Inhibitors of phosphodiesterase 1 enhance paclitaxel cytotoxicity in a MDA-MB-231-derived cell line by promoting microtubule stabilization

Abstract

Resistance of tumor cells to chemotherapy remains a critical obstacle to effective cancer treatment. Although paclitaxel is one of the most commonly used chemotherapeutic agents for treating triple-negative breast cancer (TNBC), the mechanisms underlying paclitaxel resistance are not fully understood. We previously found that phosphodiesterase 1C (PDE1C) was substantially upregulated in a paclitaxel-resistant T50RN cell clone established from the human TNBC cell line MDA-MD-231. In this study, we aimed to explore whether and how PDE1C modulates resistance to paclitaxel in T50RN cells. Our results showed that depletion of PDE1C enhanced paclitaxel cytotoxicity, and that pharmacological inhibition of PDE1 potentiated paclitaxel-induced antiproliferative and antimitotic effects in T50RN cells. Additionally, intracellular cyclic adenosine monophosphate (cAMP) levels were lower in T50RN cells than in parental MDA-MB-231 cells. PDE1 inhibition restored the cAMP level, suggesting that cAMP-degrading activity of PDE1 is elevated in the T50RN cells. Similar to PDE1 inhibitors, the cell permeable cAMP analog 8‑bromo-cAMP or the adenylate cyclase activator forskolin increased cAMP levels and concurrently augmented paclitaxel-induced cytotoxicity and spindle abnormalities in T50RN cells. Furthermore, PDE1 inhibitors, forskolin, and an agonist of the cAMP downstream effector EPAC enhanced paclitaxel-mediated microtubule (MT) stabilization. Thus, PDE1 inhibition may act through cAMP/EPAC signaling to facilitate MT stabilization and potentiate the antiproliferative and antimitotic effects of paclitaxel in T50RN cells. Upon PDE1 inhibition, paclitaxel-treated T50RN cells exhibited signs of endoplasmic reticulum (ER) stress and apoptosis. Together, our in vitro findings indicate that PDE1C overexpression contributes to paclitaxel resistance.