Immunofluorescence localization of prolyl 4-hydroxylase isoenzymes and type I and II collagens in bone tumours: type I enzyme predominates in osteosarcomas and chondrosarcomas, whereas type II enzyme predominates in their benign counterparts

Prolyl 4-hydroxylase is the key enzyme of synthesis of collagens. Hydroxylation of a sufficient number of proline residues to hydroxyproline is necessary for the stability of triple helices in collagenous proteins, because non-hydroxylated non-triple-helical collagen polypeptide chains are degraded intracellularly. We studied 15 primary chondrosarcomas and osteosarcomas, 17 benign bone tumours and one case of fibrous dysplasia and chordoma using immunofluorescence staining with antibodies against the α(I) and α(II) subunits of type I and II prolyl 4-hydroxylases, and with antibodies against collagen types I and II. Type I prolyl 4-hydroxylase was found to be the predominant isoenzyme in both types of bone sarcoma, whereas the type II enzyme was more readily expressed by benign tumours. A feature of collagen staining, that was common to both sarcoma types, was that collagen types I and II were mainly found within cancer cells and were rarely present extracellularly. Extracellular collagen staining was more obvious in benign tumours. The results show that expression of prolyl 4-hydroxylase isoenzymes is altered in bone sarcomas as compared with normal bone tissue. Chondrous cells, which normally express mainly the type II isoenzyme, switch their expression pattern to that of type I. The findings provide evidence that type I is the major isoenzyme in malignant bone tumours, and probably in malignant neoplasms in general. The pattern of enzyme expression is considered to be associated with dedifferentiation of cancer cells.