PDF file - 128K, Two mechanistically-distinct mTOR inhibitors, rapamycin and INK-128, show differential abilities to inhibit signaling components upstream of mTORC1. SKBR3 cells were serum-starved for 40-48 h. Rapamycin (50 nM) or INK (50 nM) was added to the cells for 30 minutes prior to the additional treatment of cells with or without 1 nM HRG for 30 min. Cells were then harvested, lysed, and then lysates were analyzed by Western blotting for relative levels of phospho-mTOR (S2448), phospho-TSC2 (T1462), phopsho-AKT (T308), phospho-AKT (S473) and the corresponding total proteins.
ARTICLE ABSTRACT
Overexpression of the receptor tyrosine kinase HER2/ErbB2 (ERBB2) has been linked to a poor prognosis for patients with breast cancer; thus, its activity is a central target for cancer therapy. Likewise, overexpression of heregulin (HRG/NRG1), a growth factor responsible for ErbB2 activation, has also been shown to be a driver of breast cancer progression. Although ErbB2 inhibitors offer a major advancement in the treatment of ErbB2-dependent breast cancers, patients are highly susceptible to developing clinical resistance to these drugs. Therefore, a detailed understanding of the molecular mechanism that underlies HRG/ErbB2-induced tumorigenesis is essential for the development of effective therapeutic strategies for this subset of patients with breast cancer. Here, it was demonstrated that HRG promoted anchorage-independent breast cancer cell growth more potently than EGF, and that the HRG-dependent activation of phosphoinositide 3-kinase and mTORC1 are necessary events for cell transformation. Functional evaluation of two distinct mTOR (MTOR) inhibitors, rapamycin and INK-128, on HRG-dependent signaling activities, uncovered a necessary role for mTORC2 in the regulation of the AKT/TSC2/mTORC1 axis by affecting the phosphorylation of AKT at the PDK1(PDPK1)-dependent site (T308) as well as at the mTORC2-dependent site (S473). The elimination of Rictor (RICTOR), a critical component of mTORC2, is detrimental to both the activation of mTORC1 and HRG-mediated cellular transformation. Similar results were obtained in multiple breast cancer model systems, highlighting an important role for mTORC2 in HRG/ErbB2-dependent breast cancer.Implications: These findings suggest the potential benefits of targeting mTORC2 in HRG/ErbB2-induced breast cancer. Mol Cancer Res; 12(6); 940–52. ©2014 AACR.
