No abstract

Skin aging generally results in a reduction in the amount of connective tissue, and concomitant disorganization of its structure. The most abundant connective proteins are members of the collagen family, with more than 20 different collagen types identified to date. Collagen makes up 70-80% of the dry weight of the skin, and contributes to the stability and structural integrity of tissues. A number of additional functional roles are carried out by additional protein domains. [1][2][3][4] The deposition of collagen is finely controlled and is dependent on the physiological status of the body. Thus, the control of collagen metabolism could potentially be very useful in a variety of therapeutic and cosmetic applications.Alpha-ketoglutarate (a-KG), a rate-determining intermediate in the Krebs cycle, plays a crucial role in cellular energy metabolism. It also functions as a source of glutamate and glutamine, and as an ammonium ion scavenger in hepatocytes, via its transformation into glutamate.5-7) Moreover, enteric feeding of a-KG was found to increase circulating plasma levels of such hormones as insulin, growth hormone, and insulin like growth factor-1 (IGF-1). 8,9) It has been reported that a-KG is involved in collagen metabolism through a variety of mechanisms. First, a-KG is a cofactor of prolyl-4-hydroxylase (P4H). P4H is located within the endoplasmic reticulum (ER), and catalyze the formation of 4-hydroxyproline, which is essential for formation of the collagen triple helix. Incomplete hydroxylation of proline residues within the repeated amino acid motif: any amino acid-proline-glycine (X-Pro-Gly), results in incomplete formation of the collagen triple helix. Incorrectly folded triple helices are not secreted into cytoplasm, and are subsequently degraded in the ER.10-12) Second, a-KG contributes to collagen synthesis by increasing the pool of proline residues via glutamate.13) Proline is a primary substrate for collagen synthesis, and plays a significant role in collagen metabolism. As seen in Fig. 1, proline is generated through the conversion of pyrroline 5-carboxylate (P5C), an intermediate in the interconversion of proline, ornithine and glutamate. Recently, it was shown that in addition to being a source of proline residues through the P5C-pathway, P5C activated collagen production through the activation of prolidase, a key enzyme in proline recyling.14) This is significant, Alpha-ketoglutarate is a key intermediate in the Krebs cycle, and a rate-limiting cofactor of prolyl-4-hydroxylase. It also has a potent effect on increasing the proline pool during collagen production, but the details underlying the boosting effect on collagen production by a a-ketoglutarate remain as yet unreported. To investigate the effects of a a-ketoglutarate on procollagen production and wrinkle formation, we conducted experiments in cultured human dermal fibroblasts and UVB-irradiated hairless mice. Based on ELISA measurements, a a-ketoglutarate (10 m mM) stimulated procollagen production in fibroblasts by 25.6؎4.6% compare...