Karl Houglum

Activation of hepatic stellate cells by TGF alpha and collagen type I is mediated by oxidative stress through c-myb expression.

Excessive production of collagen type I is a major contributor to hepatic fibrosis. Activated (myofibroblastic), but not quiescent, hepatic stellate cells (lipocytes) have a high level of collagen type I and alpha-smooth muscle actin expression. Therefore, stellate cell activation is a critical step in hepatic fibrosis. Here we show that quiescent stellate cells were activated by the generation of free radicals with ascorbate/FeSO4 and by malondialdehyde, a product of lipid peroxidation. In addition, stellate cell activation by collagen type I matrix and TGF alpha was blocked by antioxidants, such as d-alpha-tocopherol and butylated hydroxytoluene. Moreover, oxidative stress, TGF alpha and collagen type I markedly stimulated stellate cell entry into S-phase, NFkB activity, and c-myb expression, which were prevented by antioxidants. c-myb antisense oligonucleotide blocked the activation and proliferation of stellate cells induced by TGF alpha. Nuclear extracts from activated, but not from quiescent, stellate cells formed a complex with the critical promoter E box of the alpha-smooth muscle actin gene, which was disrupted by c-myb and NFkB65 antibodies, and competed by c-myb and NFkB cognate DNA. c-Myb expression was also stimulated in activated stellate cells in carbon tetrachloride-induced hepatic injury and fibrogenesis. This study indicates that oxidative stress plays an essential role, through the induction of c-myb and NFkB, on stellate cell activation.

Malondialdehyde and 4-hydroxynonenal protein adducts in plasma and liver of rats with iron overload.

In hepatic iron overload, iron-catalyzed lipid peroxidation has been implicated in the mechanisms of hepatocellular injury. Lipid peroxidation may produce reactive aldehydes such as malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE), which may form aldehyde-protein adducts. We investigated whether lipid peroxidation occurred in rats fed a diet containing 3% carbonyl iron for 5-13 wk, and if this resulted in the formation of MDA- and 4-HNE- protein adducts. Chronic iron feeding resulted in hepatic iron overload (greater than 10-fold) and concomitantly induced a 2-fold increase in hepatic lipid peroxidation. Using an antiserum specific for MDA-lysine protein adducts, we demonstrated by immunohistochemistry the presence of aldehyde-protein adducts in the cytosol of periportal hepatocytes, which co-localized with iron. In addition, MDA- and 4-HNE-lysine adducts were found in plasma proteins of animals with iron overload. Only MDA adducts were detected in albumin, while other plasma proteins including a approximately 120-kD protein had both MDA and 4-HNE adducts. In this animal model of hepatic iron overload, injury occurs primarily in periportal hepatocytes, where MDA-lysine protein adducts and excess iron co-localized.

Lipid peroxidation in hepatic steatosis in humans is associated with hepatic fibrosis and occurs predominately in acinar zone 3.

Hepatic steatosis has been shown to be associated with lipid peroxidation and hepatic fibrosis in a variety of liver diseases including non‐alcoholic fatty liver disease. However, the lobular distribution of lipid peroxidation associated with hepatic steatosis, and the influence of hepatic iron stores on this are unknown. The aim of this study was to assess the distribution of lipid peroxidation in association with these factors, and the relationship of this to the fibrogenic cascade.

Proliferation of hepatic stellate cells is inhibited by phosphorylation of CREB on serine 133.

Proliferating, activated, hepatic stellate cells have a high level of collagen type I expression. Therefore, stellate cell proliferation is a critical step in hepatic fibrosis. Here we show that proliferation of activated primary rat stellate cells was blocked by elevation of cAMP with 8 Br-cAMP or isomethylbutyl xanthine, a phosphodiesterase inhibitor, and by stimulation of Ca2+ fluxes with the Ca2+ ionophore A-23187. Because phosphorylation of CREB on Ser133 is an important mediator of cAMP-protein kinase (PKA) and Ca2+-calmodulin kinase II (CAMK-II) activation, we tested whether CREB-PSer133 was essential for stellate cell quiescence. Nuclear extracts from quiescent, but not from activated, stellate cells contained CREB-PSer133. Moreover, the phosphorylation of CREB on Ser133 was stimulated in activated cells by inducing the activity of PKA or CAMK-II. In addition, coexpression of CREB and either a constitutively active PKA or a constitutively active CAMK-II inhibited the proliferation of activated stellate cells. In contrast, expression of CREB alone, PKA or CAMK-II alone, CREB-Ala 133 (which lacks the Ser133 phosphoacceptor) with PKA or CAMK-II, or CREB with inactive PKA or CAMK-II mutants did not affect stellate cell proliferation, suggesting that CREB-PSer133 is necessary for blocking the stellate cell cycle. Conversely, expression of a trans-dominant negative CREB-Ala 133 mutant (which competes with CREB/CREB-PSer133 for cognate DNA binding sites and presumably for protein interactions) induced a greater than fivefold entry into S-phase of quiescent stellate cells, compared with control cells expressing either beta-galactosidase or wt CREB, indicating that CREB-PSer133 may be indispensable for the quiescent stellate cell phenotype. This study suggests that PKA and CAMK-II play an essential role on stellate cell activation through the induction of CREB phosphorylation on Ser133, and provides potential approaches for the treatment of hepatic fibrogenesis in patients with chronic liver diseases.

Pentoxifylline blocks hepatic stellate cell activation independently of phosphodiesterase inhibitory activity

Activated, but not quiescent, hepatic stellate cells (lipocytes) have a high level of collagen type I and smooth muscle actin (SMA) gene expression. Therefore, stellate cell activation is a critical step in hepatic fibrosis. The mechanisms leading to stellate cell activation in vivo are unknown. The characteristic hepatic oxidative stress cascade induced in rats by CCl4 markedly stimulated stellate cell entry into S phase, nuclear factor (NF)-κB activity, and c- myb expression. These changes were prevented by pentoxifylline, which also decreased CCl4-induced hepatic injury. As expected, cAMP-mediated phosphorylation of CREB-Ser133 was induced in vivo in stellate cells by pentoxifylline but not by its metabolite 5, an N-1 carboxypropyl derivative, which lacks phosphodiesterase inhibitory activity. Stellate cell nuclear extracts from CCl4-treated, but not from control, animals formed a complex with the critical promoter E box of the α-SMA gene, which was disrupted by c- myb antibodies and competed with by c- myb cognate DNA. Treatment with pentoxifylline or metabolite 5 prevented the molecular abnormalities characteristic of stellate cell activation induced by CCl4. These results suggest that induction of c- myb plays an important role in the in vivo activation of stellate cells. Pentoxifylline blocks stellate cell activation in vivo independently of its inhibitory effects on phosphodiesterases by interfering with the oxidative stress cascade and the activation of NF-κB and c- myb.

LAP (NF-IL6) transactivates the collagen alpha 1(I) gene from a 5' regulatory region.

Although collagen is known to enhance hepatocyte differentiation and hepatocytes produce collagen in vivo, the transcriptional factors responsible for collagen type I gene expression in hepatic cells are not known. LAP (Liver Activator Protein) is a member of the C/EBP family, which in differentiated hepatocytes contributes to the high levels of liver-specific gene expression. In this study we show that LAP binds to the collagen alpha 1(I) promoter at both reverse CCAAT motifs and activates transcription. Furthermore, an upstream element, collagen element I (-370/-344), which shares homology with the LAP binding cis-element of the albumin promoter (9 of 13 bp) is described. This collagen element I stimulates transcription in both orientations and when placed in front of either a homologous or a heterologous chimeric report construct. These experiments suggest that LAP may be important in the expression of collagen in differentiated hepatocytes through both the promoter and a newly described upstream element.