The ErbB2 receptor tyrosine kinase plays a critical role in a variety of developmental processes, and its aberrant activation may contribute to the progression of some breast and ovarian tumors. ASGP2, a transmembrane glycoprotein found on the surface of the highly metastatic ascites 13762 rat mammary adenocarcinoma cell line, is constitutively associated with ErbB2 in these cells and in mammary tissue from pregnant rats. Expression studies indicate that ASGP2 interacts directly and specifically with ErbB2 through one of its epidermal growth factor-like domains and that the co-expression of the two proteins in the same cell dramatically facilitates their direct stable interaction. Ectopic expression of ASGP2 in human melanoma tumor cells potentiates the response of endogenous ErbB2 to the neuregulin-1 growth factor. These observations point to a novel intramembrane mechanism for the modulation of receptor tyrosine kinase activity.ErbB2 (also known as Neu) is a 185-kDa cell surface transmembrane receptor tyrosine kinase that mediates the growth or differentiation of a variety of cultured cells and contributes to the proper development of cardiac and neural tissues during gestation (1-4). Its overexpression in numerous human tumors, including breast and ovarian tumors, correlates with earlier patient relapse and poor prognosis (5, 6). The observation that ErbB2 overexpression stimulates its protein-tyrosine kinase activity (7), together with the observation that activated alleles of the erbB2 gene induce metastatic tumors when expressed in murine mammary epithelium (8), suggest that the activation of ErbB2 kinase activity may play an important role in tumorigenesis or tumor progression.The protein-tyrosine kinase activity of ErbB2 may be activated by several soluble, diffusible ligands that possess epidermal growth factor (EGF) 1 -like domains. For example, EGF, transforming growth factor-␣, and amphiregulin are all capable of stimulating ErbB2 activity by binding to the related EGF receptor and promoting its heterodimerization with ErbB2 (9, 10). Likewise, the neuregulins (NRGs) bind to the ErbB3 and ErbB4 receptors and stimulate ErbB2 activity through receptor heterodimerization mechanisms (11,12). However, no molecularly characterized diffusible ligand has been demonstrated to act on ErbB2 directly, and it has been suggested that the primary function of this protein is to augment signaling through the ErbB receptor network by acting as an auxiliary co-receptor (13)(14)(15). In this context factors that influence the activity or availability of ErbB2 could have a significant impact on the strength or specificity of signaling and ultimately the cellular response. Strong candidates for such factors are cell surface proteins that possess EGF-like domains.The autonomously proliferating and highly metastatic rat ascites 13762 mammary adenocarcinoma cell line expresses a large sialomucin complex in abundance at its cell surface. This complex consists of two noncovalently associated proteins, ASGP1 and ASGP2, which ari...

Although many effects of leptin are mediated through the central nervous system, leptin can regulate metabolism through a direct action on peripheral tissues, such as fat and liver. We show here that leptin, at physiological concentrations, acts through an intracellular signaling pathway similar to that activated by insulin in isolated primary rat hepatocytes. This pathway involves stimulation of phosphatidylinositol 3-kinase (PI3K) binding to insulin receptor substrate-1 and insulin receptor substrate-2, activation of PI3K and protein kinase B (AKT), and PI3K-dependent activation of cyclic nucleotide phosphodiesterase 3B, a cAMP-degrading enzyme. One important function of this signaling pathway is to reduce levels of cAMP, because leptin-mediated activation of both protein kinase B and phosphodiesterase 3B is most marked following elevation of cAMP by glucagon, and because leptin suppresses glucagon-induced cAMP elevation in a PI3K-dependent manner. There is little or no expression of the long form leptin receptor in primary rat hepatocytes, and these signaling events are probably mediated through the short forms of the leptin receptor. Thus, leptin, like insulin, induces an intracellular signaling pathway in hepatocytes that culminates in cAMP degradation and an antagonism of the actions of glucagon. Leptin (OB)1 is a 16-kDa protein secreted primarily from adipocytes (1-3). Rodents that are defective in leptin synthesis, the ob/ob mice, or leptin receptor function, the db/db mice, Zucker fa/fa rats, and Koletsky rats, are obese and develop hyperinsulinemia and insulin resistance similar to the metabolic abnormalities associated with type 2 diabetes. Leptin suppresses food intake (4 -7) and increases thermogenesis (8) and metabolic rate (9). These responses appear to be mediated mainly through the central nervous system, because they can be achieved by intracerebroventricular injections of leptin (10, 11). Leptin has also been shown to have a wide repertoire of peripheral effects, some of which are mediated through the central nervous system, and others that are induced through a direct action on target tissues. The latter include direct inhibition of insulin secretion and gene expression in pancreatic ␤-cells, stimulation of fatty acid oxidation in adipocytes, and stimulation of angiogenesis and T-cell proliferation (12-16).Molecular cloning of the leptin receptors (OB-R) has revealed that they are single membrane-spanning receptors with homology to members of the cytokine receptor superfamily (10, 17). The different leptin receptors arise from alternative splicing, and although the extracellular domains of these splice variants are identical, differences are apparent in the intracellular signaling domain. The splice variants containing transmembrane domains can be divided into two groups: one group that has short 32-97 amino acid residue intracellular domains (OB-Ra, -Rc, -Rd, -Rf, and a unique form expressed in hematopoietic cells, termed Rg here) and another group (OB-Rb) that has a long 302-residue intrace...

Muc4/sialomucin complex (SMC) is a multifunctional glycoprotein complex which can repress apoptosis in transfected tumor cells. Its transmembrane subunit acts as an intramembrane ligand for the receptor tyrosine kinase ErbB2 to induce the phosphorylation of ErbB2 and, by acting synergistically with the ErbB3 ligand neuregulin, can potentiate the phosphorylation of ErbB2 and ErbB3. In the present study we show that Muc4/ SMC alone robustly induces the phosphorylation of ErbB2 to enhance the tyrosine phosphate epitope (Tyr1248) recognized by anti-phospho-ErbB2. Although this tyrosine phosphorylation has been implicated in cell transformation, it does not activate any of the three mitogen-activated protein kinases (MAPKs) or protein kinase B/Akt of the phosphatidyl inositol 3-kinase pathway. Instead, Muc4/SMC expression induces upregulation of the cell cycle inhibitor p27 kip , consistent with the expression of Muc4/SMC in differentiated, rather than proliferative, epithelial cells. Interestingly, a combination of Muc4/SMC and neuregulin downregulate p27 kip and activate protein kinase B/Akt. These observations suggest that Muc4/SMC acts as a regulator of differentiation by inducing a limited phosphorylation of ErbB2 and a modulator of proliferation when acting synergistically with neuregulin to induce a more extensive phosphorylation on both ErbB2 and ErbB3.

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