The voltage gated Ca2+-channel Cav3.2 and therapeutic responses in breast cancer

Background: Understanding 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. Methods: Expression 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 Ca(v)3.2 in the acquisition of trastuzumab-resistance was assessed through pharmacological inhibition and induced overexpression. Levels of Ca(v)3.2 were assessed in a panel of nonmalignant 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 Ca(v)3.2 expression (METABRIC and KM-Plotter cohort). Results: Increased mRNA of Ca(v)3.2 was a feature of both acquired and intrinsic trastuzumab-resistant SKBR3 cells. However, pharmacological inhibition of Ca(v)3.2 did not restore trastuzumab-sensitivity nor did Ca(v)3.2 overexpression induce the expression of markers associated with resistance, suggesting that Ca(v)3.2 is not a driver of trastuzumab-resistance. Ca(v)3.2 levels were significantly higher in luminal A, luminal B and HER2-enriched subtypes compared to the basal subtype. High levels of Ca(v)3.2 were associated with poor outcome in patients with oestrogen receptor positive (ER+) breast cancers, whereas Ca(v)3.2 levels were correlated positively with patient survival after chemotherapy in patients with HER2-positive breast cancers. Conclusion: Our study identified elevated levels of Ca(v)3.2 in trastuzumab-resistant SKBR3 cell lines. Although not a regulator of trastuzumab-resistance in HER2-positive breast cancer cells, Ca(v)3.2 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.