Stimulation of the Mitogen-activated Protein Kinase Cascade and Tyrosine Phosphorylation of the Epidermal Growth Factor Receptor by Hepatopoietin

Abstract

Hepatopoietin (HPO) is a novel human hepatotrophic growth factor, which specifically stimulates proliferation of cultured primary hepatocytes in vitro and liver regeneration after liver partial hepatectomy in vivo. Recently, the identification of the mitogenic effect of HPO on hepatoma cell lines and the existence of HPO-specific receptors indicate that HPO acts via its specific cell surface receptor. However, the molecular mechanism of HPO action is not fully elucidated. In this report, we examined the signal transduction events induced by HPO in hepatoma cell line (HepG2). Our results demonstrated that HPO induces phosphorylation of mitogenactivated protein kinase kinase and mitogen-activated protein kinase (MAPK) in a rapid and transient manner. HPO stimulates tyrosine phosphorylation of epidermal growth factor receptor (EGFR). Furthermore, we observed that both MAPK activation and the mitogenic effect of HPO on HepG2 cells were completely blocked by AG1478, a specific inhibitor of EGFR tyrosine kinase activity. However, the effects of HPO were not antagonized by an EGFR-blocking antibody, mAb528, which blocks the interaction between epidermal growth factor and EGFR, indicating that stimulation of tyrosine phosphorylation of EGFR by HPO was not mediated by epidermal growth factor. In contrast, genistein, a general tyrosine kinase inhibitor, significantly attenuated the tyrosine phosphorylation of EGFR in response to HPO. In conclusion, our results suggest that tyrosine phosphorylation of EGFR may play a critical role in MAPK activation and mitogenic stimulation by HPO. Hepatopoietin (HPO)1 is a novel human hepatotrophic growth factor, an orthologue of rat augmenter of liver regeneration or hepatic stimulator substance (1). In 1975, LaBrecque and Pesh (2) first reported that in the livers of weanling rats or partially hepatectomized rats, there existed a polypeptide, named hepatic stimulator substance, that could specifically stimulate DNA synthesis of hepatic cells. The existence of hepatic stimulator substance-related activities has been reported in other species including mice, cows, dogs, pigs, and humans (3). Hagiya et al. (4) cloned the cDNA of rat augmenter of liver regeneration, which is the same as rat hepatic stimulator substance. Subsequently, Giorda et al. (5) and Yang et al. (6) cloned the cDNA of human augmenter of liver regeneration or HPO by screening the cDNA library of human fetal liver. HPO encodes a novel protein with no sequence similarity to any known growth regulator. Interestingly, HPO is highly related to the yeast ERV (essential for respiration and viability) gene products. However, the functional relevance of HPO and ERV is currently unclear (7). Yang et al. (8,9) demonstrated that the recombinant human HPO stimulated proliferation of hepatocytes as well as hepatoma cells in vitro. HPO also promotes regeneration and recovery of damaged hepatocytes and rescues acute hepatic failure in vivo (8, 9). Thus, these observations support the contention that HPO is a hepatotrophic growth fa...

Liver Regeneration Associated Protein (ALR) Exhibits Antimetastatic Potential in Hepatocellular Carcinoma

“…Furthermore this isoform modulated hepatic detoxification by cross-linking growth signals to the regulation of hepatic metabolism (18). The proliferation augmenting effect of ALR was attributed to its ability to activate EGF receptor phosphorylation and the subsequent stimulation of the MAPK cascade (29). Furthermore, it has been elucidated that intracellular ALR may activate transcription factors via a MAPK independent pathway.…”

The Potentiation Role of Hepatopoietin on Activator Protein-1 Is Dependent on Its Sulfhydryl Oxidase Activity

“…In 1999 we identified the existence of HPO-specific receptor on the surface of these cells (8). Furthermore, we proposed that extracellular HPO stimulates proliferation of hepatocytes and enhances liver regeneration by activating the MAPK signaling pathway under the mediation of HPO receptor (9). Intriguingly, we further found that intracellular HPO can specifically modulate the AP-1 pathway through JAB1 via a MAPK-independent pathway and that HPO enhances the increased phosphorylation level of cJun through JAB1 but has no effect on the expression of transfected c-Jun or endogenous c-Jun N-terminal kinase nor on phosphorylation of c-Jun N-terminal kinase (10).…”

“…Since LaBrecque et al (1) first reported hepatic stimulator substance in 1975, HPO has recently been the subject of intense investigation (2)(3)(4)(5)(6)(7)(8)(9)(10). Recombinant HPO can stimulate proliferation of hepatocytes as well as hepatoma cells in vitro, promote liver regeneration and recovery of damaged hepatocytes, and rescue acute hepatic failure in vivo (6,7).…”

ALR, the multifunctional protein

“…Wang és munkatársai a "rövid" formájú (15 kDa) humán rekombináns rhALR magas affi nitású receptorait fedezték fel mind patkány-, mind humán hepatocytákon [34]. Az ALR receptorához történő kötése az EGFR foszforilá-cióját váltja ki, amely ezután a MAPK (mitogen-activated protein kinase) jelátviteli kaszkád aktiválódását okozza, amely fontos szerepet tölt be a sejtnövekedés szabályozásában [35]. Ezenkívül az ALR az EGFR-től független módon, az ERK1/2 (extracellular signal-regulated kinase 1/2) kinázok indukálásával is képes bekapcsolódni a MAPK-útvonalba [36].…”