Loss of PTEN engages ErbB3 and IGF-I receptor signaling to promote antiestrogen resistance in breast cancer.
Authors: Miller TW , Perez-Torres M , Wu H , Shyr Y , Guix M , Jiang A , Stal O , Arteaga CL 
Published: December 2010

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.&#x2028; 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.&#x2028; 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&lt;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&lt;0.0001; log rank test). Eleven genes in the PTEN signature were individually predictive of disease outcome (p&lt;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.