Nitrogen containing bisphosphonates, e.g. Zoledronic acid (ZOL), are currently used in oncological practice to reduce skeletal complications with bone metastasis of several neoplasms. Bisphosphonates (BP) inhibit bone resorption by inducing apoptosis in osteoclasts (OC). Several studies and meta-analyses report no overall beneficial effect of prescribing ZOL for specific breast and prostate cancer treatments owing to treatment failures and development of resistance towards ZOL. The bone-tumour microenvironment is a complex niche due to the presence of numerous bone resorption agonists (RAs), secreted by tumour cells. The effects of these RAs on osteoclast differentiation/function in the presence of BP are unknown. Our overall objective is to investigate the mechanisms of resistance developed by osteoclasts to overcome the therapeutic effects of ZOL in tumour microenvironments simulated using an in vitro model. OC were prepared using human peripheral blood derived mononuclear cells and the effect of ZOL and conditioned media from oral squamous cell carcinoma cell line OSCC 12 (OSCC-CM) on OC differentiation and function were examined. Quantification of TRAcP positive OC revealed a 33% reduction in OC number by ZOL (10 nM) when treated alone for 10 days, but ZOL efficacy is reduced to ½ when cultured along with OSCC-CM (20%). qPCR analyses done following 48 h treatment with ZOL +/-OSCC-CM revealed that non-receptor type protein tyrosine phosphatase (PTP), PTPN13/cyt-PTPe, expression levels were reduced by 6-fold in with ZOL treatment. In contrast, there was a 3-fold increase of PTPN13 levels with ZOL+OSCC CM treatment when compared with ZOL alone. In vitro inhibition of PTPs using Orthovanadate (10 μM) for 24 h made PTPN13 susceptible to ZOL-induced effects. Based on these results, we propose that sustained PTP elevation in OC is responsible for reduced efficacy of ZOL. PTP activation is known to up regulate anti-apoptotic, pro-survival pathways. Thus simultaneous inhibition of PTPs in osteoclasts would help overcome the development of ZOL resistance. Small molecule PTP inhibitors should be evaluated to be developed as adjuncts to ZOL therapies and would be predicted to improve the outcomes of BP treatment in tumour microenvironments.
Disclosure: The authors declared no competing interests. This work was funded by: NIH/NCRR P20 RR 020145.