Activation of STAT3/HIF-1α/Hes-1 axis promotes trastuzumab resistance in HER2-overexpressing breast cancer cells via down-regulation of PTEN

Background: Resistance to the HER2-targeted antibody trastuzumab remains to be a major clinical challenge in the treatment of HER2-positive breast cancer. Hyper-activation of STAT3 is proposed to be a predictive biomarker of trastuzumab resistance. However, the precise mechanism(s) remains poorly defined. Evidence is emerging that HIF-1 alpha, a central downstream element of STAT3 pathway, serves a pivotal role in the complex signaling network with subsequent diverse cellular events. Material and methods: We have established trastuzumab resistant SKBR3 cells (SKBR3-TR). The cell viability, apoptosis as well as western blot, siRNA transfection and co-immunoprecipitation assays were performed to evaluate the involvement of STAT3/HIF-1 alpha in modulation of trastuzumab resistance. Results: We found that in SKBR3-TR cells and conditioned medium-treated parental cells, constitutive phosphorylated STAT3 coincided with prominent up-regulation of HIF-1 alpha which was accompanied with PTEN attenuation. Moreover, the inhibition of STAT3 activation by Stattic and/or genetically STAT3 knocking down decreased HIF-1 alpha level in SKBR3-TR cells. Additionally, treatment with Stattic and/or STAT3 siRNA engendered the up-regulation of PTEN protein in STAT3-inhibited resistant cells. Restoration of PTEN was also observed following siRNA-mediated silencing of HIF-la expression. Moreover, down-regulation of HIF-1 alpha caused a reduction in the HES-1 content. Further study with HES-1 specific siRNA revealed the elevation of PTEN expression in HES-1 knock-down trastuzumab resistant cells. Conclusion: The impairment of STAT3-HIF-1 alpha-HES-1 pathway restored trastuzumab sensitivity through up regulation of PTEN protein. General significance: These findings highlighted the signal integrator role of HIF-1 alpha in STAT3-mediated trastuzumab resistance induction which would be valuable in designing more efficient chemosensitization strategies.