title: Leucine restriction inhibits chondrocyte proliferation and differentiation through mechanisms both dependent and independent of mTOR signaling.
abstract: Linear growth in children is sensitive to nutritional status. Amino acids, in particular leucine, have been shown to regulate cell growth, proliferation, and differentiation through the mammalian target of rapamycin (mTOR), a nutrient-sensing protein kinase. Having recently demonstrated a role for mTOR in chondrogenesis, we hypothesized that leucine restriction, acting through mTOR, would inhibit growth plate chondrocyte proliferation and differentiation. The effect of leucine restriction was compared with that of the specific mTOR inhibitor, rapamycin. Leucine restriction produced a dose-dependent inhibition of fetal rat metatarsal explant growth. This was accounted by reduced cell proliferation and hypertrophy but not apoptosis. mTOR activity, as reflected by ribosomal protein S6 phosphorylation, was only partially inhibited by leucine restriction, whereas rapamycin abolished S6 phosphorylation. In chondrogenic ATDC5 cells, leucine restriction inhibited cell number, proteoglycan accumulation, and collagen X expression despite minimal inhibition of mTOR. Microarray analysis demonstrated that the effect of leucine restriction on ATDC5 cell gene expression differed from that of rapamycin. Out of 1,571 genes affected by leucine restriction and 535 genes affected by rapamycin, only 176 genes were affected by both. These findings indicate that the decreased chondrocyte growth and differentiation associated with leucine restriction is only partly attributable to inhibition of mTOR signaling. Thus nutrient restriction appears to directly modulate bone growth through unidentified mTOR-independent mechanisms in addition to the well-characterized mTOR nutrient-sensing pathway.
authors.affiliation: Division of Pediatric Endocrinology and Metabolism, Rhode Island Hospital, 593 Eddy St., Providence, RI 02903, USA.
authors.forename: Mimi S
authors.initials: MS
authors.lastname: Kim
authors.forename: Ke Ying
authors.initials: KY
authors.lastname: Wu
authors.forename: Valerie
authors.initials: V
authors.lastname: Auyeung
authors.forename: Qian
authors.initials: Q
authors.lastname: Chen
authors.forename: Philip A
authors.initials: PA
authors.lastname: Gruppuso
authors.forename: Chanika
authors.initials: C
authors.lastname: Phornphutkul
mesh_terms: D000818:Animals; D000903:Antibiotics, Antineoplastic / Q000494:pharmacology; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology; D002455:Cell Division / Q000187:drug effects / Q000502:physiology; D019902:Chondrocytes / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D007930:Leucine / Q000378:metabolism / Q000494:pharmacology*; D008682:Metatarsal Bones / Q000166:cytology; D020411:Oligonucleotide Array Sequence Analysis; D009924:Organ Culture Techniques; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000235:genetics / Q000378:metabolism*; D011247:Pregnancy; D051381:Rats; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases
chemical_list: D000903:Antibiotics, Antineoplastic; D002352:Carrier Proteins; D017853:Phosphotransferases (Alcohol Group Acceptor); C546845:mTOR protein, rat; D058570:TOR Serine-Threonine Kinases; D007930:Leucine; D020123:Sirolimus