Evidence for the decreased expression of the latent TGF-beta binding protein and its splice form in human liver tumours

Background: Recently, a splice form of the latent TGF-beta binding protein (LTBP-1) was identified in the liver lacking potential important sequences for matrix association and proteinase cleavage (LTBP-1D, -1 Delta 53). For a better understanding of the unknown (patho)physiological role, the expres sion levels of LTBP-1D and LTBP-1 (full length) were investigated in normal and malignant human liver on the mRNA and protein level. Methods: Normal l iver (5 specimens), hepatocellular carcinoma (4 specimens) and fibrolamella r carcinoma (2 specimens) were examined by quantitative reverse transcripti on-polymerase chain reaction and immunohistochemistry, for which specific a ntibodies were generated. Results: The mRNA levels of LTBP-1/-1D in maligna nt liver tissues are decreased in comparison to normal liver-more so in HCC than in FLC. This finding was confirmed by a strong decrease of immunostai ning of LTBP-1/-1D in neoplastic parenchymal cells of HCC and FLC. However. the intensity of LTBP-1 (full length) protein staining was increased in th e extracellular matrix of the carcinomas, while LTBP-1D was not detectable in the matrix. Conclusion: Since TGF-fl is known to be over-expressed in li ver tumours, the results suggest its enhanced synthesis without binding to LTBP-1. This probably influences the availability of bioactive TGF-beta in the tumour tissue. The missing matrix localization of LTBP-1D indicates tha t the hinge region containing a heparin-binding site is essential for the b inding of LTBP-1 in the extracellular matrix. LTBP-1D may fulfil specific f unctions for the latency of matrix-unbound TGF-beta.