Metalloprotease-mediated GH receptor proteolysis and GHBP shedding. Determination of extracellular domain stem region cleavage site

Natural Soluble Interleukin-15Rα Is Generated by Cleavage That Involves the Tumor Necrosis Factor-α-converting Enzyme (TACE/ADAM17)

This study shows that the high affinity alpha-chain of the interleukin (IL)-15 receptor exists not only in membrane-anchored but also in soluble form. Soluble IL-15Ralpha (sIL-15Ralpha) can be detected in mouse sera and cell-conditioned media by enzyme-linked immunosorbent assay and by immunoprecipitation and Western blotting. This protein has a molecular mass of about 30 kDa because of the presence of a single N-glycosylation site, which is reduced to 26 kDa after N-glycosidase treatment. Transmembrane IL-15Ralpha is constitutively converted into its soluble form by proteolytic cleavage that involves tumor necrosis factor-alpha-converting enzyme (TACE), and this process is further enhanced by phorbol 12-myristate 13-acetate (PMA) stimulation. The hydroxamate GW280264X, which is capable of blocking TACE and the closely related disintegrin-like metalloproteinase 10 (ADAM10), effectively inhibited both spontaneous and PMA-inducible cleavage of IL-15Ralpha, whereas GI254023X, which preferentially blocks ADAM10, was ineffective. Overexpression of TACE but not ADAM10 in COS-7 cells enhanced the constitutive and PMA-inducible cleavage of IL-15Ralpha. Moreover, murine fibroblasts deficient in TACE but not ADAM10 expression exhibited a significant reduction in the spontaneous and inducible IL-15Ralpha shedding, whereas a reconstitution of TACE in these cells restored the release of sIL-15Ralpha, thereby suggesting that TACE-mediated proteolysis may represent a major mechanism for sIL-15Ralpha generation in mice. The existence of natural sIL-15Ralpha offers novel insights into the complex biology of IL-15 and envisages a new level for therapeutic intervention.

Loss of ectodomain shedding due to mutations in the metalloprotease and cysteine-rich/disintegrin domains of the tumor necrosis factor-alpha converting enzyme (TACE)

Tumor necrosis factor-alpha converting enzyme (TACE), a multidomain protease essential for development and disease, releases the ectodomains from many transmembrane proteins in a regulated fashion. To understand the mechanism underlying the regulation of TACE activity, we sought to identify the cause of ectodomain shedding deficiencies in two mutated CHO sublines designated M1 and M2. Transfection of expression vectors for human and mouse TACE restored ectodomain shedding of TNF-alpha and TGF-alpha, suggesting that defects in the TACE gene contribute to the phenotype of M1 and M2 cells. The overall levels of endogenous TACE forms in M1 cells were significantly lower than those found in their parental cells, whereas only TACE zymogen, but not its mature form, was detectable in M2 cells. Molecular analyses suggested that M1 cells contained only one expressible TACE allele encoding an M435I point mutation in the catalytic center of the protease, and M2 cells produced two TACE variants with distinct point mutations in the catalytic domain (C225Y) and the cysteinerich/disintegrin domain (C600Y). Overexpression of the C225Y and C600Y TACE by transient transfection largely compensated for maturation defects in the variants but failed to restore TNF-alpha and TGF-alpha release in the shedding-defective CHO cell lines and fibroblasts derived from TACE-null mouse embryo. Further mutagenesis and functional analyses demonstrated that Cys(600) was absolutely essential for ectodomain shedding, suggesting that Cys(600), similar to Cys(225), participates in disulfide bonding, which is critical for both the processing and catalysis of TACE.

The growth hormone receptor interacts with its sheddase, the tumour necrosis factor-alpha-converting enzyme (TACE)

Proteolysis of the GHR (growth hormone receptor) occurs at the cell surface and results in the release of its extracellular domain, the GHBP (growth hormone-binding protein). TACE (tumour necrosis factor-alpha-converting enzyme) has been identified as a putative protease responsible for GHR cleavage. However, GHR-TACE interaction has not been observed until now. Here, we identified TACE in Chinese hamster cells and confirmed processing and cell-surface expression. Interaction between GHR and TACE was only observed after growth hormone binding. As the growth hormone-GHR(2) complex is a poor substrate for TACE, we conclude that the GHR-TACE interaction precedes proteolysis, and is transient. Furthermore, we demonstrate that TACE is present in endosomes, where it partly co-localizes with endocytosed growth hormone ligand.

Janus kinase 2 determinants for growth hormone receptor association, surface assembly, and signaling

GH signaling depends on functional interaction of the GH receptor (GHR) and the cytoplasmic tyrosine kinase, Janus kinase 2 (JAK2), which possesses a C-terminal kinase domain, a catalytically inactive pseudokinase domain just N-terminal to the kinase domain, and an N-terminal half shown by us and others to harbor elements for GHR association. Computational analyses indicate that JAKs contain in their N termini ( approximately 450 residues) divergent FERM domains. FERM domains (or subdomains within them) in JAKS may be important for associations with cytokine receptors. For some cytokine receptors, JAK interaction may be required for receptor surface expression. We previously demonstrated that a JAK2 mutant devoid of its N-terminal 239 residues (JAK2-Delta1-239) did not associate with GHR and could not mediate GH- induced signaling. In this report we employ a JAK2-deficient cell line to further define N-terminal JAK2 regions required for physical and functional association with the GHR. We also examine whether JAK2 expression affects cell surface expression of the GHR. Our results suggest that FERM motifs play an important role in the interaction of GHR and JAK2. While JAK2 expression is not required for detectable surface GHR expression, an increased JAK2 level increases the fraction of GHRs that achieves resistance to deglycosylation by endoglycosidase H, suggesting that the GHR-JAK2 association may enhance either the receptor's efficiency of maturation or its stability. Further, we report evidence for the existence of a novel GH-inducible functional interaction between JAK2 molecules that may be important in the mechanism of GH-triggered JAK2 signaling.

Physiologic and pathologic events mediated by intramembranous and juxtamembranous proteolysis

Intramembranous proteolysis (IP) is a recently recognized mechanism for transmembrane signal transduction that involves proteolysis of transmembrane proteins within their membrane-spanning domains. Juxtamembranous proteolysis (JP) is similar, but proteolytic cleavage of a transmembrane protein occurs at a site close to, but not within, the transmembrane domain of the target protein. In both IP and JP, a soluble domain of a transmembrane protein is released from its membrane tether. This domain can then transmit a signal either locally or at some distance from the site of cleavage. In certain signaling pathways, JP and IP are linked. JP on one side of the membrane results in secondary IP, which then releases a signaling domain from the membrane. Whereas well-characterized proteases such as caspases, the proteasome, and metalloprotease disintegrins, have been implicated in JP, three families of multipass membrane proteases (MpMPs) have now been shown to carry out IP. Recent studies of events mediated by IP and JP indicate that they regulate key cellular signaling events including pathways involved in sterol regulation, cell fate selection, and growth regulation. Moreover, IP and JP have important roles in certain diseases such as Alzheimer's disease. Because some of the proteases mediating IP and JP can be selectivity inhibited, inhibitors targeting these proteases are likely to alter both physiologic and pathologic events triggered by IP and JP.