Bryon Petersen - Academia.edu (original) (raw)

Papers by Bryon Petersen

Liver International, Sep 14, 2011

Background-Somatostatin is a pleiotropic peptide, exerting a variety of effects through its recep... more Background-Somatostatin is a pleiotropic peptide, exerting a variety of effects through its receptor subtypes. Recently, somatostatin has been shown to act as a chemoattractant for hematopoietic progenitor cells and hepatic oval cells (HOC) via receptor subtype 2 and subtype 4 (SSTR4), respectively. Aims-we investigated the in vivo effect of somatostatin/ SSTR4 on HOC migration in the injured liver model of rats and the type of signaling molecules associated with the chemotactic function. Methods-Migration assay, HOC transplantation and PI3K signaling were assessed with or without somatostatin and an analogue of somatostatin (TT232) that specifically binds to SSTR4. Results-TT232 was shown to have an anti-migratory action on HOC induced by somatostatin in vitro. In HOC transplantation experiments, a lower number of donor-derived cells was detected in TT232-treated animals, as compared to control animals. Activation of PI3K was observed in HOC exposed to somatostatin, and this activation was suppressed by either anti-SSTR4 antibody or TT232-pretreatment. In addition, a PI3K inhibitor abrogated the motility of HOC. Conclusion-Together, these data suggest that somatostatin stimulates the migration of HOC within injured liver through SSTR4, and this action appears to be mediated by the PI3K pathway.

Cloning and Stem Cells, Dec 1, 2002

Stromal derived factor-1 alpha (SDF-1alpha) and its receptor CXCR4 have been shown to play a role... more Stromal derived factor-1 alpha (SDF-1alpha) and its receptor CXCR4 have been shown to play a role in the systematic movement of hematopoietic stem cells (HSC) in the fetal and adult stages of hematopoiesis. Under certain physiological conditions liver oval cells can participate in the regeneration of the liver. We have shown that a percentage of oval cells are of hematopoietic origin. Others have shown that bone marrow derived stem cells can participate in liver regeneration as well. In this study we examined the role of SDF-1alpha and its receptor CXCR4 as a possible mechanism for oval cell activation in oval cell aided liver regeneration. In massive liver injury models where oval cell repair is involved hepatocytes up-regulate the expression of SDF-1alpha, a potent chemoattractant for hematopoietic cells. However, when moderate liver injury occurs, proliferation of resident hepatocytes repairs the injury. Under these conditions SDF-1alpha expression is not up-regulated and oval cells are not activated in the liver. In addition, we show that oval cells express CXCR4, the only known receptor for SDF-1alpha. Lastly, in vitro chemotaxis assays demonstrated that oval cells migrate along a SDF-1alpha gradient which suggests that the SDF-1alpha/CXCR4 interaction is a mechanism by which the oval cell compartment could be activated and possibly recruit a second wave of bone marrow stem cells to the injured liver. In conclusion, these experiments begin to shed light on a possible mechanism, which may someday lead to a better understanding of the hepatic and hematopoietic interaction in oval cell aided liver regeneration.

Stem Cells, Apr 1, 2006

Reports of neural transdifferentiation of mesenchymal stem cells (MSCs) suggest the possibility t... more Reports of neural transdifferentiation of mesenchymal stem cells (MSCs) suggest the possibility that these cells may serve as a source for stem cell–based regenerative medicine to treat neurological disorders. However, some recent studies controvert previous reports of MSC neurogenecity. In the current study, we evaluate the neural differentiation potential of mouse bone marrow–derived MSCs. Surprisingly, we found that MSCs spontaneously express certain neuronal phenotype markers in culture, in the absence of specialized induction reagents. A previously published neural induction protocol that elevates cytoplasmic cyclic AMP does not upregulate neuron-specific protein expression significantly in MSCs but does significantly increase expression of the astrocyte-specific glial fibrillary acidic protein. Finally, when grafted into the lateral ventricles of neonatal mouse brain, MSCs migrate extensively and differentiate into olfactory bulb granule cells and periventricular astrocytes, without evidence of cell fusion. These results indicate that MSCs may be “primed” toward a neural fate by the constitutive expression of neuronal antigens and that they seem to respond with an appropriate neural pattern of differentiation when exposed to the environment of the developing brain.

Introduction: Obesity and type 2 diabetes are risk factors for liver cancer due to a transition f... more Introduction: Obesity and type 2 diabetes are risk factors for liver cancer due to a transition from steatosis to nonalcoholic steatohepatitis (NASH), resulting in liver fibrosis. If left untreated, liver fibrosis can progress into cirrhosis leading to hepatocellular carcinoma (HCC). Connective tissue growth factor (CTGF) is a pro-fibrotic matricellular protein that is highly expressed during hepatocarcinogenesis. This study aims to investigate the involvement of CTGF in nonalcoholic steatohepatitis (NASH) and liver cancer development during diabetic conditions induced by streptozotocin (STZ) treatment and the feeding of a high fat diet (HFD) in mice. Methods: Transgenic male mice expressing green fluorescent protein (GFP) under the control of Ctgf promoter (Ctgfp-GFP), liver specific CTGF knockouts (CtgfΔhep/Δhep), and control mice that contained two alleles of floxed Ctgf (Ctgffloxed/floxed) were given STZ (200 ng/pup) at postnatal day 2 (P2) through subcutaneous injection and were fed HFD at postnatal week four to induce NASH and HCC. The treated animals were harvested at postnatal week 5, 12, and 20 for early steatosis, fibrosis and liver cancer development respectively. CTGF expression was analyzed by immunofluorescent staining, Western analysis, and qRT-PCR. In addition, the mouse liver cancer RT2 profiler PCR array was screened to identify cancer-related genes that were differentially expressed during NASH and HCC development comparing CtgfΔhep/Δhep and Ctgffloxed/floxed mice. Results: All three types of animals developed NASH and HCC in response to STZ and HFD feeding. Liver pathologies ranging from steatosis, NASH, liver fibrosis, and nodule formation, to intratumoral angiogenesis were observed as discrete molecular and histological stages in the STZ/HFD induced NASH-HCC model. Immunofluorescent analysis of Ctgfp-GFP reporter mice showed induction of the Ctgf gene in vascular endothelial cells, biliary epithelial cells, hepatocytes, and inflammatory cells in STZ/HFD livers. We utilized the mouse liver cancer RT2 profiler PCR array and compared the expression of 90 liver cancer related genes between CtgfΔhep/Δhep and Ctgff/f tumors that developed after 12-week HFD feeding. 12 upregulated genes and 10 downregulated genes were identified in Ctgfk/k mice compared to Ctgff/f animals. Conclusion: We successfully established a NASH-HCC model combining STZ and HFD treatment. CTGF expression was found in multiple cell types including endothelial cells, cholangiocytes, inflammatory cells, and hepatocytes. Liver specific deletion of CTGF was associated with differential expression of groups of genes involved in cell proliferation and apoptosis. The functional relationship between these genes and CTGF in liver cancer development in the setting of metabolic syndrome needs to be characterized in future studies. Citation Format: Liya Pi, Marda Jorgensen, Seh-Hoon Oh, Altin Gjymishka, Bryon E. Petersen. The involvement of connective tissue growth factor in nonalcoholic steatohepatitis and liver cancer development under diabetic condition. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 5107.

American Journal of Pathology, May 1, 2016

miRNAs are involved in liver regeneration, and their expression is dysregulated in hepatocellular... more miRNAs are involved in liver regeneration, and their expression is dysregulated in hepatocellular carcinoma (HCC). Connective tissue growth factor (CTGF), a direct target of miR-133b, is crucial in the ductular reaction (DR)/oval cell (OC) response for generating new hepatocyte lineages during liver injury in the context of hepatotoxin-inhibited hepatocyte proliferation. Herein, we investigate whether miR-133b regulation of CTGF influences HCC cell proliferation and migration, and DR/OC response. We analyzed miR-133b expression and found it to be down-regulated in HCC patient samples and induced in the rat DR/OC activation model of 2-acetylaminofluorene with partial hepatectomy. Furthermore, overexpression of miR-133b via adenoviral system in vitro led to decreased CTGF expression and reduced proliferation and Transwell migration of both HepG2 HCC cells and WBF-344 rat OCs. In vivo, overexpression of miR-133b in DR/OC activation models of 2-acetylaminofluorene with partial hepatectomy in rats, and 3,5-diethoxycarbonyl-1,4-dihydrocollidine in mice, led to down-regulation of CTGF expression and OC proliferation. Collectively, these results show that miR-133b regulation of CTGF is a novel mechanism critical for the proliferation and migration of HCC cells and OC response.

American Journal of Pathology, Jun 1, 2010

Digestive and Liver Disease, May 1, 2008

E-biomed, May 17, 2000

Hepatic oval “stem” cells were first described in 1956 by E. Farber (1), and are considered to be... more Hepatic oval “stem” cells were first described in 1956 by E. Farber (1), and are considered to be a cell capable of developing into hepatocytes or bile duct epithelial cells. Other investigators suggest that they are the progeny of a hepatic stem cell. Numerous in vivo and in vitro studies have documented a central role of oval cells in liver biology and carcinogenesis (2–4). In addition to the effects seen in the liver, oval cells (or cells very similar to oval cells) have been implicated in the architecture of the regenerating pancreas after injury (5–6) and they may be involved in the regeneration of other organs as well. While there is a precise purpose for this cell population in the liver, the mechanism(s) by which oval cells activate, proliferate and differentiate are poorly understood. Despite forty years of research and 300 publications on hepatic oval “stem” cells their ontogeny remains a mystery. “Oval cell proliferation” describes the occurrence of nonparenchymal cells detected in large numbers at early stages of carcinogenesis. The population is heterogeneous and contains cells that differ in their differentiation state and potential in their lineage commitment. “Oval cell compartment” is also used to describe the cells invading the parenchyma after the administration of certain carcinogens (7–8). Farber (1991) argued that although oval cells might be the progenitors of hepatocelluar carcinomas (HCC) induced by 3 9 methyl-4 9 -dimethyl-amino-azo-benzene (DMAB), this conclusion can not be generalized to other models of hepatocarcinogenesis (3). It is his view that cell lineage analyses cannot be achieved in a culture system(s) and that the markers used in studies of hepatocarcinogenesis can be considered reliable lineage markers for work done in vivo (3). Farber also reported that in each oval cell compartment model the oval cells will express different isoenzyme profiles, however, certain hepatic alpha-fetoprotien (AFP) and biliary cytokeratin-19 (CK-19) markers will always be expressed (3). In the DMAB model, the single most important conclusion provided by this data is that hepatocytes could be generated from nonparenchymal progenitor cells (3,9). This model establishes the key principle that some type of progenitor cell exists even if it turns out that precursor cell differentiation occurs only infrequently. Evarts et al. (1987 and 1989) show that the oval cell compartment is activated extensively in a 2-Acetylaminofluorene/two thirds partial-hepatecomy (2-AAF/PHx) model. This model is a variation of the SoltFarber protocol (2,10) and has become the industry standard. Figure 1 represents a timeline for this type of protocol. Under these conditions hepatocyte proliferation is heavily suppressed, and it was possible to label oval cells with [3H]-thymidine. This procedure allowed them the ability to follow the fate of the labeled cells. Four to five days after [3H]-thymidine injection, the label was found in basophilic hepatocytes expressing AFP and albumin (2). The authors concluded that there was a precursor-product relationship between the labeled cells of the oval cell compartment and the basophilic hepatocytes. Further studies, presented by the same investigators , show that proliferation of the oval cells and hepatocyte differentiation at early stages of carcinogenesis are associated with the activation of the stellate or Ito cells, which may reg-

Journal of Cell Communication and Signaling, Dec 5, 2022

Liver fibrosis is the common outcome of many chronic liver diseases, resulting from altered cell-... more Liver fibrosis is the common outcome of many chronic liver diseases, resulting from altered cell-cell and cell-matrix interactions that promote hepatic stellate cell (HSC) activation and excessive matrix production. This study aimed to investigate functions of cellular communication network factor 2 (CCN2)/Connective tissue growth factor (CTGF), an extracellular signaling modulator of the CYR61/CTGF/Nov (CCN) family, in liver fibrosis. Tamoxifen-inducible conditional knockouts in mice and hepatocyte-specific deletion of this gene in rats were generated using the Cre-lox system. These animals were subjected to peri-central hepatocyte damage caused by carbon tetrachloride. Potential crosstalk of this molecule with a new profibrotic pathway mediated by the Slit2 ligand and Roundabout (Robo) receptors was also examined. We found that Ccn2/Ctgf was highly upregulated in periportal hepatocytes during carbon tetrachloride-induced hepatocyte damage, liver fibrosis and cirrhosis in mice and rats. Overexpression of this molecule was observed in human hepatocellular carcinoma (HCC) that were surrounded with fibrotic cords. Deletion of the Ccn2/Ctgf gene significantly reduced expression of fibrosis-related genes including Slit2, a smooth muscle actin (SMA) and Collagen type I during carbon tetrachloride-induced liver fibrosis in mice and rats. In addition, Ccn2/Ctgf and its truncated mutant carrying the first three domains were able to interact with the 7th-9th epidermal growth factor (EGF) repeats and the C-terminal cysteine knot (CT) motif of Slit2 protein in cultured HSC and fibrotic murine livers. Ectopic expression of Ccn2/Ctgf protein upregulated Slit2, promoted HSC activation, and potentiated fibrotic responses following chronic intoxication by carbon tetrachloride. Moreover, Ccn2/Ctgf and Slit2 synergistically enhanced activation of phosphatidylinositol 3-kinase (PI3K) and AKT in primary HSC, whereas soluble Robo1-Fc chimera protein could inhibit these activities. These observations demonstrate conserved cross-species functions of Ccn2/Ctgf protein in rodent livers. This protein can be induced in hepatocytes and contribute to liver fibrosis. Its novel connection with the Slit2/Robo signaling may have therapeutic implications against fibrosis in chronic liver disease.

The FASEB Journal, Feb 26, 2022

Yes‐associated protein (YAP), a central effector in the Hippo pathway, is involved in the regulat... more Yes‐associated protein (YAP), a central effector in the Hippo pathway, is involved in the regulation of organ size, stem cell self‐renewal, and tissue regeneration. In this study, we observed YAP activation in patients with alcoholic steatosis, hepatitis, and cirrhosis. Accumulation of this protein in the nucleus was also observed in murine livers that were damaged after chronic‐plus‐single binge or moderate ethanol ingestion combined with carbon tetrachloride intoxication (ethanol/CCl4). To understand the role of this transcriptional coactivator in alcohol‐related liver injury, we knocked out the Yap1 gene in hepatocytes of floxed homozygotes through adeno‐associated virus (AAV8)‐mediated deletion utilizing Cre recombinase. Yap1 hepatocyte‐specific knockouts (KO) exhibited hemorrhage, massive hepatic necrosis, enhanced oxidative stress, elevated hypoxia, and extensive infiltration of CD11b+ inflammatory cells into hepatic microenvironments rich for connective tissue growth factor (Ctgf) during ethanol/CCl4‐induced liver damage. Analysis of whole‐genome transcriptomics indicated upregulation of genes involved in hypoxia and extracellular matrix (ECM) remodeling, whereas genes related to hepatocyte proliferation, progenitor cell activation, and ethanol detoxification were downregulated in the damaged livers of Yap1 KO. Acetaldehyde dehydrogenase (Aldh)1a1, a gene that encodes a detoxification enzyme for aldehyde substrates, was identified as a potential YAP target because this gene could be transcriptionally activated by a hyperactive YAP mutant. The ectopic expression of the human ALDH1A1 gene caused increase in hepatocyte proliferation and decrease in hepatic necrosis, oxidative stress, ECM remodeling, and inflammation during ethanol/CCl4‐induced liver damage. Taken together, these observations indicated that YAP was crucial for liver repair during alcohol‐associated injury. Its regulation of ALDH1A1 represents a new link in liver regeneration and detoxification.

Investigative Ophthalmology & Visual Science, 2014

Liver Research, 2017

The liver possesses an extraordinary ability to regenerate after injury. Hepatocyte-driven liver ... more The liver possesses an extraordinary ability to regenerate after injury. Hepatocyte-driven liver regeneration is the default pathway in response to mild-to-moderate acute liver damage. When replication of mature hepatocytes is blocked, facultative hepatic progenitor cells (HPCs), also referred to as oval cells (OCs) in rodents, are activated. HPC/OCs have the ability to proliferate clonogenically and differentiate into several lineages including hepatocytes and bile ductal epithelia. This is a conserved liver injury response that has been studied in many species ranging from mammals (rat, mouse, and human) to fish. In addition, improper HPC/OC activation is closely associated with fibrotic responses, characterized by myofibroblast activation and extracellular matrix production, in many chronic liver diseases. Matrix remodeling and metalloprotease activities play an important role in the regulation of HPC/OC proliferation and fibrosis progression. Thus, understanding molecular mechanisms underlying HPC/OC activation has therapeutic implications for rational design of anti-fibrotic therapies.

Cancer Research, 2015

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Introduction: Co... more Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Introduction: Connective tissue growth factor (CTGF) is a secreted pro-angiogenic protein within the CCN (Cyr61/CTGF/Nov) family and modulates multiple angiogenic pathways through its broad binding ability to cysteine knot motifs presents in key angiogenic factors including vascular endothelial growth factor (VEGF)-A. Overexpression of CTGF has been found in many cancers such as Lewis lung carcinoma (LLC), cholangiocarcinoma (CC), and hepatocellular carcinoma (HCC). In an effort to understand the molecular action of CTGF, we identified a disintegrin and metalloproteinase with thrombospondin type I repeat 7 (ADAMTS7) as a CTGF binding protein in yeast two-hybrid analysis. This enzyme belongs to the ADAMTS family capable of cleaving extracellular matrix (ECM) components and extracellular regulatory molecules. Here, we show that ADAMTS7 binds to and degrades CTGF during cancer development. Methods: Adamts7 knockout mice were used to investigate the importance of ADAMTS7 in CTGF interaction and cleavage during LLC development. Adamts7+/+ and Adamts7−/− mice were subcutaneously implanted with LLC cells. Tumor growth was quantitatively measured within two weeks after implantation. Tumor angiogenesis was assessed utilizing a microvascular density assay. In addition, Adamts7 expression was tracked via X-gal staining of LLC tumors grown in Adamts7−/− mice that contained a LacZ trap-in cassette in the targeted Adamts7 allele. VEGF-A, CTGF, and ADAMTS7 were determined at both the mRNA and protein levels. Results: Adamts7 induction as indicated by the β-galactosidase activity in X-gal staining was found in intra-tumor stromal cells of LLC tumors grown in Adamts7−/− mice. The association of CTGF and ADAMTS7 proteins in protein lysates of LLC tumors grown in Adamts7+/+ mice was confirmed in immunoprecipitation assays. Higher levels of VEGF-A, CTGF and ADAMTS7 mRNAs were found in LLC tumors from Adamts7−/− animals as compared to Adamts7+/+ controls. Slower rate of CTGF turnover, faster growth of LLC tumors, and higher microvascular density were also found in Adamts7−/− animals than controls. Conclusion: These observations confirmed the binding and processing of CTGF by ADAMTS7 during LLC growth. The host-derived ADAMTS7 appears to regulate CTGF turnover and provides a protective effect towards aberrant LLC tumorigenesis and angiogenesis. Citation Format: Liya Pi, Bryon E. Petersen. Interaction between connective tissue growth factor and a disintegrin and metalloproteinase with thrombospondin type I repeats 7 during cancer development. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2388. doi:10.1158/1538-7445.AM2015-2388

Liver Regeneration, 2015

Normal healthy livers have highly regenerative capabilities, but diseased livers have reduced (or... more Normal healthy livers have highly regenerative capabilities, but diseased livers have reduced (or lack the) ability to regenerate. When hepatocytes are prevented from initiating the regenerative response due to chronic or overwhelming injury then the liver stem cell compartment is activated and gives rise to the liver progenitor cell (oval cell) population. Oval cells are bipotential cells that are capable of differentiating into either hepatocyte or biliary lineages. In this chapter, we will review several aspects of liver progenitor cells in liver regeneration including cell markers, animal models, molecular regulation, their role in fibrosis, and possible therapeutic targets.

The FASEB Journal, 2012

Blood vessels are formed during development and tissue repair through a plethora of modifiers tha... more Blood vessels are formed during development and tissue repair through a plethora of modifiers that coordinate efficient vessel assembly in various cellular settings. Here we used the yeast 2-hybrid approach and demonstrated a broad affinity of connective tissue growth factor (CCN2/CTGF) to C-terminal cystine knot motifs present in key angiogenic regulators Slit3, von Willebrand factor, platelet-derived growth factor-B, and VEGF-A. Biochemical characterization and histological analysis showed close association of CCN2/CTGF with these regulators in murine angiogenesis models: normal retinal development, oxygen-induced retinopathy (OIR), and Lewis lung carcinomas. CCN2/CTGF and Slit3 proteins worked in concert to promote in vitro angiogenesis and downstream Cdc42 activation. A fragment corresponding to the first three modules of CCN2/CTGF retained this broad binding ability and gained a dominant-negative function. Intravitreal injection of this mutant caused a significant reduction in vascular obliteration and retinal neovascularization vs. saline injection in the OIR model. Knocking down CCN2/CTGF expression by short-hairpin RNA or ectopic expression of this mutant greatly decreased tumorigenesis and angiogenesis. These results provided mechanistic insight into the angiogenic action of CCN2/CTGF and demonstrated the therapeutic potential of dominant-negative CCN2/CTGF mutants for antiangiogenesis.

Laboratory Investigation, May 1, 2017

During progression to type 1 diabetes, insulin-producing β-cells are lost through an autoimmune a... more During progression to type 1 diabetes, insulin-producing β-cells are lost through an autoimmune attack resulting in unrestrained glucagon expression and secretion, activation of glycogenolysis, and escalating hyperglycemia. We recently identified a protein, designated islet homeostasis protein (IHoP), which specifically co-localizes within glucagon-positive α-cells and is overexpressed in the islets of both post-onset non-obese diabetic (NOD) mice and type 1 diabetes patients. Here we report that in the αTC1.9 mouse α-cell line, IHoP was released in response to high-glucose challenge and was found to regulate secretion of glucagon. We also show that in NOD mice with diabetes, major histocompatibility complex class II was upregulated in islets. In addition hyperglycemia was modulated in NOD mice via suppression of IHoP utilizing small interfering RNA (IHoP-siRNA) constructs/approaches. Suppression of IHoP in the pre-diabetes setting maintained normoglycemia, glyconeolysis, and fostered β-cell restoration in NOD mice 35 weeks post treatment. Furthermore, we performed adoptive transfer experiments using splenocytes from IHoP-siRNA-treated NOD/ShiLtJ mice, which thwarted the development of hyperglycemia and the extent of insulitis seen in recipient mice. Last, IHoP can be detected in the serum of human type 1 diabetes patients and could potentially serve as an early novel biomarker for type 1 diabetes in patients.

Gut, Jul 1, 2003

Background and aim: Liver regeneration after severe liver damage depends in part on proliferation... more Background and aim: Liver regeneration after severe liver damage depends in part on proliferation and differentiation of hepatic progenitor cells (HPCs). Under these conditions they must be able to withstand the toxic milieu of the damaged liver. ATP binding cassette (ABC) transporters are cytoprotective efflux pumps that may contribute to the preservation of these cells. The aim of this study was to determine the ABC transporter phenotype of HPCs. Methods: HPC activation was studied in rats treated with 2-acetylaminofluorene (2-AAF) followed by partial hepatectomy (PHx). ABC transporter gene expression was determined by real time detection reverse transcription-polymerase chain reaction in isolated HPCs, hepatocytes, cholangiocytes, and cultured progenitor cell-like RLF φ 13 cells and by immunohistochemistry of total liver samples. ABC transporter efflux activity was studied in RLF φ 13 cells by flow cytometry. Results: 2-AAF/PHx treated animals showed increased hepatic mRNA levels of the genes encoding multidrug resistance proteins Mdr1b, Mrp1, and Mrp3. Immunohistochemistry demonstrated expression of Mrp1 and Mrp3 proteins in periportal progenitor cells and of the Mdr1b protein in periportal hepatocytes. Freshly isolated Thy-1 positive cells and cultured RLF φ 13 progenitor cells highly expressed Mrp1 and Mrp3 mRNA while the hepatocyte specific transporters Mdr2, Bsep, Mrp2, and Mrp6 were only minimally expressed. Blocking Mrp activity by MK-571 resulted in accumulation of the Mrp specific substrate carboxyfluorescein in RLF φ 13 cells. Conclusion: HPCs express high levels of active Mrp1 and Mrp3. These may have a cytoprotective role in conditions of severe hepatotoxicity.