Phospholipase D and extracellular signal-regulated kinase in hepatic stellate cells: effects of platelet-derived growth factor and extracellular nucleotides

Abstract

We have previously provided evidence suggesting that phosphatidic acid, possibly derived from the hydrolysis of phosphatidylcholine by phospholipase D (PLD), is involved in platelet-derived growth factor (PDGF)-mediated increases in extracellular signal-regulated kinase (ERK) activity and DNA synthesis in rat hepatic stellate cells (HSC), the primary fibrogenic cells of the liver. A recent study has shown the presence of P2Y nucleotide receptors on HSC that are coupled to contraction and synthesis of the matrix component, alpha1-procollagen, leading to the suggestion that they may represent a new therapeutic target in the treatment of liver fibrosis. However, although extracellular nucleotides have been shown to stimulate both PLD and ERK, and to elicit proliferation of fibrogenic cells outside the liver, their effect on these parameters in HSC have not yet been investigated. PLD activity was determined by [3H]choline release and [3H]phosphatidylbutanol production, ERK activity by Western blotting, and DNA synthesis by [3H]thymidine incorporation. We report here, for the first time in HSC, that extracellular nucleotides stimulate PLD activity and a sustained activation of ERK. However, in contrast to PDGF, nucleotides had negligible effects on DNA synthesis. Moreover, the effects of PDGF and nucleotides on PLD and ERK were not additive, suggesting activation of the same PLD isoform and pool of ERK. The data demonstrate that nucleotide-stimulated PLD and ERK activities are not coupled to DNA synthesis in HSC. Instead, these responses may be linked to other phenotypic changes associated with activated HSC such as increases in contraction, motility, or extracellular matrix deposition.

Purinergic Signalling: Therapeutic Developments

“…Liver fibrosis followed by cirrhosis is a common cause of liver failure. HSC are the main fibrogenic cells of the liver, which express nucleotide receptors that are functional ( Kruglov et al, 2007 ) and mediate phospholipase D activity ( Benitez-Rajal et al, 2006 ).…”

Purinergic signaling in hepatic disease

“…Similarly, purinergic receptor activation by different nucleotides (UTP being the most potent) in HSCs induced the synthesis of phosphatidic acid (PA) through activation of both phospholipase D (PLD) and the extracellular mitogenactivated kinases (ERK) [66]. However, the exact role of these signals promoting the phenotypic appearance of MFB is not well understood.…”

Cytokine Production and Signaling in Stellate Cells

“…As a result, inhibitors of ERK 1/2 efficiently inhibit cell growth and limit the further activation of molecules dependent on ERK activation, such as c-fos and AP-1 [11]. Other pathways regulated by ERK 1/2 in response to PDGF include STAT-1 and phosphatidic acid, a lipid second messenger that provides additional proliferative signals [12,13]. More recently, human antigen R, an RNA-binding protein, has been identified as a target of PDGF receptor activation and ERK signaling.…”