Abstract
PDZ domain containing 1 (PDZK1) is a scaffold protein that plays a role in the fate of several proteins. Estrogen can induce PDZK1 gene expression; however, our recent report showed that PDZK1 expression in the breast cancer cell line MCF-7 is indirect and involves insulin-like growth factor (IGF)-1 receptor function. Such a relationship was established in cell culture systems and human breast cancer tissues. Here we show that overexpression of PDZK1 promoted an increase in cyclin D1 and enhanced anchorage-independent growth of MCF-7 cells in the absence of 17β-estradiol, suggesting that PDZK1 harbors oncogenic activity. Indeed, PDKZ1 overexpression enhanced epidermal growth factor receptor (EGFR)-stimulated MEK/ERK1/2 signaling and IGF-induced Akt phosphorylation. PDZK1 appeared to play this role, in part, by stabilizing the integrity of the growth promoting factors Akt, human epidermal growth factor receptor 2 (Her2/Neu) and EGFR. Increased Akt levels occurred via a decrease in the ubiquitination of the kinase. PDZK1 overexpression was associated with resistance to paclitaxel/5-fluorouracil/etoposide only at low concentrations. Although the increased stability of Akt was sensitive to heat shock protein 90 (HSP90) inhibition, increased levels of the cochaperone cell division cycle 37 (Cdc37), as well as its ability to bind PDZK1, appear to play a larger role in kinase stability. Using human tissue microarrays, we show strong positive correlation between PDZK1, Akt and Cdc37 protein levels, and all correlated with human breast malignancy. There were no positive correlations between PDZK1 and Cdc37 at the mRNA levels, confirming our in vitro studies. These results demonstrate a relationship between PDZK1, Akt and Cdc37, and potentially Her2/Neu and EGFR, in breast cancer, representing a new axis that can be targeted therapeutically to reduce the burden of human breast cancer.