The Markedly Increased PAI1 in Obesity Induces a Compensatory Increase of Hepatocyte Tpa Expression By Activating a LRP1-CREB1 Pathway

Tissue-type plasminogen activator (tPA) initiates fibrinolysis, the primary mechanism that dissolves a thrombus. We have shown that in lean mice the hepatocytes maintain a basal level of circulating tPA that influences fibrinolysis in the occurrence of a vessel injury. As a serine protease, tPA is inhibited by the serpin plasminogen activator inhibitor 1 (PAI1). Both PAI1 and tPA which can be produced by hepatocytes, which is important for regulating energy metabolism and is sensitive to metabolic stress. In obesity, despite an increase in plasma tPA, blood tPA activity and fibrinolysis are reduced primarily due to a larger increase in PAI1. However, the source and regulatory mechanism of this phenomena are unknown.

Palmitate treatment of human primary hepatocytes causes an increase in tPA mRNA/protein, reflecting the situation in the obese liver. This increase, however, is overcompensated by a larger increase in PAI1 mRNA, resulting in decreased net tPA activity in the cell culture medium. Silencing PAI1 mRNA with si-SERPINE1 in these cells increased tPA activity in the culture medium. Surprisingly, silencing PAI1 also prevented the increase of tPA mRNA induced by palmitate. Building on this observation, we further treated the cells with active recombinant PAI1 (rPAI1), which induced tPA expression. Next, we fed PAI1^fl/flmice with a diet-induced obesity (DIO) diet for three months and silenced their hepatic PAI1 using AAV8-TBG-Cre, which inactivates ~95% floxed gene expression in hepatocytes but not in other cell types of the liver or other tissues. We found that while plasma tPA activity was increased in hepatocyte-PAI1 knockout mice, plasma tPA protein concentration was reduced. In other words, reducing hepatocyte PAI1 in obesity stopped the compensatory increase in liver tPA mRNA and protein. Thus, PAI1 induces tPA, which is likely a compensatory response.

To explore the mechanism whereby rPAI1 induces tPA, we considered CREB1, as it is a transcription activator of tPA expression in endothelial cells and is expressed in hepatocytes. We found that CREB1 was activated (phosphorylated) by rPAI1 in human primary hepatocytes and that silencing CREB1 prevented the elevated tPA expression induced by rPAI1. Next, we silenced hepatic CREB1 in obese mice by treating DIO diet-fed CREB1^fl/flmice with AAV8-TBG-Cre, which blocked the compensatory increase in tPA and thus worsened the impaired fibrinolysis in obesity.

LDL receptor-related protein 1 (LRP1), the major cellular receptor of PAI1, can stimulate CREB1 transcription activity in neurons and adipocytes. As LRP1 is expressed on the surface of hepatocytes, we hypothesized that PAI1 activates CREB1 through LRP1-mediated signaling to increase tPA expression in obesity. Indeed, silencing LRP1 with si-LRP1 stopped the increase of tPA expression induced by rPAI1 in human primary hepatocytes. Interestingly, treating the cells with a mutant rPAI1 lacking the LRP1-interacting heparin-binding domain was unable to induce tPA expression.

In summary, hepatocyte tPA mRNA/protein is increased in obesity, but this increase is ultimately overcompensated by a larger increase in PAI1, resulting in decreased plasma tPA activity and fibrinolysis. The markedly increased PAI1, through activating LRP1-CREB1 signaling pathway, drives an increase in hepatic tPA expression as a "compensatory" pathway in obesity. Preventing this compensatory pathway in obesity resulted in the worsening of fibrinolysis. Therapeutic boosting of this compensatory pathway may provide novel strategies to restore effective fibrinolysis in obesity.