The alpha 1-adrenergic receptors activate a phospholipase C enzyme by coupling to members of the large molecular size (approximately 74 to 80 kilodaltons) G alpha h family of guanosine triphosphate (GTP)-binding proteins. Rat liver G alpha h is now shown to be a tissue transglutaminase type II (TGase II). The transglutaminase activity of rat liver TGase II expressed in COS-1 cells was inhibited by the nonhydrolyzable GTP analog guanosine 5'-O-(3-thiotriphosphate) or by alpha 1-adrenergic receptor activation. Rat liver TGase II also mediated alpha 1-adrenergic receptor stimulation of phospholipase C activity. Thus, G alpha h represents a new class of GTP-binding proteins that participate in receptor signaling and may be a component of a complex regulatory network in which receptor-stimulated GTP binding switches the function of G alpha h from transglutamination to receptor signaling.

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Effectors involved in G protein-coupled receptor signaling modulate activity of GTPases through GTPaseactivating protein or guanine nucleotide exchanging factor (GEF). Phospholipase C␦1 (PLC␦1) is an effector in tissue transglutaminase (TGII)-mediated ␣ 1B -adrenoreceptor (␣ 1B AR) signaling. We investigated whether PLC␦1 modulates TGII activity. PLC␦1 stimulated GDP release from TGII in a concentration-dependent manner, resulting in an increase in GTP␥S binding to TGII. PLC␦1 also inhibited GTP hydrolysis by TGII that was independent from the ␣ 1B AR. These results indicate that PLC␦1 is GEF for TGII and stabilizes the GTP⅐TGII complex. When GEF function of PLC␦1 was compared with that of the ␣ 1B AR, the ␣ 1B AR-mediated GTP␥S binding to TGII was greater than PLC␦1-mediated binding and was accelerated in the presence of PLC␦1. Thus, the ␣ 1B AR is the prime GEF for TGII, and GEF activity of PLC␦1 promotes coupling efficacy of this signaling system. Overexpression of TGII and its mutants with and without PLC␦1 resulted in an increase in ␣ 1B AR-stimulated Ca 2؉ release from intracellular stores in a TGIIspecific manner. We conclude that PLC␦1 assists the ␣ 1B AR function through its GEF action and is primarily activated by the coupling of TGII to the cognate receptors.

Leucine-rich glioma inactivated (LGI)3 is a secreted protein member of LGI family. We previously repo rted that LGI3 was upregulated in adipose tissues from obese mice and suppressed adipogenesis through its receptor, a disintegrin and metalloproteinase domain-containing protein 23 (ADAM23). We demonstrated that LGI3 regulated tumor necrosis factor-α and adiponectin, and proposed that LGI3 may be a pro-inflammatory adipokine involved in adipose tissue inflammation. In this study, we analyzed adipokine and cytokine profiles in LGI3 knockout mice and demonstrated that multiple factors were increased or decreased in the adipose tissues and plasma of the LGI3 knockout mice. Phosphoprotein array analysis revealed increases in the phosphorylation levels of Akt, AMP-activated protein kinase (AMPK), Bad, extracellular signal-regulated kinase (Erk)1/2, glycogen synthase kinase 3α (GSK3α), phosphatase and tensin homolog (PTEN) and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1) in the LGI3treated 3T3-L1 pre-adipocytes. Treatment with LGI3 increased the expression of various inflammatory genes in pre-adipocytes, adipocytes and macrophages. Integrative functional enrichment analysis for all LGI3-regulated gene products suggested their involvement in a number of biological processes, including cancer, inflammatory response, response to wounding, as well as cell proliferation and differentiation. Protein interaction network analysis of LGI3-regulated gene products revealed that 94% of the gene products formed a cluster of interaction networks. Taken together, these results support the critical involvement of LGI3 in the cytokine network by interplaying with multiple adipokines, cytokines and signaling proteins.

Leucine-rich glioma inactivated 3 (LGI3) is a secreted protein that belongs to LGI/epitempin family. LGI3 is highly expressed in brain in a transcriptionally and developmentally regulated manner. Here we found that LGI3 induced neurite outgrowth in Neuro-2a cells and dorsal root ganglia explants. LGI3 treatment or overexpression increased neurite outgrowth and knockdown of LGI3 by siRNA had opposite effect. LGI3 treatment increased phosphorylation of Akt and a 125-kDa protein. Immunoprecipitation identified the 125-kDa protein as focal adhesion kinase (FAK). LGI3 overexpression increased phospho-Akt, phospho-FAK and FAK protein. Inhibition of Akt activation by PI3 kinase inhibitor attenuated LGI3-induced FAK phosphorylation and neurite outgrowth. Taken together, we propose that LGI3 is a neuritogenic factor whose signaling pathway involves Akt-mediated FAK activation.

Leucine-rich glioma inactivated 3 (LGI3) is known to be expressed mainly in the brain. However, the expression and physiological roles of LGI3 in skin cells remain unknown. In this study, it was found for the first time that LGI3 is expressed mostly by normal human keratinocytes. Furthermore, ELISA analysis showed that HaCaT human keratinocytes increased LGI3 secretion after exposure to ultraviolet B (UVB) in a time-and dose-dependent manner. We next investigated the possible role of LGI3 in keratinocytes. LGI3 (50 ng/ml) increased survival of HaCaT cells by 20% after UVB irradiation (150 mJ/cm 2 ). It was also found that LGI3 stimulates the phosphorylation of Akt, which is involved in the cell survival-signalling cascade.Furthermore, LGI3 led to the phosphorylation of MDM2 and subsequent p53 degradation. Taken together, the data suggest that LGI3 may regulate p53 levels and that keratinocyte-derived LGI3 may act as a novel cytokine for skin homoeostasis.Abbreviations: BSA, bovine serum albumin; ELISA, enzyme-linked immunosorbent assay; EPTP, epitempin; LGI3, leucine-rich glioma inactivated 3; PVDF, polyvinylidene fluoride; RT-PCR, reverse transcription polymerase chain reaction; UVB, ultraviolet B.