Activation of the oval cell compartment occurs in the liver when hepatocytes are functionally compromised and/or unable to divide. Our goal is to investigate the systemic signals responsible for determining the efficiency of oval cell mediated liver regeneration, focusing on the Notch signaling cascade. The established oval cell induction protocol of 2-Acetylaminofluorine (2AAF) implantation followed by 70% surgical resection of the liver (PH) was employed in a rat model. This oval cell induction model was further combined with injections of GSI XX to examine the effects of Notch inhibition on stem cell-aided regeneration of the liver. Notch signaling was found to be upregulated at the peak of oval cell induction during 2AAF-PH alone. Treatment with GSI XX led to interruption of the Notch signal, as shown by a decrease in expression of Hes1. While there was a robust oval cell response seen at day 11 post-PH, there was a measurable delay in differentiation when Notch was inhibited. This was confirmed morphologically as well as by immunohistochemistry for the oval cell markers AFP, OV-6 and CK19. The hepatocytes seen at day 22 demonstrated an enhanced hepatocellular mitoinhibition index (p21Waf1/Ki67), suggestive of dysregulated proliferation and cell cycle progression. Moreover these hepatocytes exhibited decreased expression of hepatocyte functional markers such as cytochrome p450 and glucose-6-phosphatase-alpha. Taken together these results identify the Notch signaling pathway as a potent regulator of differentiation and proliferation in oval cells and is necessary for functional repair of the liver by oval cells.
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