Elevated prolidase activity in keloids: correlation with type I collagen turnover

Background Keloid pathogenesis involves an altered balance of extracellular matrix metabolism, mainly accumulation of type I collagen. This could be due to excessive synthesis or decreased degradation of matrix, or a combination of both processes. Prolidase, an imidodipeptide-cleaving cytosolic enzyme, plays an important role in the collagen catabolic process by recycling proline for collagen synthesis. Collagen accumulation in keloids is due to an imbalance in the steady state of collagen turnover. Objectives To investigate prolidase activity and its role in the steady state of collagen turnover between normal skin and keloid tissue and their derived fibroblasts. Methods Ten sets of keloid and normal skin tissues and their derived fibroblasts were employed. Measurements were made of tissue prolidase activity, free proline level, and concentrations of the collagen synthesis product aminoterminal propeptide of type I procollagen (PINP) and the collagen degradative product carboxyterminal telopeptide of type I collagen (ICTP). Also, synthesis of collagens type I and III and matrix metalloproteinases 1 and 2 was investigated using Western blot analysis. Results Keloid tissues had a significant increase in prolidase activity, up to fourfold that in normal skin. The elevated prolidase activity was accompanied by an increase in tissue PINP and ICTP concentrations in keloid; in addition, the collagen turnover index (PINP/ICTP) was higher in keloids. Conclusions The combination of elevated prolidase activity and associated higher collagen synthesis to degradation ratio in keloids suggests a possible metabolic process for the excessive accumulation of type I collagen in keloids.