Pre-clinical studies suggest that the p38 MAPK signaling pathway plays a detrimental role in cardiac remodeling, but its role in cardiac fibroblast (CF) function is not well defined. We aimed to identify the p38 MAPK subtypes expressed by human CF, study their activation in response to proinflammatory cytokines, and determine which subtypes were important for expression of specific cytokines and matrix metalloproteinases (MMPs). Quantitative real-time RT-PCR analysis of mRNA levels in human CF cultured from multiple patients revealed a consistent pattern of expression with p38 alpha being most abundant, followed by p38 gamma, then p38 delta and only low expression of p38 beta (3% of p38 alpha mRNA levels). Immunoblotting confirmed marked protein expression of p38 alpha, gamma and beta, with little or no expression of p38 beta. Phospho-ELISA and combined immunoprecipitation/immunoblotting techniques demonstrated that the proinflammatory cytokines IL-1 alpha and TNF alpha selectively activated p38 alpha and p38 gamma, but not p38 delta. Selective p38 alpha siRNA gene silencing reduced IL-1 alpha-induced IL-6 and MMP-3 mRNA expression and protein secretion, without affecting IL-1 alpha-induced IL-I beta and MMP-9 mRNA expression. In conclusion, human CF express the alpha, gamma and delta subtypes of p38 MAPK, and the alpha subtype is important for IL-1 alpha-induced IL-6 and MMP-3 expression in this cell type. (C) 2012 Elsevier Inc. All rights reserved.