The potential mechanism for glutamine-induced collagen biosynthesis in cultured human skin fibroblasts.

Although glutamine (Gln) is known as an important stimulator of collagen biosynthesis in collagen-producing cells, the mechanism and endpoints by which it regulate the process remain largely unknown. Intermediates of Gln interconversion: glutamate (Glu) and pyrroline-5-carboxylate (P5C) stimulate collagen biosynthesis in cultured cells but evoke different maxima of collagen biosynthesis stimulating activity at different times of incubation. P5C was found to be the most potent stimulator of collagen biosynthesis after 6 h of incubation (approx. three-fold increase); after 12 h, it induced increase in collagen biosynthesis to 260%, while at 24 h, the process was decreased to approximately 80% of control values. Glu induced increase in collagen biosynthesis to approximately 180%, 400% and 120% of control values, after 6, 12 and 24 h, respectively, suggesting that after 12 h of incubation, Glu was the most potent stimulator of collagen biosynthesis. Glu was also the most potent stimulator of type I procollagen expression at this time. After 6, 12 and 24 h incubation, Gln induced collagen biosynthesis to approximately 112, 115 and 230% of control values, respectively. Since prolidase is known to be involved in collagen metabolism, the enzyme activity assay was performed in fibroblasts cultured in the presence of Gln, Glu and P5C. While Gln and Glu required 24 h for maximal stimulation of prolidase activity, P5C induced it after 6-12 h. The data suggest that P5C induced collagen biosynthesis and prolidase activity in a shorter time than Gln and Glu. We considered that P5C directly stimulates the processes, while Gln acts through its intermediate-P5C. Reduction of P5C to proline is coupled to the conversion of glucose-6-phosphate (G6P) to 6-phospho-gluconate, catalyzed by G6P dehydrogenase. We have found that dehydroepiandrosterone (DHEA), a potent inhibitor of G6P dehydrogenase, inhibited a stimulatory effect of P5C on collagen synthesis, expression of type I collagen and prolidase activity. Our results postulate a potential mechanism of glutamine-induced collagen biosynthesis through its intermediate - P5C. P5C-dependent activation of nucleotide biosynthesis, prolidase activity and P5C conversion into proline may contribute to the stimulation of collagen biosynthesis.