Loss of PTEN engages ErbB3 and IGF-I receptor signaling to promote antiestrogen resistance in breast cancer.

Abstract

Abstract Abstract #73 We sought to determine whether loss of the lipid phosphatase PTEN confers resistance to antiestrogens in estrogen receptor (ER)-positive breast cancer cells and primary tumors. Stable knockdown of PTEN expression (PTEN-kd) with shRNA in MCF-7, T47D, and MDA-361 ER+ human breast cancer cells resulted in upregulation of PI3K and P-Akt, resistance to tamoxifen and faslodex, and estrogen-independent growth. Upon PTEN-kd, ER levels were maintained in MCF-7 cells but markedly reduced in T47D and MDA-361 cells. While PTEN-kd upregulated ER transcriptional reporter activity in MCF-7 cells, opposite effects were seen in T47D and MDA-361 cells.
 We evaluated mechanisms of PI3K activation in PTEN-kd cells by immunoprecipitating the p85 regulatory subunit of PI3K and examining p85-bound tyrosine-phosphorylated adaptors or receptors. PTEN-kd increased the binding of p85 to IRS-1 (MCF-7) and ErbB3 (T47D). PTEN-kd increased PI3K pathway sensitivity to IGF-I (MCF-7) and endogenous ligand (T47D, MDA-361). PTEN-kd cells had increased and prolonged activation of IGF-IR and ErbB3, thus implicating PTEN in the regulation of signaling upstream of PI3K. Further, PTEN-kd increased non-genomic, estrogen-induced signaling via IGF-IR by increasing p85-IRS-1 binding and activation of PI3K/Akt in MCF-7 cells. Inhibition of PI3K and mTOR with the small molecule BEZ235, of IGF-IR with the small molecule AEW541, and/or of ErbB2-mediated activation of ErbB3 with lapatinib restored the growth inhibitory effect of antiestrogens.
 We generated gene expression signatures of PTEN loss in each cell line by microarray analyses. Comparison of these signatures to the Connectivity Map (Science 313:1929, 2006) suggested activation of PI3K, as signatures of PTEN loss negatively connected with those induced by the PI3K inhibitors wortmannin and LY294002 (all p<0.06). We found a 24-gene signature of PTEN loss common to all 3 lines, which we used to score the gene expression profiles of tumors from a cohort of 268 patients with ER+ breast cancer treated with adjuvant tamoxifen for 5 yrs and a median follow-up of 9.1 years (BMC Genomics 9:239, 2008). Patients with tumors exhibiting a signature of PTEN loss had shorter relapse-free survival (p<0.0001; log rank test). Eleven genes in the PTEN signature were individually predictive of disease outcome (p<0.05). In a separate, smaller cohort of patients with ER+ cancers treated with adjuvant tamoxifen (n=34), undetectable PTEN in tumor cells (measured by IHC) correlated with shorter relapse-free survival vs. tumors with detectable PTEN (p=0.06). These data suggest that 1) PTEN loss confers antiestrogen resistance to ER+ breast cancer by genomic and non-genomic mechanisms; 2) PTEN loss is permissive for activation of IGF-IR and ErbB3 signaling; 3) inhibition of IGF-IR and/or ErbB signaling pathways overcomes the resistance to antiestrogens conferred by PTEN loss; and 4) a gene expression signature reflective of loss of PTEN and/or absence of PTEN protein can predict poor patient outcome after adjuvant hormonal therapy. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 73.