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Fetuins are serum proteins with diverse functions including the regulation of
osteogenesis and inhibition of unwanted mineralization. Besides the
alpha2-Heremans and Schmid glycoprotein/fetuin-A, the recently identified
fetuin-B is a second member of the fetuin family [Olivier, Soury, Risler, Smih,
Schneider, Lochner, Jouzeau, Fey and Salier (1999) Genomics 57, 352-364;
Olivier, Soury, Ruminy, Husson, Parmentier, Daveau and Salier (2000) Biochem. J.
350, 589-597], which belongs to the cystatin superfamily. We compared the
expressions of fetuin-B and fetuin-A at the RNA level and established that both
genes are most highly expressed in liver tissue. Like fetuin-A, fetuin-B mRNA is
also highly expressed in tongue and placenta tissues. We demonstrated for the
first time that fetuin-B is also expressed at the protein level in sera and
several organs of mouse, rat and human. We isolated contiguous genomic clones
containing both fetuin-B and fetuin-A genes, indicating that these genes are
closely linked at the genome level. The close proximity of both these genes may
explain our observation that fetuin-B expression was decreased in
fetuin-A-deficient mice. Unlike fetuin-A, the amount of fetuin-B protein in
human serum varied with gender and was higher in females than in males.
Functional analysis revealed that fetuin-B, similarly to fetuin-A, is an
inhibitor of basic calcium phosphate precipitation, albeit less active when
compared with fetuin-A. Therefore fetuin-B may have a function that is partly
overlapping, if not identical, with the function of fetuin-A.
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