Although inactivation of the PTEN gene has been implicated in the development of resistance to the HER2 targeting antibody trastuzumab, the mechanisms mediating this resistance remain elusive. We generated trastuzumab resistant cells by knocking down PTEN expression in HER2 overexpressing breast cancer cell lines and demonstrate that development of trastuzumab resistance in these cells is mediated by activation of an IL-6 inflammatory feedback loop leading to expansion of the cancer stem cell (CSC) population. Long term trastuzumab treatment generates highly enriched CSCs which display an EMT phenotype secreting over 100-fold more IL-6 than parental cells. An IL-6 receptor antibody interrupted this inflammatory feedback loop reducing the cancer stem cell population resulting in decreased tumor growth and metastasis in mouse xenographs. These studies demonstrate that trastuzumab resistance may be mediated by an IL-6 inflammatory loop and suggest that blocking this loop may provide alternative strategy to overcome trastuzumab resistance.

Although current breast cancer treatment guidelines limit the use of HER2 blocking agents to tumors with HER2 gene amplification, recent retrospective analyses suggest that a wider group of patients may benefit from this therapy. Utilizing breast cancer cell lines, mouse xenograft models and matched human primary and metastatic tissues, we demonstrate that HER2 is selectively expressed in and regulates self-renewal of the cancer stem cell population in ER+, HER2− luminal breast cancers. Although trastuzumab had no effects on the growth of established luminal breast cancer mouse xenografts, administration after tumor inoculation blocked subsequent tumor growth. HER2 expression is increased in luminal tumors grown in mouse bone xenografts, as well as in bone metastases from breast cancer patients compared to matched primary tumors. Furthermore this increase in HER2 protein expression was not due to gene amplification but rather was mediated by RANK-ligand in the bone microenvironment. These studies suggest that the clinical efficacy of adjuvant trastuzumab may relate to the ability of this agent to target the cancer stem cell population in a process that does not require HER2 gene amplification. Furthermore these studies support a cancer stem cell model in which maximal clinical benefit is achieved when cancer stem cell targeting agents are administered in the adjuvant setting.

BackgroundOccasionally, breast cancer relapses more than 5 years after initial treatment, sometimes with highly aggressive disease in such late-recurring patients. This study investigated predictors of recurrence after more than 5 years in operable breast cancer.MethodsWe retrospectively analyzed data from patients with recurrent breast cancer treated at Siriraj Hospital. Patients were divided into those whose relapse times were longer or shorter than 5 years. Factors that predicted late recurrence were analyzed in both the overall population and the luminal subgroup. Patterns of relapse, changes in biomarkers, and time to disease progression after first relapse were also recorded.ResultsWe included 300 women whose breast cancers recurred between 2005 and 2013, of whom 180 had recurrence within 5 years of diagnosis and 120 later than 5 years (median time to recurrence: 45.43 months; range: 4.4–250.3 months). Tumors larger than 2 cm, lymph node metastasis, and high nuclear grade were related with early recurrence. Estrogen receptor-positive, progesterone receptor-positive, and HER2− disease predicted late recurrence. Almost all late-relapsing patients with luminal tumors had high estrogen receptor (ER+) titers (≥50 %) and HER2− disease. Liver and brain were the most common early recurrence sites. Biomarkers did not significantly change by time of recurrence.ConclusionsER+/PR+ and HER2− patients have higher risk of recurrence later than 5 years, especially in patients with high ER titer and low nuclear grade. Larger and node-positive tumors had higher risk of early recurrence.