Abstract
Phosphatidylinositol 3‐kinase (PI3‐K) is a growth factor‐activated transforming lipid (and protein) kinase, involved in cell motility and invasion, that has multiple effectors. Relatively little is known about its expression and enzymatic activity in human breast cancer. Since growth factor receptors are amplified in breast cancer, and the tumor suppressor PTEN may be mutated in human breast cancer, it was hypothesized that PI3‐K and its downstream effectors would be activated in this disease. In 11 resected tumors analyzed for expression of this kinase, a mean 3‐fold increase in protein expression was observed over the corresponding adjacent control tissue. Using an in vitro lipid kinase assay of the immunoprecipitated PI3‐K protein, a greater than 2‐fold increase in activation was observed. These changes were observed in the absence of an activation of either protein kinase B (PKB, akt1) or p70 S6 kinase (p70 S6K). However, p21‐activated kinase (Pak), p38 mitogen‐activated protein kinase (p38 MAPK) and mitogen‐activated protein kinase‐activated protein kinase 2 (MAPKAPK 2) were all overexpressed and demonstrated increased enzyme activity. It may be concluded that aberrant mitogenic signaling in human breast cancer in vivo involves Pak, p38 MAPK and MAPKAPK2 downstream of PI3‐K, but neither of PKB or p70 S6K. It is proposed that this pathway may serve as a useful targeting nexus for investigation of small molecule inhibitors in human breast cancer. © 2001 Wiley‐Liss, Inc.