GALNT3 encodes UDP-N-acetyl-alpha-d-galactosamine: polypeptide N-acetylgalactosaminyl-transferarase 3 (ppGalNacT3), a glycosyltransferase which has been suggested to prevent proteolysis of FGF23, a potent phosphaturic protein. Accordingly, loss-of-function mutations in GALNT3 cause hyperphosphatemic familial tumoral calcinosis (HFTC), a rare autosomal recessive disorder manifesting with increased kidney reabsorption of phosphate, resulting in severe hyperphosphatemia and widespread ectopic calcifications. Although these findings definitely attribute a role to ppGalNacT3 in the regulation of phosphate homeostasis, little is currently known about the factors regulating GALNT3 expression. In addition, the effect of decreased GALNT3 expression in peripheral tissues has not been explored so far. In the present study, we demonstrate that GALNT3 expression is under the regulation of a number of factors known to be associated with phosphate homeostasis, including inorganic phosphate itself, calcium and 1,25-dihydroxyvitamin D(3). In addition, we show that decreased GALNT3 expression in human skin fibroblasts leads to increased expression of FGF7 and of matrix metalloproteinases, which have been previously implicated in the pathogenesis of ectopic calcification. Thus, the present data suggest that ppGalNacT3 may play a role in peripheral tissues of potential relevance to the pathogenesis of disorders of phosphate metabolism.