Changes in transcripts of basement components during rat liver development: increase in laminin messenger RNAs in the neonatal period

Regulation of hepatocyte cell cycle progression and differentiation by type I collagen structure

Cell behavior is strongly influenced by the extracellular matrix (ECM) to which cells adhere. Both chemical determinants within ECM molecules and mechanical properties of the ECM network regulate cellular response, including proliferation, differentiation, and apoptosis. Type I collagen is the most abundant ECM protein in the body with a complex structure that can be altered in vivo by proteolysis, cross-linking, and other processes. Because of collagen's complex and dynamic nature, it is important to define the changes in cell response to different collagen structures and its underlying mechanisms. This chapter reviews current knowledge of potential mechanisms by which type I collagen affects cell behavior, and it presents data that elucidate specific intracellular signaling pathways by which changes in type I collagen structure differentially regulate hepatocyte cell cycle progression and differentiation. A network of polymerized fibrillar type I collagen (collagen gel) induces a highly differentiated but growth-arrested phenotype in primary hepatocytes, whereas a film of monomeric collagen adsorbed to a rigid dish promotes cell cycle progression and dedifferentiation. Studies presented here demonstrate that protein kinase A (PKA) activity is significantly elevated in hepatocytes on type I collagen gel relative to collagen film, and inhibition of this elevated PKA activity can promote hepatocyte cell cycle progression on collagen gel. Additional studies are presented that examine changes in hepatocyte cell cycle progression and differentiation in response to increased rigidity of polymerized collagen gel by fiber cross-linking. Potential mechanisms underlying these cellular responses and their implications are discussed.

Evidence for a role of insulin in hepatocytic differentiation of human hepatoma BC1 cells

To examine the effect of insulin on hepatocytic differentiation, we took advantage of the properties of the newly established human hepatoma BC1 cell line to maintain quiescence after confluency and to progressively acquire in culture (3 weeks after confluency) an hepatocytic phenotype, as assessed by expression of specific hepatic genes (Le Jossicet al., 1995). In BC1 cells cultured in the presence of insulin (1 μM: ), expression of albumin and transferrin mRNA and protein occurs earlier than in cells cultured in its absence (1 weekvs 2 weeks). Moreover, at any time considered, the level of the two hepatic markers was higher (2- to 3-fold) in the former than in untreated cells. The beneficial effect of insulin on hepatocytic differentiation of BC1 cells was paralleled by: i) modest increases in insulin receptor (IR) mRNA level and IR binding activity, and ii) a 6-fold increase in sensitivity to insulin for stimulation of glycogenesis. These results provide the first evidence for insulin's ability to exert a positive effect on hepatocytic differentiation. The beneficial effect of insulin probably results both from increased IR expression and binding activity and from alteration at post-receptor levels.

Partial cloning of the M subunit of laminin from adult rat lipocytes: expression of the M subunit by cells isolated from normal and injured liver

Laminin is a heterotrimeric glycoprotein found in the perisinusoidal space of adult rat liver. The principal cellular source of laminin in liver is the lipocyte, with its three subunits measuring 324, 200 and 200 kD. The large subunit of lipocyte-derived laminin is distinct from the A subunit of murine laminin (440 kD); its size suggests that it represents a peptide, called M, recently cloned from human placenta. Using oligonucleotide primers derived from the human M-subunit cDNA, we amplified a 445-bp sequence encoding a fragment of M-laminin from adult rat lipocytes. The rat cDNA is 90% homologous to the human M-subunit cDNA and recognizes an mRNA in lipocytes measuring about 10 kb. M-subunit transcripts were identified only in lipocytes from normal adult liver; they could not be identified in hepatocytes, endothelial cells or Kupffer cells. Lipocytes were screened for M-subunit protein with a polyclonal M antiserum. Cells stained specifically for the M-subunit after 36 hr in primary culture; the protein was also identified in freshly isolated cells by means of immunoblotting. To determine whether lipocytes alter their expression of the laminin M subunit during liver injury, we monitored M-subunit mRNA in these cells at various intervals after carbon tetrachloride administration. M-subunit transcripts increased twofold within 12 hr of toxin exposure, returning to below baseline by 48 hr. The results indicate that lipocytes produce the M subunit of laminin in place of A. Production of this subunit by lipocytes may facilitate cell growth and reorganization during liver regeneration.