Pharmacological Inhibition of PP2A Overcomes Nab-Paclitaxel Resistance by Downregulating MCL1 in Esophageal Squamous Cell Carcinoma (ESCC)

SIMPLE SUMMARY: Paclitaxel-based chemotherapy has been introduced to treat esophageal squamous cell carcinoma. While its initial efficacy has been clinically established, the development of resistance is inevitable. To understand the paclitaxel resistance mechanism, we developed nanoparticle albumin-bound paclitaxel (nab-PTX)-resistant esophageal squamous cell lines from their sensitive counterparts. We found that resistant cells express higher levels of protein phosphatase 2A (PP2A), oxidative phosphorylation, and anti-apoptotic protein MCL1 than their counterparts. Therapeutically, the PP2A inhibitor LB-100 synergistically sensitized resistant esophageal squamous cells to nab-PTX both in vitro and in vivo. Therefore, our data suggest that LB-100 may potentially overcome nab-PTX resistance in a clinical setting. ABSTRACT: Paclitaxel-based chemotherapy is a treatment option for advanced esophageal squamous cell carcinoma (ESCC). However, the development of chemoresistance leads to treatment failure, and the underlying mechanism remains elusive. We investigated the mechanisms of nanoparticle albumin-bound paclitaxel (nab-PTX) resistance by establishing three nab-PTX resistant ESCC cell lines. Proteomics analysis revealed higher oxidative phosphorylation (OXPHOS) in resistant cell line DR150 than in its parental cell line KYSE150, which is likely caused by stabilized anti-apoptotic protein MCL1. Additionally, we discovered the elevated activity of protein phosphatase 2A (PP2A), the phosphatase that dephosphorylates and stabilizes MCL1, in nab-PTX resistant cell lines. Pharmacological inhibition of PP2A with small molecule compound LB-100 decreased MCL1 protein level, caused more apoptosis in nab-PTX resistant ESCC cell lines than in the parental cells in vitro, and significantly inhibited the tumor growth of nab-PTX resistant xenografts in vivo. Moreover, LB-100 pretreatment partially restored nab-PTX sensitivity in the resistant cell lines and synergistically inhibited the tumor growth of nab-PTX resistant xenografts with nab-PTX. In summary, our study identifies a novel mechanism whereby elevated PP2A activity stabilizes MCL1 protein, increases OXPHOS, and confers nab-PTX resistance, suggesting that targeting PP2A is a potential strategy for reversing nab-PTX resistance in patients with advanced ESCC.