Prolyl 4-hydroxylase isoenzymes I and II have different expression patterns in several human tissues.
Prolyl 4-hydroxylase plays a central role in the synthesis of all collagens. We have previously reported that the recently identified Type II isoenzyme is its main form in chondrocytes and possibly in capillary endothelial cells, while Type I is the main form in many other cell types. We report here that the Type II isoenzyme is clearly the main form in capillary endothelial cells and also in cultured umbilical vein endothelial cells, whereas no Type I isoenzyme could be detected in these cells by immunostaining or Western blotting. The Type II isoenzyme was also the main form in cells of the developing glomeruli in the fetal kidney and tubular structures of collecting duct caliber in both fetal and adult kidney, in occasional sinusoidal structures and epithelia of the bile ducts in the liver, and in some cells of the decidual membrane that probably represented invasive cytotrophoblasts in the placenta. Osteoblasts in a fetal calvaria, i.e., a bone developing by intramembranous ossification, stained strongly for both types of isoenzyme. The Type I isoenzyme was the main form in undifferentiated interstitial mesenchymal cells of the developing kidney, for example, and in fibroblasts and fibroblastic cells in many tissues. Skeletal myocytes and smooth muscle cells appeared to have the Type I isoenzyme as their only prolyl 4-hydroxylase form. Hepatocytes expressed small amounts of the Type I enzyme and very little if any Type II, the Type I expression being increased in malignant hepatocytes and cultured hepatoblastoma cells. The data suggest that the Type I isoenzyme is expressed especially by cells of mesenchymal origin and in developing and malignant tissues, whereas the Type II isoenzyme is expressed, in addition to chondrocytes and osteoblasts, by more differentiated cells, such as endothelial cells and cells of epithelial structures. (J Histochem Cytochem 49:1143-1153, 2001)