Age-dependent variations of the biosyntheses of fibronectin and fibrous collagens in mouse skin.

Skin explant cultures from hairless mice of increasing age were incubated with radioactive precursors in order to determine the age-dependent variations of the biosyntheses of fibronectin and fibrous collagens (types I and III). Total collagen synthesis expressed as a percentage of total protein synthesis did not vary with age but, if expressed as micrograms hydroxyproline per mg wet weight of skin, decreased by about 30% between 2 and 22 months of age. Hydroxylation of collagen, expressed as the ratio of 3H-hypro over 3H (pro + hypro) incorporated in freshly synthesized collagen, decreased with age by about 40% between 2 and 22 months of age. The proportion of type III collagen expressed as % of type I + type III collagens increased progressively with age by about 25% at 12 months to 60% at 22 months of age. Fibronectin biosynthesis, determined by immunoprecipitation of 35S-methionine labeled peptides in SDS-extracts of skin increased progressively with age from about 2% of total incorporated radioactivity in fibronectin at 2 months to 4% at 22 months. Plasma fibronectin, of hepatic origin, was shown already to increase with age in humans. It appears thus that the expression of genes coding for extracellular matrix macromolecules is under age-dependent regulation. This regulation appears to be different for the investigated macromolecules.