Arginine reverses growth hormone resistance through the inhibition of toll-like receptor 4-mediated inflammatory pathway.

OBJECTIVE: Growth hormone stimulates growth by increasing insulin-like growth factor 1 expression and secretion. In the presence of insufficient nutrients, GH increases, whereas IGF-1 expression becomes severely suppressed, leading to GH resistance. This study aimed to explore the effect of arginine (Arg) on GH resistance during malnutrition and to describe its underlying mechanism. METHODS: C57BL/6J mice were injected intraperitoneally with Arg for 1h or subjected to caloric restriction with Arg supplement in drinking water for 18days. HepG2 cells were exposed to different Arg concentrations for 24h. Signaling pathway agonists/inhibitors, siRNA, and overexpression plasmids were used to investigate the underlying molecular mechanism. Liver-specific toll-like receptor (TLR4) knockout mice were utilized to clarify the role of TLR4 in Arg-induced IGF-I expression and secretion. RESULTS: Arg inhibited the TLR4 downstream pathway by binding to TLR4 and consequently activated Janus kinase 2/signal transducer and activator of transcription 5 signaling pathway. As a result, IGF-1 transcription and secretion increased. Arg activity was absent in liver-specific TLR4 knockout mice and was greatly suppressed in liver with overexpressed TLR4, suggesting that hepatic TLR4 was required and sufficient to induce GH resistance. By contrast, the mammalian target of rapamycin pathway was unnecessary for Arg activity. Arg not only significantly increased IGF-1 expression and secretion under acute fasting and chronic CR conditions but also attenuated body weight loss. CONCLUSIONS: Our results demonstrate a previously unappreciated pathway involving Arg that reverses GH resistance and alleviates malnutrition-induced growth restriction through the inhibition of TLR4-mediated inflammatory pathway.

Nuclear localization of STAT5A modified with O-linked N-acetylglucosamine and early involution in the mammary gland of Hirosaki hairless rat.

Hirosaki hairless rat (HHR) is a mutant strain spontaneously derived from Sprague-Dawley rats (SDR), and its inheritance is autosomal recessive. In addition to hair loss, female HHRs show involution of the mammary gland at an early stage of lactation. In the present study we investigated the mammary gland development in HHR. Morphological examinations revealed that HHR mammary glands are underdeveloped in virgins and exhibit distended alveoli on day 1 of lactation (L1), followed by involution. Milk secretion was observed on L1 in HHR. Whey acidic protein and other proteins were increased in milk of HHR and heterozygous rats on SDS-polyacrylamide gel electrophoresis. Terminal deoxynucleotide transferase-mediated dUTP nick-end labeling assay revealed apoptosis induction in HHRs at an early stage of lactation. By Western blotting, signal transducer and activator of transcription (STAT) 5A levels in cytoplasmic and nuclear fractions of the mammary glands were not different between HHR and SDR on L1 and L7. Nuclear localization of STAT5A in HHR and SDR was confirmed by immunohistochemistry. Tyr-phosphorylated STAT5A was not detected in HHR but was detected in SDR nuclear fractions. Several proteins modified with O-linked N-acetylglucosamine (O-GlcNAc) were detected in HHR nuclear extract on L1, although not in SDR or heterozygous rats by Western blotting. When HHR nuclear extract was applied to wheat germ agglutinin-agarose, a part of STAT5A was recovered in bound fractions. STAT5A of SDR or heterozygous rat nuclei were not bound to the lectin. Electrophoretic mobility shift assay revealed that STAT5A modified with O-GlcNAc is bound to the STAT5-responsive element. These results indicate that the mammary glands of HHR showed terminal differentiation for a short period, followed immediately by involution. In HHR, STAT5A is modified with O-GlcNAc but is not Tyr-phosphorylated. This type of glycosylation is suggested to be involved in the transient activation of STAT5A in HHR.