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 s pace of adult rat liver. The principal cellular source of laminin in l iver is the lipocyte, with its three subunits measuring 324, 200 and 2 00 kD. The large subunit of lipocyte-derived laminin is distinct from the A subunit of murine laminin (440 kD); its size suggests that it re presents a peptide, called M, recently cloned from human placenta. Usi ng oligonucleotide primers derived from the human M-subunit cDNA, we a mplified 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-subu nit transcripts were identified only in lipocytes from normal adult li ver; they could not be identified in hepatocytes, endothelial cells or Kupffer cells. Lipocytes were screened for M-subunit protein with a p olyclonal M antiserum. Cells stained specifically for the M-subunit af ter 36 hr in primary culture; the protein was also identified in fresh ly isolated cells by means of immunoblotting. To determine whether lip ocytes alter their expression of the laminin M subunit during liver in jury, we monitored M-subunit mRNA in these cells at various intervals after carbon tetrachloride administration. M-subunit transcripts incre ased twofold within 12 hr of toxin exposure, returning to below baseli ne 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.