Cyclin-dependent kinase (CDK) inhibitor p27

The molecular mechanisms by which the anti-HER2 antibodies trastuzumab and its murine equivalent 4D5 inhibit tumor growth and potentiate chemotherapy are not fully understood. Inhibition of signaling through the phosphatidylinositol 3-kinase (PI3K)-AKT pathway may be particularly important. Treatment of breast cancer cells that overexpress HER2 with trastuzumab inhibited HER2-HER3 association, decreased PDK1 activity, reduced Thr-308 and Ser-473 phosphorylation of AKT, and reduced AKT enzymatic activity. To place the role of PI3K-AKT in perspective, gene expression was studied by using Affymetrix microarrays and real time reverse transcription-PCR. Sixteen genes were consistently down-regulated 2.0 -4.9-fold in two antibody-treated breast cancer cell lines. Fourteen of the 16 genes were involved in three major functional areas as follows: 7 in cell cycle regulation, particularly of the G 2 -M; 5 in DNA repair/replication; and 2 in modifying chromatin structure. Of the 16 antibody-regulated genes, 64% had roles in cell growth/maintenance and 52% contributed to the cell cycle. Direct inhibition of PI3K with an inhibitor markedly reduced expression of 14 genes that were also affected by the antibody. Constitutive activation of AKT1 blocked the effect of the anti-HER2 antibody on cell cycle arrest and on eight differentially expressed genes. The antibody enhanced docetaxel-induced growth inhibition but did not increase the fraction of apoptotic cells induced with docetaxel alone. In contrast, the antibody plus docetaxel markedly down-regulated two genes, HEC and DEEPEST, required for passage through G 2 -M. Thus, anti-HER2 antibody preferentially affects genes contributing to cell cycle progression and cell growth/maintenance, in part through the PI3K-AKT signaling. Transcriptional regulation by anti-HER2 antibody through PI3K-AKT pathway may potentiate the growth inhibitory activity of docetaxel by affecting cell cycle progression.The human epidermal growth factor receptor 2 (HER2, 1 also known as c-Neu or ErbB-2) encodes a 185-kDa transmembrane tyrosine kinase growth factor receptor. The ligand that binds to the homodimers of HER2 has not yet been identified. Rather, HER2 functions as a preferred co-receptor to form heterodimers with HER1 (epidermal growth factor receptor), HER3, or HER4. Of these heterodimers, HER2-HER3 is particularly important for intracellular signaling (1). HER2 signaling has been linked to a variety of cellular responses to growth factors under both normal and pathophysiological conditions. HER2 signaling is required not only during normal development of the mammary gland but also during development of the glia, neurons, and heart (1, 2). Amplification of the HER2 gene and overexpression of HER2 protein have been documented in ϳ30% of breast and 15% of ovarian cancers (3). In many (but not all) reports, HER2 overexpression has been associated with a more aggressive course of disease. Although the underlying mechanisms for this association are still not well characterized, HER2 overexpression...