HER2 is a tyrosine kinase receptor causally involved in cancer. A subgroup of breast cancer patients with particularly poor clinical outcomes expresses a heterogeneous collection of HER2 carboxy-terminal fragments (CTFs). However, since the CTFs lack the extracellular domain that drives dimerization and subsequent activation of full-length HER2, they are in principle expected to be inactive. Here we show that at low expression levels one of these fragments, 611-CTF, activated multiple signaling pathways because of its unanticipated ability to constitutively homodimerize. A transcriptomic analysis revealed that 611-CTF specifically controlled the expression of genes that we found to be correlated with poor prognosis in breast cancer. Among the 611-CTF-regulated genes were several that have previously been linked to metastasis, including those for MET, EPHA2, matrix metalloproteinase 1, interleukin 11, angiopoietin-like 4, and different integrins. It is thought that transgenic mice overexpressing HER2 in the mammary glands develop tumors only after acquisition of activating mutations in the transgene. In contrast, we show that expression of 611-CTF led to development of aggressive and invasive mammary tumors without the need for mutations. These results demonstrate that 611-CTF is a potent oncogene capable of promoting mammary tumor progression and metastasis.HER2 (ErbB2) is a type I transmembrane protein that belongs to the epidermal growth factor receptor (EGFR, ErbB1, HER1) family. Two additional members, HER3 and -4 (ErbB3 and -4), complete this family. When an EGF-like ligand binds to HER1, -3, or -4, its extracellular domain adopts the socalled open conformation, which allows the formation of homo-or heterodimers (5). Despite not binding any ligand, HER2 readily interacts with other ligand-bound HER receptors because its extracellular domain is constitutively in an open conformation (10).At the cell surface, dimerization of the extracellular domains leads to interaction between the intracellular kinases of the HER receptors and subsequent transphosphorylation of tyrosine residues in the C-terminal tails. The phosphotyrosines act as docking sites for proteins that initiate signals which are transduced to the nucleus through different pathways, including the mitogen-activated protein kinases (MAPKs), phosphoinositide-3-kinase-activated Akt, Src, and phospholipase C gamma (PLCgamma) pathways. These signaling circuitries control the expression of target genes that act coordinately to modify key aspects of cellular biology, including proliferation, migration, survival, and differentiation (7).In addition to the canonical mode, HER receptors or fragments of them are capable of direct signaling. For example, a nuclear carboxy-terminal fragment (CTF) encompassing the entire cytoplasmic domain of HER4 has been shown to regulate gene transcription (22,39). The CTF of HER4 is generated at the plasma membrane by the sequential action of two types of proteolytic enzymes known as the alpha-and gammasecretases. Alpha-secre...

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CD43 or leukosialin is a transmembrane sialoglycoprotein, whose extracellular domain participates in cell adhesiveness and the cytoplasmic tail regulates a variety of intracellular signal transduction pathways involved in cell proliferation. CD43 is abundantly expressed on the surface of hematopoietic cells, but CD43 expression is also frequently found in the tumor cells of nonhematopoietic origin. In the early stages of some tumors, the accumulation of tumor suppressor protein p53 has been described. Here, we show that the expression of CD43 causes the induction of functionally active p53 protein.Moreover, we found that the activation of p53 by CD43 is mediated by tumor suppressor protein ARF. The coexpression of CD43 and ARF in ARF-null mouse embryonic fibroblasts resulted in programmed cell death, but that was not the case when CD43 alone was expressed in these cells. These data provide the first evidence of the connection between p53-and CD43-dependent pathways.

CD43 is an abundant transmembrane sialoglycoprotein in leukocyte-type cell lines, but it has also been suggested to be present in colon adenomas and colon carcinomas. We have now shown that CD43 is expressed in a variety of cell lines of different origins (CaSKI, A549, 293, MTSV1-7, MCF7, HT-1080, Jurkat, K562, COLO 205, HT-29, Caco-2, DLD-1 and SW480). The level of expression of CD43 mRNA was analyzed by reverse transcriptase-polymerase chain reaction and that of the protein by immunoprecipitation and Western blot, flow cytometry and confocal microscopy using two monoclonal anti-CD43 antibodies (L10 and 4D2). As all cell lines expressed CD43, it is suggested that CD43 has a more fundamental function than previously believed and thus cannot be considered only as a specific leukocyte marker.

CD43 is a transmembrane molecule that contains a 123-aminoacids-long cytoplasmic tail and a highly O-glycosylated extracellular domain of mucin type. Endogenous CD43 expressed in COLO 205, K562 and Jurkat cells revealed a membrane-associated, 20 kDa CD43-specific cytoplasmic tail fragment (CD43-CTF) upon inhibition of gamma-secretase. This fragment was formed by an extracellular cleavage, as it was not accumulated after treating cells with 1,10-phenanthroline, a metalloprotease inhibitor. When CD43 was transfected into HEK-293 cells expressing dominant-negative PS1 (presenilin-1), the CD43-CTF was accumulated, but not in cells with wild-type PS1. Owing to its accumulation in the presence of a non-functional PS variant, it may thus be a novel gamma-secretase substrate. This CTF is formed by an extracellular cleavage close to the membrane, is a fragment that can be concluded to be a substrate for gamma-secretase. However, the intracellular gamma-secretase product has not been possible to detect, suggesting a quick processing of this product. During normal growth the CTF was not found without gamma-secretase inhibition, but when the cells (COLO 205) were very confluent the fragment could be detected. The intracellular domain of CD43 has previously been shown to contain a functional nuclear localization signal, and has been suggested to be involved in gene activation. From this and the present results, a novel way to explain how mucin-type molecules may transduce intracellular signals can be proposed.