Hepatopoietin (HPO) is a novel polypeptide mitogen specific for hepatocytes and hepatoma cell lines, which is derived from liver and supports its regeneration. To determine whether HPO acts via a receptor-based signal transduction, recombinant human hepatopoietin was labeled by iodination and used to characterize its binding activity by specific displacement test and Scatchard analysis in primarily cultured rat hepatocytes and human hepatoma Hep-G2 cells. The binding was saturable and specific because it was replaceable by HPO but not by epidermal growth factor, transforming growth factor-␣, or insulin. Scatchard analysis indicated the presence of a single class of high affinity receptor with dissociation constant (K d ) of 2 and 0.7 pM, and a receptor density of about 10,000 sites/cell and 55,000 sites/cell in the rat hepatocytes and human hepatoma cells, respectively. The K d values were consistent with the half-maximum dose of HPO activity. Affinity cross-linking of the receptor with 125 I-HPO revealed a polypeptide of molecular mass approximately 90 kDa by SDS-polyacrylamide gel electrophoresis. Thus, the molecular mass of the HPO receptor was calculated to be about 75 kDa. These data demonstrated the existence of an HPO receptor in hepatocytes and hepatoma cells, which may account for biological effect.Previous studies implicate that a small molecule derived from liver itself specifically stimulates hepatocytes proliferation and supports liver regeneration (1-3). In 1975, LaBrecque et al. (3) first reported that in the liver of a weaning rat and the regenerating liver of a partially hepatectomized rat, there existed hepatic stimulator substance (HSS) 1 that could specifically stimulate DNA synthesis in hepatic cells. Other groups have also carried out extensive research on HSS derived from other species (2). At the same time, experiments and clinical research on human fetal liver cells demonstrated its therapeutic effect on hematopoietic diseases and severe liver diseases (2, 4). Since the 1980s, we began to isolate and purify the effective component from fetal liver. We identified hepatic stimulatory activity in the fraction with molecular size ranging from 10 to 30 kDa of human fetal liver lysate (5-7). The activity was target-specific, which was different from various well known nonspecific hepatic stimulators such as insulin, EGF, insulinlike growth factor, and TGF-␣. The characteristics of the effective component derived from human fetal liver were consistent with those of HSSs derived from other species, suggesting that the effective component could be the human-derived homologue of the animal's HSS. Then, we purified this activity and demonstrated that the biological activity of its pure form is identical to those of the crude form and consistent with those of animal-derived HSSs, but evidently different from those of serum-derived hepatocyte growth factor (8). The factor was named as hepatopoietin (HPO). Later, we proved that HPO is encoded by mRNA of fetal liver (9) and further cloned (10) its ...

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...

Many growth factors and cytokines are involved in liver regeneration. Of them, only hepatopoietin (HPO)/ALR (augmenter of liver regeneration) is a specifically hepatotrophic factor originally identified from the cytosol of regenerating or hyperplastic hepatic cells. Previous reports indicate that extracellular HPO triggers the MAPK pathway by binding its specific receptor on the cell surface. However, its function in the cytosol of hepatocytes is unclear. Here we identified that JAB1 (Jun activation domain-binding protein 1), a co-activator of AP-1, which is essential for liver regeneration, specifically interacts with intracellular HPO. JAB1 colocalizes with HPO in nuclei of hepatic cells or COS-7 cells. As an intracrine factor, the intracellular function of HPO is to increase c-Jun phosphorylation independent of c-Jun amino-terminal kinase (JNK), extracellular signal-regulated kinase (ERK) -1 and -2, and leads to potentiation of JAB1-mediated AP-1 activation. Amino acids 1-63 of HPO molecule are sufficient to bind to JAB1, but the full-length HPO is necessary for its intracellular signaling. Taken together, these results elucidate a novel mechanism of intracrine cytokine signaling by specifically modulating the AP-1 pathway through JAB1, in a MAPK-independent fashion.

The social relationships that individuals experience at different life stages have a non-negligible influence on their lives, and this is particularly true for group living animals. The long lifespan of many primates makes it likely that these animals have various tactics of social interaction to adapt to complex changes in environmental or physical conditions. The different strategies used in social interaction by individuals at different life stages, and whether the position (central or peripheral) or role (initiator or recipient) of an individual in the group social network changes with age, are intriguing questions that remain to be investigated. We used social network analysis to examine age-related differences in social interaction patterns, social roles, and social positions in three affiliative social networks (approach, allogrooming, and social play) in a group of wild rhesus macaques (Macaca mulatta). Our results showed that social interaction patterns of rhesus macaques differ between age classes in the following ways: i) young individuals tend to allocate social time to a high number of groupmates, older individuals prefer to focus on fewer, specific partners; ii) as they grow older, individuals tend to be recipients in approach interactions and initiators in grooming interactions; and iii) regardless of the different social interaction strategies, individuals of all ages occupy a central position in the group. These results reveal a possible key role played by immature individuals in group social communication, a little-explored issue which deserves closer investigation in future research.

Tumor endothelial marker 8 (TEM8) was discovered as a cell membrane protein that is predominantly expressed in tumor endothelium and identified as a receptor for anthrax toxin. We developed an antibody-like molecule that consists of the protective antigen (PA)-binding domain of human TEM8 linked to the Fc portion of human immunoglobulin G1 (TEM8-Fc). This engineered protein bound to PA in a divalent cation-dependent manner and efficiently protected J774A.1 macrophage-like cells against anthrax toxin challenge in a dose-dependent manner. TEM8-Fc suppressed the growth and metastasis of xenograft human tumors in athymic nude mice (control versus 10 mg/kg TEM8-Fc, mean tumor weight: LS-180, 1.72 versus 0.16 g, difference = 1.56 g, 95% confidence interval [CI] = 0.96 to 2.16 g; P<.001; MCF-7, 1.12 versus 0.08 g, difference = 1.04 g, 95% CI = 0.77 to 1.31 g; P<.001; HepG2, 1.28 versus 0.35 g, difference = 0.93 g, 95% CI = 0.60 to 1.25 g; P<.001). Furthermore, TEM8 interacted with the M2 isoenzyme of pyruvate kinase (M2-PK), which has an important role in tumor growth and metastasis. TEM8-Fc is a novel therapeutic antibody-like agent in the management of solid tumors that may act by trapping M2-PK.