Contact Inhibition of Hepatocyte Growth Regulated by Functional Association of the c-Met/Hepatocyte Growth Factor Receptor and LAR Protein-tyrosine Phosphatase

Abstract

Inhibition of cell proliferation regulated by cell-cell contact is a fundamental characteristic of normal cells by which cellular adhesion successfully maintains formation and highly organized tissue architecture. On the other hand, the loss of contact inhibition is associated with cellular transformation, which is a precursor to malignant disorder (1-5). However, the molecular machineries involved in contact regulation are not fully understood.Although contact inhibition is a general characteristic of normal cells, the extent to which cells are inhibited by cell-cell contact differs significantly depending on cell types. Hepatocytes in primary culture provide a most appropriate experimental model in investigating contact inhibition (6 -9). Mature hepatocytes, which are terminally differentiated cells, undergo proliferation depending on exogenous growth factors. Proliferation of primary cultured hepatocytes is strictly regulated by cell-cell contact. When hepatocytes are cultured at sparse cell density, the cells undergo DNA synthesis in response to growth factors. However, when the cells are in a confluent density, DNA synthesis is mostly inhibited even in the presence of growth factors.Hepatocyte growth factor (HGF) 2 plays pivotal roles in hepatocyte proliferation through the specific receptor c-Met tyrosine kinase (10 -14). Disruption of the HGF-c-Met pathway in hepatocytes results in impaired liver regeneration (15, 16). c-Met tyrosine phosphorylation is a triggering event that forms phosphotyrosine-mediated activation cascades of the downstream signaling molecules of c-Met to elicit hepatocyte proliferation in response to HGF stimulation (14, 17). Conversely, it has been proposed that protein-tyrosine phosphatase (PTP) activities are involved in regulating the balance of tyrosine phosphorylation states of receptor tyrosine kinases, including c-Met and the possible downstream signal mediators, such as Shc, the Stat families, phospholipase C␥1, ␤-catenin, ␥-catenin, and p120 CAS in the normal liver (18). However, the precise role of PTPs in hepatocyte proliferation is not well clarified.In the present study, focusing on the activation states of c-Met, we investigated the mechanisms by which tight cell-cell contact in primary hepatocytes prevents mitogenic response to HGF. We found that LAR, a transmembrane protein-tyrosine phosphatase, plays a definitive role in inactivation, i.e. tyrosine dephosphorylation of c-Met through their physical interaction, which specifically occurs in hepatocytes under the confluent condition. Additionally, specific suppression of LAR expression prolonged tyrosine phosphorylation of c-Met and released contact inhibition. Our results indicate that regulation of the activation states of c-Met by LAR underlies contact inhibition of the mitogenic response to HGF in hepatocytes. MATERIALS AND METHODSCell Culture-Hepatocytes were prepared from 7-week-old male Sprague-Dawley rats by in situ perfusion of the liver with collagenase, as described elsewhere (6,19). The cells were cultured o...

From Tpr-Met to Met, tumorigenesis and tubes

“…Several phosphatases have been identified that target the Met receptor. These include the receptor phosphatases DEP1 (Palka et al, 2003) and LAR, where LAR inactivates the Met receptor, resulting in contact inhibition, and thereby preventing the mitogenic response of primary rat hepatocytes (Machide et al, 2006). In addition, Met is a target for the nonreceptor phosphatase PTP1B, and PTP1B null mice are protected from liver damage in part through elevated phosphorylation of Met (Sangwan et al, 2006).…”

Receptor-type Protein Tyrosine Phosphatase β (RPTP-β) Directly Dephosphorylates and Regulates Hepatocyte Growth Factor Receptor (HGFR/Met) Function

“…In addition to our finding that RPTP-␤ dephosphorylates Met, other RPTPs have been shown to influence Met tyrosine phosphorylation (44,60,61). DEP-1 has been shown to preferentially dephosphorylate tyrosines 1349 and 1365 (44).…”

“…PTP1B and TCPTP have been shown to dephosphorylate tyrosine 1234/ 1235 of Met (61). Using only the antisense method, Kulas et al (60) have shown that knockdown of RPTP-LAR increases tyrosine phosphorylation of insulin receptor, insulin receptor substrate-1, EGFR, and Met. The mechanism for the observed increased phosphorylation was not determined.…”

Autocrine stimulation of human hepatocytes triggers late DNA synthesis and stabilizes long-term differentiation in vitro

“…EGFr and c-Met both mediate mitogenic signalling, and contribute to cell-cycle progression (24,25), therefore serving as representative parameters to evaluate the growth activity of human hepatocytes. Notably, distinct c-Met up-regulation was only seen when hepatocytes were treated daily with new culture medium but did not occur in the autocrine system where the medium was not renewed.…”