The Matrix Biochemistry Section focuses its research on the functions of five major noncollagenous proteins first found associated with the mineralized matrix of bones and teeth. These include: bone sialoprotein (BSP);osteopontin (OPN);dentin matrix protein 1 (DMP1);dentin sialophosphoprotein (DSPP);and matrix extracellular phosphoglycoprotein (MEPE). We have made a strong case for the genetic relatedness of these seemingly different proteins and there is increasing acceptance of the SIBLING (Small Integrin-Binding LIgand, N-linked Glycoprotein) family concept. The genes encoding these proteins are all clustered in a tandem fashion within a short (400,000 basepair) region of human chromosome 4. Because the same genes are clustered together in all mammals studied, they are likely to be the result of gene duplications and subsequent divergence more than 180 million years ago. One of the conserved motifs among the different SIBLING members (and among animal species) is the integrin-binding tripeptide, arginine-glycine-aspartate (RGD), used to bind the SIBLINGs to the cell surface. In recent years, we have shown that at least three members of the SIBLING family bind and activate different members of the matrix metalloproteinase (MMP) family and may modify MMP activities by bridging these otherwise soluble proteins to cell surfaces. This year, in a continuing collaboration with Dr. Fedarko of Johns Hopkins University, we studied the effects of BSP on both MMP-2's natural inhibitors as well as synthetic inhibitors developed to be anti-metastasis cancer drugs. In both cases, BSP enabled the MMP-2 to function even in the presence of its inhibitors. One interesting implication of this work is that drugs which were designed to interfere with cancer metastasis (including formation of new blood vessels that feed the growing tumors) by blocking MMP activities, should now be rescreened for their ability to function in the presence of SIBLINGs because these proteins are known to be synthesized by many tumors. We have also shown that another SIBLING, DSPP, is elevated in the blood of prostate cancer patients. The results suggest that the combination of DSPP and the well-known PSA serum assays may complement each other and lead to a more accurate diagnosis. The current study also provides data suggesting that serum DSPP levels may be useful in the future for: 1) following the progress of prostate cancer;2) judging the success of intervention protocol shortly after it has been performed;as well as 3) the recurrence of cancer months or years after treatment. The Section has also continued its studies on the biochemistry and genetics of DSPP. This protein (possibly the most acidic and hydrophilic protein made by humans) is the most abundant noncollagenous protein found in dentin. Indeed, all of the known dentin-specific genetic diseases in humans have been shown to involve mutations in the DSPP gene. Although several families with dentinogenesis imperfecta (DGI) or the less severe dentin dysplasia (DD) have been reported to have mutations in the first 5% (amino-terminus) of the DSPP protein sequence, the mutations of most patients could not be found. By modifying several biochemical and microbiological techniques, we were able to uncover the mutations in the technically challenging region of DSPP for all of the problematical DD and DGI families whose DNA was available to us. In each case, the patients had lost small amounts of DNA in their DSPP gene causing the normally soluble protein to become completely insoluble. This insoluble form of DSPP was hypothesized to interfere with the production of normal dentin and to cause the teeth to become easily damaged and worn. Using our newly published insights and techniques, other laboratories will now be able to uncover the mutations in their DGI and DD patients. We have continued our studies into the physiological roles of the conserved integrin-binding tripeptide, RGD, in the SIBLINGs. We are particularly intrigued with the fact that a single cell often simultaneously makes all five SIBLINGs and we wondered how the cell differentially responds to each of them. Furthermore, because an abundant blood protein (complement Factor H) appears to quickly bind and inactivate SIBLINGs, the functions of these proteins would appear to be limited to very short distances. In a recent publication, we showed that cells can respond differently to at least three of the five SIBLING proteins by using specific combinations of RGD-binding integrins. Cells required specific integrins to attach to a SIBLING-related matrix while they used other varieties when they migrated to a new location. Together these studies show the spectrum of biological functions that the SIBLING family of proteins may play in both health and disease.
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