Novel non-TGF-beta-binding splice variant of LTBP-4 in human cells and tissues provides means to decrease TGF-beta deposition

Small latent TGF-beta consists of latency associated peptide (LAP) bound to the 25 kDa TGF-beta by noncovalent interactions. Small latent TGF-beta is secreted from cells and deposited into the extracellular matrix as covalent complexes with its binding proteins, LTBPs. Four LTBPs have been molecular ly cloned and their structures contain repetitive sequences. The 3(rd) 8-Cy s repeats of LTBP-1, -3 and -4 are able to associate with small latent TGF- beta. We analyzed by RT-PCR the expression of LTBPs 1-4 in a panel of cultu red human cell lines including fibroblasts of different origin, endothelial cells and immortalized keratinocytes. LTBPs were expressed in an overlappi ng manner, but differences in their expression levels were detected. SV-40 transformed human embryonic lung fibroblasts contained less of the mRNAs fo r the LTBPs, suggesting that malignant transformation leads to decrease in LTBP expression. A novel alternatively spliced form of LTBP-4 lacking the 3 (rd) 8-Cys repeat (LTBP-4 Delta8-Cys3(rd)) was identified. LTBP-4 Delta8-Cy s3(rd) does not bind TGF-beta and it was found to be expressed in the same tissues as the full length LTBP-4. The exon-intron structure of LTBP-4 arou nd the 3rd 8-Cys repeat was similar to those of LTBP-2 and -3. LTBP-4 Delta 8-Cys3(rd) was produced by alternative splicing over two exons. In addition , HL-60 promyelocytic leukemia cells expressed a splice variant lacking onl y one exon of this region. The expression of the non-TGF-beta -binding vari ant of LTBP-4 may be important for the regulation of TGF-beta deposition in tissues. Since LTBPs are a part of the extracellular matrix microfibrils, the LTBP-4 Delta8-Cys3(rd) protein may also be involved in various structur al functions not related to TGF-beta signaling.