Protein Kinases in Human Breast Carcinoma

Abstract : This project focuses on the biology of the Rak protein tyrosine kinase in human breast cancer. Rak is a 54 kDa protein tyrosine kinase expressed in epithelial cells. Rak resembles the proto-oncogene Src structurally, but Rak lacks an amino-terminal myristylation site and localizes to the nuclear and perinuclear regions of the cell. We report here that expression of Rak in breast cancer cell inhibits growth and causes U1 arrest of the cell cycle. This growth inhibition is kinase-dependent, but does not require the Rak S112 or SH3 domains. Rak also binds to the pRb retinoblastoma tumor suppressor protein, but Rak inhibits growth even in cells that lack pRb. These results are consistent with Rak functioning as a regulator of cell growth that is distinct from the Src-related kinase family. We have examined the expression of Rak in human breast cancer cell lines and tissues using anti-Rak monoclonal and polyclonal antibodies as well as Taqman analysis of mRNA expression. We have found that Rak is variably expressed within the breast cancer tumors with approximately 1/3 of the tumors overexpressing Rak. We have also produced an adenoviral Rak construct which has confirmed the growth inhibitory properties, suggested by our UFP-Rak constructs. The adenoviral expression of Rak has led to loss of adherence along with U1 arrest of the breast cancer cell lines.