Objective: The objective of this study was to investigate the role of p38-C/EBP beta signaling in leiomyoma cells and myometrial cells challenged with Activin A, and to identify specifically the isoform of p38 MAPK that mediates the effects of Activin A. Methods: The immortalization human leiomyoma cells (HuLM) and human myometrial cells (HM), and mouse myometrial tissues were treated with Activin A (4 nM) in response to p38 alpha/beta inhibition (10 mu M SB202190) or depletion (p38 alpha/beta-targeting siRNA or p38 beta muscle specific-knock out mice). p38 MAPK signaling molecules (p-p38 and p-C/EBP beta) and ECM components (COL1A1 and/or FN) were analyzed by Western blotting. Results: Activin A induced ECM accumulation in leiomyoma cells and myofibroblastic transformation in myometrical cells specifically by p38 beta MAPK. Conclusion: This study is the first to demonstrate that activation of C/EBP beta by p38 beta MAPK may contribute to tumorigenesis and progression of Activin A-induced leiomyoma. Specific p38 beta inhibition may represent a novel and promising intervention for leiomyoma. (C) 2020 Elsevier Inc. All rights reserved.