ANALYSIS OF THE EXPRESSION PATTERN OF THE LATENT TRANSFORMING-GROWTH-FACTOR-BETA BINDING-PROTEIN ISOFORMS IN NORMAL AND DISEASED HUMAN LIVER REVEALS A NEW SPLICE VARIANT MISSING THE PROTEINASE-SENSITIVE HINGE REGION

Latent transforming growth factor beta binding protein (LTBP), a compo nent of the extracellular matrix (ECM) of various tissues, is importan t for the secretion of TGF-beta and, furthermore, for the storage of T GF-beta in ECM. The proteolytic cleavage of LTBP is assumed to he the prerequisite for the activation of TGF-beta. We investigated the mRNA expression pattern of the three LTBP isoforms (LTBP-1, -2, -3) and the protein distribution of the components of the large latent TGF-beta c omplex, namely LTBP-1 and -2, latency-associated protein (LAP), and TG F-beta, in human liver using reverse-transcription polymerase chain re action (RT-PCR) and immunhistochemical alkaline phosphatase anti-alkal ine phosphatase (APAAP) staining. Parts of explanted livers diagnosed as hepatitis B, hepatitis C, primary biliary cirrhosis (PBC), and prim ary sclerosing cholangitis (PSC) and normal liver tissue were examined . LTBP transcripts were detected in the same manner in all liver speci mens. interestingly, we found a new splice variant of LTBP-1 (LTBP-1D) , in which the sequence coding for the proteinase-sensitive hinge regi on is deleted. The corresponding parts of the human LTBP-2 and LTBP-3 cDNA coding for the hinge region were sequenced and show neither simil ar proteinase cleavage sites nor deleted cDNA sequences. The proposed proteinase cleavage site of mouse LTBP-3 seems not to be conserved in the human LTBP-3 gene. By immunohistochemistry, LTBP-1, -2, and LAP we re detectable in normal and diseased livers and showed a different sta ining pattern for both LTBP isoforms. By contrast, TGF-beta showed a s potted staining pattern in diseased livers only, predominantly in the area of parenchymal cells that are close to fibrotic tissue. This stro ngly suggests the release of active TGF-beta from preexisting latent c omplexes. The LTBP-1D splice variant, which is probably less sensitive against proteolytic degradation and therefore may protect TGF-beta fr om activation, may have importance for modulating the biological activ ity of TGF-beta in normal and diseased liver.