Abstract
BackgroundUnderstanding the cause of therapeutic resistance and identifying new biomarkers in breast cancer to predict therapeutic responses will help optimise patient care. Calcium (Ca2+)-signalling is important in a variety of processes associated with tumour progression, including breast cancer cell migration and proliferation. Ca2+-signalling is also linked to the acquisition of multidrug resistance. This study aimed to assess the expression level of proteins involved in Ca2+-signalling in an in vitro model of trastuzumab-resistance and to assess the ability of identified targets to reverse resistance and/or act as potential biomarkers for prognosis or therapy outcome.MethodsExpression levels of a panel of Ca2+-pumps, channels and channel regulators were assessed using RT-qPCR in resistant and sensitive age-matched SKBR3 breast cancer cells, established through continuous culture in the absence or presence of trastuzumab. The role of Cav3.2 in the acquisition of trastuzumab-resistance was assessed through pharmacological inhibition and induced overexpression. Levels of Cav3.2 were assessed in a panel of non-malignant and malignant breast cell lines using RT-qPCR and in patient samples representing different molecular subtypes (PAM50 cohort). Patient survival was also assessed in samples stratified by Cav3.2 expression (METABRIC and KM-Plotter cohort).ResultsIncreased mRNA of Cav3.2 was a feature of both acquired and intrinsic trastuzumab-resistant SKBR3 cells. However, pharmacological inhibition of Cav3.2 did not restore trastuzumab-sensitivity nor did Cav3.2 overexpression induce the expression of markers associated with resistance, suggesting that Cav3.2 is not a driver of trastuzumab-resistance. Cav3.2 levels were significantly higher in luminal A, luminal B and HER2-enriched subtypes compared to the basal subtype. High levels of Cav3.2 were associated with poor outcome in patients with oestrogen receptor positive (ER+) breast cancers, whereas Cav3.2 levels were correlated positively with patient survival after chemotherapy in patients with HER2-positive breast cancers.ConclusionOur study identified elevated levels of Cav3.2 in trastuzumab-resistant SKBR3 cell lines. Although not a regulator of trastuzumab-resistance in HER2-positive breast cancer cells, Cav3.2 may be a potential differential biomarker for survival and treatment response in specific breast cancer subtypes. These studies add to the complex and diverse role of Ca2+-signalling in breast cancer progression and treatment.Electronic supplementary materialThe online version of this article (doi:10.1186/s12935-016-0299-0) contains supplementary material, which is available to authorized users.
Highlights
Understanding the cause of therapeutic resistance and identifying new biomarkers in breast cancer to predict therapeutic responses will help optimise patient care
Cav3.2 levels were significantly higher in luminal A, luminal B and HER2-enriched subtypes compared to the basal subtype
Whilst we found no evidence for a direct role of Cav3.2 in driving or reversing trastuzumab-resistance, our data suggest that Cav3.2 may be an informative prognosis marker in ER+ breast cancer patients for overall survival and in HER2-positive breast cancer patients with chemotherapy
Summary
Understanding the cause of therapeutic resistance and identifying new biomarkers in breast cancer to predict therapeutic responses will help optimise patient care. The monoclonal antibody trastuzumab (Herceptin®) represents a major advancement in the treatment of HER2-positive breast cancers [2, 3]. Resistance to tyrosine kinase inhibitors in lung cancer is attributed to the acquisition of an EGFR T790M mutation, and evidence suggests that a combination of irreversible tyrosine kinase inhibitors and an antiEGFR antibody is effective in overcoming this particular resistance mechanism [9]. Different mechanisms may be responsible for acquired and intrinsic trastuzumab-resistance These mechanisms include, but are not limited to, mutations in ERBB2 resulting in the expression of a modified HER2 receptor altering trastuzumab binding [10, 11] and upregulation of proteins that sterically hinder trastuzumab binding [12, 13]. Many of the aforementioned studies have been evaluated in breast cancer cell lines established from HER2-positive breast cancer cells cultured in the presence of trastuzumab, including the HER2-positive SKBR3 cell line [15, 16, 21]
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