for the development and progression of breast cancer. Overexpression of growth factors or their receptors has been widely investigated as a potential pathway of breast malignancy. PCDGF is a novel growth factor characterized in our laboratory. There is significant evidence to suggest a role of PCDGF in human cancers. The purpose of this study was to investigate the role of PCDGF on tumorigenicity, estrogen dependence, endocrine therapy resistance and metastatic potential in human breast cancer. A model system to study the role of PCDGF and estrogen independence was developed by overexpressing PCDGF in human breast cancer MCF-7 cells and cultivating them in estrogen depleted condition. Results presented here show that PCDGF overexpression confers resistance to tamoxifen and fulvestrant in both in vitro and in vivo. PCDGF overexpression and estrogen depletion downregulate estrogen receptor alpha isoform, resulting in estrogen unresponsive cell growth. In addition, doxorubicin resistance was observed in PCDGF overexpressing cells. We found that PCDGF prevents apoptosis induced by tamoxifen, fulvestrant and doxorubicin. The key event in this process is that PCDGF maintains the upregulation of bcl-2 when treated with these agents. In addition, tamoxifen resistant cells express higher level of PCDGF. PCDGF transcriptionally activates estrogen inducible genes such as progesterone receptor and vascular endothelial growth factor (VEGF) via an ERa-dependent pathway. Tumors originated from PCDGF overexpressing cells express higher level of VEGF and angiopoietin-1. In addition, treatment with tamoxifen, in cooperation with PCDGF, stimulates VEGF expression in vitro and in vivo. This may explain why tamoxifen stimulates tumor growth from the PCDGF overexpressing cells but inhibits it in the wild-type cells. PCDGF induces cell migration through matrigel and stimulates matrix metalloprotease-9 secretion, suggesting an important role in metastasis. In summary, these studies provide a possible mechanism of antiestrogens and doxorubicin resistance in human breast cancer by overexpression of PCDGF. The results also show that PCDGF expression correlates with higher tumorigenicity and promotes angiogenesis and metastasis in human breast cancer.