Mechanisms of hepatocyte adhesion to liver biomatrix proteins

Abstract

Laminin (Ln) is the most important cell-adhesion protein besides fibronectin. Recent data have suggested that liver sinusoidal Lns differ in chain composition from that of the Engelbert-Holmes-Swarm tumor (Ln-1). We compared the adhesion of freshly isolated rat hepatocytes to Ln-1 and a matrix extracted with dilute acetic acid from rat liver biomatrix (LBP). We demonstrated that Ln in LBP is the main adhesion protein for hepatocyte attachment. Close to 70% of cells attached to LBP after 15 min. at 37{dollar}\sp\circ{dollar}C. Cell adhesion was Mg{dollar}\sp{lcub}++{rcub}{dollar} and Mn{dollar}\sp{lcub}++{rcub}{dollar}-dependent and Ca{dollar}\sp{lcub}++{rcub}{dollar}- and insulin independent. EDTA prevented cell attachment. Synthetic peptides containing adhesion sequences present in Ln-1 (RGD and YIGSR) or an antibody to a cell-binding domain (SIKVAV) of the {dollar}\alpha{dollar}l chain did not prevent adhesion to LBP. LBP also has cell specificity since hepatocytes adhere to it preferentially as compared to other epithelial and mesenchymal cell lines.;Using LBP-affinity chromatography, LBP-binding proteins were isolated. Antibodies raised to these proteins immunodetected mainly components of 55 and 45 kDa with occasional cross-reactivity to a 68 kDa protein. These proteins are membrane associated rather than integral membrane components. Immunoprecipitation of {dollar}\sp{lcub}125{rcub}{dollar}I surface-labeled hepatocytes and immunohistochemical analyses of liver sections and cultured hepatocytes confirmed their membrane localization. With the exception of heart, proteins of similar molecular masses were found in kidney, spleen and lung. Affinity purified immunoglobulins to the LBP-binding proteins inhibited hepatocyte attachment to LBP but not to Ln-1. Incubation of liver membrane associated proteins with Ln-1 showed that Ln-1 co-precipitated with membrane components of 55 and 68 kDa that were recognized by antibodies to LBP-binding proteins.;In conclusion, Ln is the main attachment protein in LBP and differs from Ln-1 since it lacks an {dollar}\alpha{dollar}1 chain. Three membrane associated proteins of 68, 55 and 45 kDa bind to LBP, and at least two of these proteins (68 and 55 kDa) bind to Ln-1. Due to differences in chain composition between Ln-1 and LBP-laminin and in cell adhesion to the two substrata, further studies are needed to determine the nature of the receptors, the actual chain composition and domains of liver laminin to which hepatocytes adhere.