Matrix proteases play a critical role in cell invasion and migration, including the process of angiogenesis. The ability of specific factors to induce angiogenic responses correlates with their stimulation of matrix protease synthesis and release. Using an in vivo angiogenesis assay, the endothelial cell response to known angiogenic factors, basic fibroblast growth factor (bFGF) and adipocyte conditioned medium, was blocked by an inhibitor of matrix metalloproteinase activity, TIMP-1. The TIMP effect was mediated, at least in part, through the inhibition of endothelial cell migration, as determined by the ability of TIMP to block chemotaxis in a Boyden chamber assay. These results indicate that the inhibition of migration is a direct effect on the endothelial cells and does not require accessory cells. An additional observation was that the RNA levels for TIMP were significantly reduced in differentiated adipocytes, compared to undifferentiated F442A controls. Therefore, the acquisition of an angiogenic phenotype may involve not only the induction of positive factors, but also the suppression of angiogenesis inhibitors.

The erythrocyte anion transport protein (band 3) mediates two distinct cellular functions: it provides plasma membrane attachment sites for the erythroid cytoskeletal network, and it also functions as the anion transporter between the erythrocyte cytoplasm and extracellular milieu. We previously showed that two chicken band 3 polypeptides are encoded by two different mRNAs with different translation initiation sites. Here we show that these two band 3 mRNAs are transcribed from two separate promoters within a single gene. In addition, the two pre-mRNAs are differentially spliced, leading to fusion with coding exons used in common in the two mRNAs. The chicken erythrocyte band 3 gene is therefore the first example of a gene that has two promoters within a single locus which function equally efficiently in one cell type at the same developmental stage.

The chicken erythrocyte anion transport protein (band 3 of the erythrocyte cytoskeleton) is a central component taking part in two widely divergent functions of erythroid cells; it is a primary determinant of cytoskeletal architecture and responsible for electroneutral Cl-/HCO3- exchange across the plasma membrane. To analyze interesting aspects of the developmental regulation of this gene, we have cloned the cDNA and genomic counterparts of the erythroid-specific anion transport protein. We show that a single genetic locus for band 3 encodes two different erythroid cell-specific mRNAs, with different translational initiation sites, which predict polypeptides of sizes very close to those observed in vivo. In vitro translation and immune precipitation of synthetic mRNA derived from one putative fully encoding cDNA clone demonstrate that this clone gives rise to a protein which is identical in size and antigenicity to bona fide chicken erythroid band 3.