Adenosine A2AReceptor Occupancy Stimulates Collagen Expression by Hepatic Stellate Cells via Pathways Involving Protein Kinase A, Src, and Extracellular Signal-Regulated Kinases 1/2 Signaling Cascade or p38 Mitogen-Activated Protein Kinase Signaling Pathway

Prior studies indicate that adenosine and the adenosine A_2A receptor play a role in hepatic fibrosis by a mechanism that has been proposed to involve direct stimulation of hepatic stellate cells (HSCs). The objective of this study was to determine whether primary hepatic stellate cells produce collagen in response to adenosine (via activation of adenosine A_2A receptors) and to further determine the signaling mechanisms involved in adenosine A_2A receptor-mediated promotion of collagen production. Cultured primary HSCs increase their collagen production after stimulation of the adenosine A_2A receptor in a dose-dependent fashion. Likewise, LX-2 cells, a human HSC line, increases expression of procollagen αI and procollagen αIII mRNA and their translational proteins, collagen type I and type III, in response to pharmacological stimulation of adenosine A_2A receptors. Based on the use of pharmacological inhibitors of signal transduction, adenosine A_2A receptor-mediated stimulation of procollagen αI mRNA and collagen type I collagen expression were regulated by signal transduction involving protein kinase A, src, and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (erk), but surprisingly, adenosine A_2A receptor-mediated stimulation of procollagen αIII mRNA and collagen type III protein expression depend on the activation of p38 mitogen-activated protein kinase (MAPK), findings confirmed by small interfering RNA-mediated knockdown of src, erk1, erk2, and p38 MAPK. These results indicate that adenosine A_2A receptors signal for increased collagen production by multiple signaling pathways. These results provide strong evidence in support of the hypothesis that adenosine receptors promote hepatic fibrosis, at least in part, via direct stimulation of collagen expression and that signaling for collagen production proceeds via multiple pathways.