Deregulation of growth factor, circadian clock, and cell cycle signaling in regenerating hepatocyte...

Activation of the nuclear receptors constitutive androstane receptor, pregnane X receptor, and peroxisome proliferator-activated receptor alpha results in hepatomegaly, and these nuclear receptors are implicated in the regulation of liver regeneration. Retinoid X receptor (RXR)alpha is an essential partner of these nuclear receptors. Therefore, we studied the role of hepatocyte RXRalpha in liver regeneration using partial hepatectomy model. The results showed that hepatocyte RXRalpha deficiency caused an approximately 20-hour delay in hepatocyte proliferation after partial hepatectomy. Several pathways, including growth factors and the circadian cell cycle, were impaired due to hepatocyte RXRalpha deficiency. In addition, the expression patterns of hepatocyte growth factor, fibroblast growth factor 2, platelet-derived growth factor, and transforming growth factor alpha were altered due to lack of RXRalpha. Furthermore, the peroxisome proliferator-activated receptor alpha/brain and muscle Arnt-like protein 1/Rev-erbalpha/P21 pathway was compromised, and Cry1/Cry2 and Wee1/Per1 expression was deregulated in regenerating RXRalpha-null livers. Accordingly, the expression and regulation of cyclin D1/Cyclin- dependent Kinase (Cdk)4, cyclin E1/Cdk2, cyclin A2/Cdk2, and cyclin B1/Cdk1 after partial hepatectomy were altered in regenerating RXRalpha-null livers. Hepatocyte RXRalpha deficiency also affected the basal, as well as regeneration-induced cyclin E1 expression levels. Activation of RXRalpha by retinoic acids increased the cyclin E1 promoter activity indicating retinoic acid-mediated signaling positively controls cyclin E1 gene expression. As many of these observed changes were not documented in the regenerating livers of other nuclear receptor knockout mice, these observed effects may be hepatocyte RXRalpha specific.