Identification of Y589 and Y599 in the juxtamembrane domain of Flt3 as ligand-induced autophosphorylation sites involved in binding of Src family kinases and the protein tyrosine phosphatase SHP2.

Early signal relay steps upon ligand binding to the receptor tyrosine kinase Flt3 (ie, sites of Flt3 autophosphorylation and subsequent docking partners) are mainly unresolved. By immunoprecipitation of specific tryptic peptides contained in the juxtamembrane region of human Flt3 and subsequent radiosequencing, we identified the tyrosine residues 572, 589, 591, and 599 as in vivo autophosphorylation sites. Focusing on Y589 and Y599, we examined Flt3 ligand (FL)-mediated responses in wild-type-Flt3-(WT-Flt3-), Y589F-Flt3-, and Y599F-Flt3-expressing 32D cells. Compared with WT-Flt3-32D cells upon ligand stimulation, 32D-Y589F-Flt3 showed enhanced Erk activation and proliferation/survival, whereas 32D-Y599F-Flt3 cells hereby displayed substantially diminished responses. Both pY589 and pY599 were identified as association sites for signal relay molecules including Src family kinases and SHP2. Consistently, 32D-Y589F-Flt3 and 32D-Y599F-Flt3 showed decreased FL-triggered activation of Src family kinases. Interference with the Src-dependent negative regulation of Flt3 signaling may account for the enhanced mitogenic response of Y589F-Flt3. Y599 was additionally found to interact with the protein tyrosine phosphatase SHP2 in a phosphorylation-dependent manner. As Y599F-Flt3-32D was unable to associate with and to phosphorylate SHP2 and since silencing of SHP2 in WT-Flt3-expressing cells mimicked the Y599F-Flt3 phenotype, we hypothesize that recruitment of SHP2 to pY599 contributes to FL-mediated Erk activation and proliferation.

Hierarchy of ADAM12 binding to integrins in tumor cells.

ADAMs (a disintegrin and metalloprotease) comprise a family of cell surface proteins with protease and cell-binding activities. Using different forms and fragments of ADAM12 as substrates in cell adhesion and spreading assays, we demonstrated that alpha9beta1 integrin is the main receptor for ADAM12. However, when alpha9beta1 integrin is not expressed--as in many carcinoma cells--other members of the beta1 integrin family can replace its ligand binding activity. In attachment assays, the recombinant disintegrin domain of ADAM12 only supported alpha9 integrin-dependent tumor cell attachment, whereas full-length ADAM12 supported attachment via alpha9 integrin and other integrin receptors. Cells that attached to full-length ADAM12 in an alpha9 integrin-dependent manner also attached to ADAM12 in which the putative alpha9beta1 integrin-binding motif in the disintegrin domain had been mutated. This attachment was mediated through use of an alternate beta1 integrin. We also found that cell spreading in response to ADAM12 is dependent on the apparent level of integrin activation. Binding of cells to ADAM12 via the alpha9beta1 integrin was Mn(2+)-independent and resulted in attachment of cells with a rounded morphology; attachment of cells with a spread morphology required further activation of the alpha9beta1 integrin. We demonstrated that phosphoinositide-3-kinase appears to be central in regulating alpha9beta1 integrin cell spreading activity in response to ADAM12.

Regulation of ADAM12 cell-surface expression by protein kinase C epsilon.

The ADAM (a disintegrin and metalloprotease) family consists of multidomain cell-surface proteins that have a major impact on cell behavior. These transmembrane-anchored proteins are synthesized as proforms that have (from the N terminus): a prodomain; a metalloprotease-, disintegrin-like-, cysteine-rich, epidermal growth factor-like, and transmembrane domain; and a cytoplasmic tail. The 90-kDa mature form of human ADAM12 is generated in the trans-Golgi through cleavage of the prodomain by a furin-peptidase and is stored intracellularly until translocation to the cell surface as a constitutively active protein. However, little is known about the regulation of ADAM12 cell-surface translocation. Here, we used human RD rhabdomyosarcoma cells, which express ADAM12 at the cell surface, in a temporal pattern. We report that protein kinase C (PKC) epsilon induces ADAM12 translocation to the cell surface and that catalytic activity of PKCepsilon is required for this translocation. The following results support this conclusion: 1) treatment of cells with 0.1 microM phorbol 12-myristate 13-acetate (PMA) enhanced ADAM12 cell-surface immunostaining, 2) ADAM12 and PKCepsilon could be co-immunoprecipitated from membrane-enriched fractions of PMA-treated cells, 3) RD cells transfected with EGFP-tagged, myristoylated PKCepsilon expressed more ADAM12 at the cell surface than did non-transfected cells, and 4) RD cells transfected with a kinase-inactive PKCepsilon mutant did not exhibit ADAM12 cell-surface translocation upon PMA treatment. Finally, we demonstrate that the C1 and C2 domains of PKCepsilon both contain a binding site for ADAM12. These studies show that PKCepsilon plays a critical role in the regulation of ADAM12 cell-surface expression.

ADAM 12 protease induces adipogenesis in transgenic mice.

ADAM 12 (meltrin-alpha) is a member of the ADAM (a disintegrin and metalloprotease) family. ADAM 12 functions as an active metalloprotease, supports cell adhesion, and has been implicated in myoblast differentiation and fusion. Human ADAM 12 exists in two forms: the prototype membrane-anchored protein, ADAM 12-L, and a shorter secreted form, ADAM 12-S. Here we report the occurrence of adipocytes in the skeletal muscle of transgenic mice in which overexpression of either form is driven by the muscle creatine kinase promoter. Cells expressing a marker of early adipogenesis were apparent in the perivascular space in muscle tissue of 1- to 2-week-old transgenic mice whereas mature lipid-laden adipocytes were seen at 3 to 4 weeks. Moreover, female transgenics expressing ADAM 12-S exhibited increases in body weight, total body fat mass, abdominal fat mass, and herniation, but were normoglycemic and did not exhibit increased serum insulin, cholesterol, or triglycerides. Male transgenics were slightly overweight and also developed herniation but did not become obese. Transgenic mice expressing a truncated form of ADAM 12-S lacking the prodomain and the metalloprotease domain did not develop this adipogenic phenotype, suggesting a requirement for ADAM 12 protease activity. This is the first in vivo demonstration that an ADAM protease is involved in adipogenesis.

ADAM12 induces actin cytoskeleton and extracellular matrix reorganization during early adipocyte differentiation by regulating beta1 integrin function.

Changes in cell shape are a morphological hallmark of differentiation. In this study we report that the expression of ADAM12, a disintegrin and metalloprotease, dramatically affects cell morphology in preadipocytes, changing them from a flattened, fibroblastic appearance to a more rounded shape. We showed that the highest levels of ADAM12 mRNA were detected in preadipocytes at the critical stage when preadipocytes become permissive for adipogenic differentiation. Furthermore, as assessed by immunostaining, ADAM12 was transiently expressed at the cell surface concomitant with the reduced activity of beta1 integrin. Co-immunoprecipitation studies indicated the formation of ADAM12/beta1 integrin complexes in these preadipocytes. Overexpression of ADAM12 at the cell surface of 3T3-L1 preadipocytes achieved by transient transfection or retroviral transduction led to the disappearance of the extensive network of actin stress fibers that are characteristic of these cells, and its reorganization into a cortical network located beneath the cell membrane. The cells became more rounded, exhibited fewer vinculin-positive focal adhesions, and adhered less efficiently to fibronectin in attachment assays. Moreover, ADAM12-expressing cells were more prone to apoptosis, which could be prevented by treating the cells with beta1-activating antibodies. A reduced and re-organized fibronectin-rich extracellular matrix accompanied these changes. In addition, beta1 integrin was more readily extracted with Triton X-100 from cells overexpressing ADAM12 than from control cells. Collectively, these results show that surface expression of ADAM12 impairs the function of beta1 integrins and, consequently, alters the organization of the actin cytoskeleton and extracellular matrix. These events may be necessary for early adipocyte differentiation.