Receptor desensitization is accomplished by accelerated endocytosis and degradation of ligand-receptor complexes. An in vitro reconstituted system indicates that Cbl adaptor proteins directly control downregulation of the receptor for the epidermal growth factor (EGFR) by recruiting ubiquitin-activating and -conjugating enzymes. We infer a sequential process initiated by autophosphorylation of EGFR at a previously identified lysosome-targeting motif that subsequently recruits Cbl. This is followed by tyrosine phosphorylation of c-Cbl at a site flanking its RING finger, which enables receptor ubiquitination and degradation. Whereas all three members of the Cbl family can enhance ubiquitination, two oncogenic Cbl variants, whose RING fingers are defective and phosphorylation sites are missing, are unable to desensitize EGFR. Our study identifies Cbl proteins as components of the ubiquitin ligation machinery and implies that they similarly suppress many other signaling pathways.

The ErbB family includes two receptors, ErbB‐1 and ErbB‐3, that respectively bind to epidermal growth factor and Neu differentiation factor, and an orphan receptor, ErbB‐2. Unlike ErbB‐1 and ErbB‐2, the intrinsic tyrosine kinase of ErbB‐3 is catalytically impaired. By using interleukin‐3‐dependent cells that ectopically express the three ErbB proteins or their combinations, we found that ErbB‐3 is devoid of any biological activity but both ErbB‐1 and ErbB‐2 can reconstitute its extremely potent mitogenic activity. Transactivation of ErbB‐3 correlates with heterodimer formation and is reflected in receptor phosphorylation and the transregulation of ligand affinity. Inter‐receptor interactions enable graded proliferative and survival signals: heterodimers are more potent than homodimers, and ErbB‐3‐containing complexes, especially the ErbB‐2/ErbB‐3 heterodimer, are more active than ErbB‐1 complexes. Nevertheless, ErbB‐1 signaling displays dominance over ErbB‐3 when the two receptors are coexpressed. Although all receptor combinations activate the mitogen‐activated protein kinases ERK and c‐Jun kinase, they differ in their rate of endocytosis and in coupling to intervening signaling proteins. It is conceivable that combinatorial receptor interactions diversify signal transduction and confer double regulation, in cis and in trans, of the superior mitogenic activity of the kinase‐defective ErbB‐3.

Ligand-induced activation of receptor tyrosine kinases (RTK) results in the initiation of diverse cellular pathways, including proliferation, differentiation and cell migration. The ErbB family of RTKs represents a model for signal diversification through the formation of homo-and heterodimeric receptor complexes. Each dimeric receptor complex will initiate a distinct signaling pathway by recruiting a different set of Src homology 2-(SH2-) containing effector proteins. Further complexity is added due to the existence of an oncogenic receptor that enhances and stabilizes dimerization but has no ligand (ErbB-2), and a receptor that can recruit novel SH-2-containing proteins, but is itself devoid of kinase activity (ErbB-3). The resulting signaling network has important implications for embryonic development and malignant transformation.

, is a seco-steroid hormone that binds with high affinity to a nuclear receptor, the vitamin D 3 receptor (VDR). This receptor selectively associates with recognition sequences in the promoter region of target genes, thereby regulating the transcription of those genes. The principal functions of 1,25(OH) 2 D 3 are the stimulation of intestinal calcium and phosphorus absorption, mediation of bone remodeling, and conservation of minerals in the kidney (for reviews, see references 46 and 63). In addition to its action in these tissues, however, 1,25(OH) 2 D 3 has been found in skin, testes, breast, muscle, pancreas, endocrine glands, thymus, and bone marrow, suggesting additional regulatory functions for the hormone. Notably, 1,25(OH) 2 D 3 appears to play an important role in modulating the growth of cells of the immune system: the hormone can induce the differentiation of myeloid leukemia cells along a monocyte/ macrophage lineage (1, 4, 6, 49) and can inhibit T-lymphocyte proliferation and activation both in vivo and in vitro (8,10,37,43,44). Interleukin-2 (IL-2), gamma interferon, and granulocytemacrophage colony-stimulating factor (GM-CSF) mRNA levels all decrease after T cells are exposed to 1,25(OH) 2 D 3 , but the molecular mechanisms mediating these changes have not been fully described (9,58,(66)(67)(68)(69)79).T-cell activation is a key step in the initiation of an immunological response. Upon receipt of the appropriate stimulus, a complex signaling cascade is initiated, resulting in cell proliferation and secretion of cytokines that enhance the immune response. One of the first genes to be expressed postactivation is the IL-2 gene. The lymphokine IL-2 exerts its influence by interacting with the IL-2 receptor on the surface of activated T cells, and this interaction is required for progression through the cell cycle (transition from G 1 to S phase). The kinetics of IL-2 induction are quite rapid, with transcripts detectable within 30 to 45 min after activation. A complex enhancer, which includes 275 bp just downstream from the transcription start site (Ϫ52 to Ϫ326), has been exhaustively studied (16,24,75). Binding sites for several ubiquitous and T-cell-specific transcription factors were defined in this region, and the proteins that bind to these sites, including Oct-1, AP1, NF-B, and NFATp/c, have been identified (65).Several agents, including the drugs cyclosporin A and FK506 (18, 51), as well as glucocorticoids (25,80,81) and retinoids (13, 19) appear to act as immunosuppressors by targeting IL-2 expression. 1,25(OH) 2 D 3 inhibits the entry of activated T cells into S phase (68); similar blocks at this point in the cell cycle have been demonstrated for other inhibitors of IL-2 synthesis, such as the synthetic glucocorticoid dexamethasone (7), further implicating IL-2 as a target for vitamin D 3 's immunosuppressive effect.These observations demonstrate an important role for 1,25(OH) 2 D 3 in the immunomodulation of T lymphocytes but not the actual mechanism by which this regulation is carried out....