EGF Regulation of HRPAP20: A Role for Calmodulin and Protein Kinase C in Breast Cancer Cells

The breast cancer cell genome is remarkably unstable, most likely due to early dysfunction of DNA replication, repair or recombination (Roskelley and Bissel, 2002). Accumulation of genetic alterations in the cells and/or stroma lead to development of a genetically diverse cell population characterized by uncontrolled cell proliferation (Witz, 2002; Gupta et al., 2006). Therefore, identification of genes that may be responsible for pre-disposition or facilitate progression of the disease may contribute to improvement of currently available therapeutic approaches in treatment of breast cancer. We previously reported the identification, cloning and functional characterization of HRPAP20, which encodes for protein designated Hormone Regulated ProliferationAssociated Protein 20 (accession number: NM_ 014165; Karp et al., 2004). Our observations indicated that HRPAP20 is a regulator of proliferation, survival, and invasion in hormoneresponsive breast cancer cells. Moreover, highly invasive, hormone unresponsive breast cell lines, such as MDA-MB-231 and tumor specimens of invasive breast adenocarcinomas from patients exhibited constitutively elevated levels of HRPAP20 (Karp et al., 2007). Results from an independent study conducted by another group suggested that HRPAP20 is a promising marker of tamoxifen resistance in women with ER alpha-positive breast tumors (Tozlu-Kara et al., 2007). Together, these observations suggested that elevated HRPAP20 may facilitate breast cancer progression toward a more malignant phenotype. Other studies from our group suggested that an interaction between HRPAP20 and calmodulin (CaM) may contribute to HRPAP20-mediated biological effects in tumor cells. Furthermore, a basic amino acid residue (K73) within the putative CaM-binding domain of HRPAP20 appeared to be important for CaM-binding to the protein. CaM has been shown to influence cell cycle control and proliferation in human breast cancer cells by activating CaM-kinase II (CaMK II) and MAPK-mediated signaling pathways (Cheung , 1980; Wang et al., 1983; Rodriguez-Mora et al., 2005). The recent identification of the ErbB2/HER2/Neu, ER-┙, and androgen receptor (AR) as CaM-binding proteins, has opened new areas of investigation in the regulation of signaling by CaM, particularly in hormone-responsive cancer (Cifuentes et al., 2004; Li et al., 2006; Maximciuc et al., 2006). Overexpression or constitutive activation of the epidermal growth factor receptor (EGFR) is frequently associated with the development and progression of a number of human cancers,