LX-2 Stellate Cells Are a Model System for Investigating the Regulation of Hepatic Vitamin A Metabolism and Respond to Tumor Necrosis Factor α and Interleukin 1β

Abstract

Hepatic stellate cells (HSC) are the major site of vitamin A (retinol) esterification and subsequent storage as retinyl esters within lipid droplets. However, retinyl esters become depleted in many pathophysiological states, including acute and chronic liver injuries. Recently, using a liver slice culture system as a model of acute liver injury and fibrogenesis, a time-dependent increase and decrease in the apparent formation of the bioactive retinoid all-<i>trans</i>-retinoic acid (<i>at</i>RA) and retinyl palmitate was measured, respectively. This coincided with temporal changes in the gene expression of retinoid metabolizing enzymes and binding proteins which preceded HSC activation. However, the underlying mechanisms that promote early changes in retinoid metabolism remain unresolved. We hypothesized that LX-2 cells could be applied to investigate differences in quiescent and activated HSC retinoid metabolism. We demonstrate that the hypermetabolic activity of activated stellate cells relative to quiescent stellate cells may be attributed to induction of <i>STRA6, RBP4, </i>and<i> CYP26A1, </i>thereby reducing intracellular concentrations of <i>at</i>RA. We further hypothesized that paracrine and autocrine cytokine signaling regulates HSC vitamin A metabolism in both quiescent and activated cells. In quiescent cells TNF-α dose-dependently down-regulated <i>LRAT</i> and <i>CRBP1</i> mRNA with EC₅₀ values of 30-50 pg/mL. Likewise, IL-1β decreased <i>LRAT</i> and <i>CRBP1</i> gene expression but with less potency. In activated stellate cells, multiple enzymes were down regulated suggesting that the full effects of altered hepatic vitamin A metabolism in chronic conditions require both paracrine and autocrine signaling events. Further, this study suggests potential for cell-type specific autocrine effects in hepatic retinoid signaling. <b>Significance Statement</b> Hepatic stellate cells (HSC) are the major site of vitamin A storage and important determinant of retinol metabolism during liver fibrogenesis. Here, two LX-2 culture methods were applied as models of hepatic retinoid metabolism to demonstrate the effects of activation status and dose-dependent cytokine exposure on the expression of genes involved in retinoid metabolism. This study suggests that compared to quiescent cells, activated HSC are hypermetabolic, have reduced apparent formation of retinoic acid which may alter downstream retinoic acid signaling.

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