Abstract 2161: Activation of the MAPK pathway in combination with PTEN loss leads to aggressive primary tumor formation

Breast cancer is the most frequent malignancy in women, and although many breast cancers are curable via surgery, approximately one quarter maintain a latent and insidious characteristic of slow growth. The loss of the tumor suppressor PTEN is associated with breast cancer stage, increased lymph node status, and disease-related death, and the high rate of loss in primary tumors suggests a potential role in initiation and/or progression of the disease. Additionally, a large fraction of breast tumors carry oncogenic mutations resulting in the hyper-activation of the MAPK/ERK cascade (20%-25% ErbB2, 5% KRAS, 2% BRAF, 1% HRAS, 1% NRAS). Hyperactivation of survival and growth pathways is considered a hallmark of many human carcinomas, including breast cancer. The overactivation of the PI3K pathway (PTEN loss) or the MAPK pathway could grant a cell the ability to circumvent inhibitory pathways. However, specific cellular alterations in human breast epithelium controlled by PTEN inactivation and/or Ras activation, which lead to early primary tumor formation, remain poorly defined. Since the current view of cancer is based on a “multi-hit” hypothesis where human cancers display a multitude of genetic and epigenetic changes, and a number of such alterations are required for tumor development, the loss of PTEN (activation of the PI3K pathway) and expression of activated K-Ras(V12) (activation of the MAPK pathway) may cooperate to promote tumorigenesis. We therefore tested the hypothesis that the activation of the MAPK pathway via activated Ras expression in a PTEN-negative background promotes tumorgenicity. Using the non-tumorigenic human mammary cells line, MCF-10A, we created MCF-10A PTEN-/- cells, MCF-10A KRas(V12) cells, and MCF-10A PTEN-/-KRas(V12) cells. We have found that each mutation, independently and collectively, greatly enhanced cellular survival and regrowth efficiency of nutrient deprived and suspended cells in vitro. Using bioluminescent mouse xenograft models, we have determined the cells with either PTEN loss or KRas(V12) expression maintain an increased persistence in vivo up to 5 weeks beyond that of the parental cells, and the combination of PTEN loss and KRas(V12) expression resulted large tumors within 4 weeks of initial injection. The combination of “one-hit” to the PI3K pathway and “one-hit” to the MAPK pathways synergized to result in aggressive tumor growth, while each individual mutation only lead to cellular persistence in vivo, a charac