Activation of STAT3/HIF-1α modulates trastuzumab resistance development in SKBR3 breast cancer cells

Resistance to the human epidermal growth factor receptor (HER2)–targeted antibody trastuzumab is a major clinical concern in the treatment of HER2-positive metastatic breast cancer. In that line, Aberrant activation of signal transducer and activator of transcription protein 3 (SATA3) is thought to potentially promote drug resistance in breast cancer. However, the precise mechanism(s) remains poorly defined. Evidence is emerging that HIF-1α, a central downstream element of STAT3 pathway, serves a pivotal role in the complex signaling network with subsequent diverse cellular events. Here in, we have established trastuzumab resistant SKBR3 cells (SKBR3-TR). MTT assay and flow-cytometry analysis were performed to determine cell viability and apoptosis. Moreover, western blot and siRNA transfection assays were performed to evaluate the involvement of STAT3/HIF-1α in modulation of trastuzumab resistance. Our results revealed that constitutive phosphorylated STAT3 was coincided with prominent up-regulation of hypoxia-inducible factor 1 alpha (HIF-1α), a STAT3 target gene, in trastuzumab resistant cells. Moreover, inhibition of STAT3 activation by 2.5 μM STAT3 inhibitor V decreased HIF-1α expression as well as restored sensitivity to trastuzumab in SKBR3-TR cells. Conversely, Reactivation of STAT3 resulted in expression and stability of HIF-1α with consequent induction of trastuzumab resistance phenotype in parental sensitive cells. Besides, further investigation demonstrated that knock down of HIF-1α by specific si-RNA partially reversed the growth inhibitory effects of trastuzumab in SKBR3-TR cells. These findings highlighted the importance of STAT3/HIF-1α axis in trastuzumab resistance induction which would be valuable in designing more efficient chemosensitization strategies for HER2-positive breast cancer patients.