HER2/ErbB2 activates HSF1 and thereby controls HSP90 clients including MIF in HER2-overexpressing breast cancer

Abstract

Overexpression of the human epidermal growth factor receptor-2 (HER2) in breast cancer strongly correlates with aggressive tumors and poor prognosis. Recently, a positive correlation between HER2 and MIF (macrophage migration inhibitory factor, a tumor-promoting protein and heat-shock protein 90 (HSP90) client) protein levels was shown in cancer cells. However, the underlying mechanistic link remained unknown. Here we show that overexpressed HER2 constitutively activates heat-shock factor 1 (HSF1), the master transcriptional regulator of the inducible proteotoxic stress response of heat-shock chaperones, including HSP90, and a crucial factor in initiation and maintenance of the malignant state. Inhibiting HER2 pharmacologically by Lapatinib (a dual HER2/epidermal growth factor receptor inhibitor) or CP724.714 (a specific HER2 inhibitor), or by knockdown via siRNA leads to inhibition of phosphoactivated Ser326 HSF1, and subsequently blocks the activity of the HSP90 chaperone machinery in HER2-overexpressing breast cancer lines. Consequently, HSP90 clients, including MIF, AKT, mutant p53 and HSF1 itself, become destabilized, which in turn inhibits tumor proliferation. Mechanistically, HER2 signals via the phosphoinositide-3-kinase (PI3K)–AKT– mammalian target of rapamycin (mTOR) axis to induce activated pSer326 HSF1. Heat-shock stress experiments confirm this functional link between HER2 and HSF1, as HER2 (and PI3K) inhibition attenuate the HSF1-mediated heat-shock response. Importantly, we confirmed this axis in vivo. In the mouse model of HER2-driven breast cancer, ErbB2 inhibition by Lapatinib strongly suppresses tumor progression, and this is associated with inactivation of the HSF1 pathway. Moreover, ErbB2-overexpressing cancer cells derived from a primary mouse ErbB2 tumor also show HSF1 inactivation and HSP90 client destabilization in response to ErbB2 inhibition. Furthermore, in HER2-positive human breast cancers HER2 levels strongly correlate with pSer326 HSF1 activity. Our results show for the first time that HER2/ErbB2 overexpression controls HSF1 activity, with subsequent stabilization of numerous tumor-promoting HSP90 clients such as MIF, AKT and HSF1 itself, thereby causing a robust promotion in tumor growth in HER2-positive breast cancer.

Heat Shock Factor 1 in Protein Homeostasis and Oncogenic Signal Integration

“…Hsf1 þ/À Neu þ mice also showed reduced association of the Hsp90 client protein c-RAF with HSP90 and reduced levels of AKT-ERK1/2 signaling (16). Similar findings with respect to stabilization of Hsp90 client proteins by Her2/HSF1 axis came from a study where inhibition of Her2 signaling resulted in the inhibition of HSF1 activity (phosphorylation) and the destabilization of HSP90 clients including MIF (macrophage migration inhibitory factor) and AKT (17). Recent studies have demonstrated that overexpression of mutant p53 also activates EGFR/Her2, leading to enhanced PI3K and MAPK signaling, which results in the phosphorylation and activation of HSF1 (32).…”

“…Experimental evidence suggests that loss of HSF1 results in the decreased stability of HSP90 client proteins (mostly kinases) and inhibits their downstream signaling (described in the more detail in the following sections; refs. 16,17).…”

ErbB2 Receptor in Breast Cancer: Implications in Cancer Cell Migration, Invasion and Resistance to Targeted Therapy

“…Downstream signaling pathways are activated upon ErbB2 receptor activation through either heterodimerization with ligand bound EGFR, ErbB3, or ErbB4 family receptors, or in presence of overexpression of ErbB2 due to gene ampliication, by ligand independent homodimerization [12]. The homo/heterodimerization promotes the receptor activation that in turn leads to tyrosine phosphorylation of the C-terminal residues.…”

Roles of HSF1 and Heat Shock Proteins in Cancer

“…Moreover, it was recently demonstrated in HER2-overexpressing breast cancer cells that HSF1 is constitutively phosphorylated and activated by the HER2-PI3K-AKT-mTOR signaling pathway and controls Hsp90 client proteins, including MIF (macrophage migration inhibitory factor, a tumor promoting protein) [25].…”