PIK3CA and breast cancer: To assess PI3K signaling activity in the panel of breast cancer cells used for the present investigation, the levels of phosphorylated forms of AKT, S6 protein kinase 1 and S6 (indicators of PI3K signaling activation), and the expression of PI3K catalytic subunit isoforms, PTEN, AKT isoforms and mTOR were examined (Figure 1).