Expression and release of the latent transforming growth factor β binding protein by hepatocytes from rat liver

In very recent studies it was established that transforming growth factor β (TGF-β), likely to be the most relevant fibrogenic cytokine and regulator of cell proliferation, differentiation, and matrix metabolism, is expressed by hepatocytes (parenchymal cell [PC]) and secreted from cultured PC in a latent form incapable of receptor binding. The structural composition of the latent TGF-β complex secreted by cultured PC is unknown. In some TGF-β expressing cell types this cytokine is released as a large molecular weight complex containing in addition to the TGF-β latency associated peptide (LAP) a disulfide bonded latent TGF-β binding protein (LTBP), of which the existence and function in liver is hitherto unknown. This study is directed to the identification of LTBP expression in rat PC. Cells were isolated from rat liver with the collagenase method and analyzed for LTBP before and during culture under standard conditions using alkaline phosphatase anti-alkaline phosphatase (APAAP) immunostainings, metabolic labeling, messenger RNA (mRNA) detection (reverse-transcription polymerase chain reaction [RT-PCR]) and sequencing, and immunoblotting of gel chromatographically separated cell extracts and conditioned media, respectively. APAAP immunostainings applying a specific polyclonal LTBP-antiserum (ab 39) indicated expression of LTBP in PC of liver in situ and freshly isolated PC but a strong expression in cultured PC. Transcripts of LTBP-1 were detected by RT-PCR and confirmed by sequence analyses. Metabolic labeling of PC with 35S-Met/Cys followed by immunoprecipitation of cell lysates with LTBP antiserum confirmed the synthesis of the high molecular mass complex of 250 kd containing LTBP with a molecular mass of 160 kd. Latent TGF-β complexes, associated with LTBP related proteins, could be separated from both extracts and conditioned media of PC by gel filtration chromatography. They confirmed the release of the large latent TGF-β complex from PC. Investigations of immunocytochemical LTBP staining under different culture conditions (TGF-β supplementation, extracellular matrices) point to concordant variations of LTBP and TGF-β expression. The results suggest a role for PC in paracrine- and autocrine-mediated effects of TGF-β, which might be important for various cell activities in healthy and diseased liver.