c-Myc functionally cooperates with Bax to induce apoptosis

c-Myc promotes apoptosis by destabilizing mitochondrial integrity, leading to the release of proapoptotic effectors including holocytochrome c. Candidate mediators of c-Myc in this process are the proapoptotic members of the Bcl-2 family. We show here that fibroblasts lacking Bak remain susceptible to c-Myc-induced apoptosis whereas bax-deficient fibroblasts are resistant. However, despite this requirement for Bax, c-Myc activation exerts no detectable effects on Bax expression, localization, or conformation. Moreover, susceptibility to c-Myc-induced apoptosis can be restored in bax-deficient cells by ectopic expression of Bax or by microinjection of a peptide comprising a minimal BH3 domain. Microinjection of BH3 peptide also restores sensitivity to c-Myc-induced apoptosis in p53-deficient primary fibroblasts that are otherwise resistant. By contrast, there is no synergy between BH3 peptide and c-Myc in fibroblasts deficient in both Bax and Bak. We conclude that c-Myc triggers a proapoptotic mitochondrial destabilizing activity that cooperates with proapoptotic members of the Bcl-2 family.     

Virally activated ras cooperates with integrin to induce tubulogenesis in sinusoidal endothelial cell lines

Four cell lines, named nonparenchymal 11 (NP11), NP26, NP31, and NP32, were established from sinusoidal endothelial cells (SECs) of rat liver. They still retained expression of receptors for vascular endothelial growth factor (VEGF), Flt-1, and kinase domain-containing receptor (KDR). NP31 and NP32 turned out to be incapable of tubulogenesis in basement membrane matrix (Matrigel), which belongs to endothelial properties, as shown by SECs in primary culture. Expression of temperature-sensitive, virally activated Ras (ts-v-Ras) restored tubulogenic behaviors back to NP31 only at permissive temperature. Matrigel induced long-lasting tyrosine phosphorylation of Shc, with recruitment of Grb-2 and microtubule-associated protein kinase (MAPK) activation in both parental NP31 and NP31 transformed by ts-v-Ras, which was blocked by anti-β1 integrin antibody. Tubulogenesis was inhibited by adenovirus-mediated expression of dominant-negative Ras in human umbilical vein endothelial cells (HUVECs). PD 098059, a selective inhibitor of MAPK kinase (MEK), nearly perfectly blocked tubulogenesis by ts-v-Ras-expressing NP31 cells at permissive temperature. Furthermore, the botulinum C3 toxin, an inhibitor for Rho, caused fragmentation of branching cords in networks formed by NP31 that expressed ts-v-Ras at permissive temperature. These data suggest that the integrin-mediated Ras signals may be necessary but are not sufficient for tubulogenesis and that an artificial expression of v-Ras might substitute for the second signal required in this system. J. Cell. Physiol. 176:223–234, 1998. © 1998 Wiley-Liss, Inc.     

Mdm2 associates with Ras effector NORE1 to induce the degradation of oncoprotein HIPK1

The Ras effector NORE1 is frequently silenced in primary adenocarcinomas, although the significance of this silencing for tumorigenesis is unclear. Here we show that NORE1 induces polyubiquitination and proteasomal degradation of oncoprotein HIPK1 by facilitating its interaction with the Mdm2 E3 ubiquitin ligase. Endogenous HIPK1 is stabilized in Nore1-deficient mouse embryonic fibroblasts, and depletion of HIPK1 in NORE1-silenced lung adenocarcinoma cells inhibits anchorage-independent cell growth and tumour formation in nude mice. These findings indicate that the control of HIPK1 stability by Mdm2-NORE1 has a major effect on cell behaviour, and epigenetic inactivation of NORE1 enables adenocarcinoma formation in vivo through HIPK1 stabilization.     

p73 cooperates with Ras in the activation of MAP kinase signaling cascade

The p73 gene is capable of inducing cell cycle arrest, apoptosis, senescence, differentiation and to cooperate with oncogenic Ras in cellular transformation. Ras can be considered as a branch point in signal transduction, where diverse extracellular stimuli converge. The intensity of the mitogen-activated protein kinase (MAPK) cascade activation influences the cellular response to Ras. Despite the fundamental role of p53 in Ras-induced growth arrest and senescence, it remains unclear how the Ras/MEK/ERK pathway induces growth arrest in the absence of p53. We report here that oncogenic Ras stabilizes p73 resulting in p73 accumulation and enhancement of its activity. p73, in turn, induces a sustained activation of the MAP kinase cascade synergizing with oncogenic Ras. We also found that inhibition of p73 function modifies the cellular outcome to Ras activation inhibiting Ras-dependent differentiation. Here, we show for the first time that there is a signaling loop between Ras-dependent MAPK cascade activation and p73 function.     

TGF-beta cooperates with TGF-alpha to induce the self-renewal of normal erythrocytic progenitors: evidence for an autocrine mechanism.

Simultaneous addition of both TGF-alpha and TGF-beta induces the sustained, long-term outgrowth of chicken erythrocytic progenitor cells, referred to as T2ECs from both chick bone marrow and 2-day-old chicken embryos. By analysis for differentiation antigens and gene expression, these cells were shown to represent very immature haematopoietic progenitors committed to the erythrocytic lineage. T2ECs differentiate into almost pure populations of fully mature erythrocytes within 6 days, when TGF-alpha and TGF-beta are withdrawn and the cells exposed to anaemic chicken serum plus insulin. Outgrowth of these cells from various sources invariably required both TGF-alpha and TGF-beta, as well as glucocorticoids. Proliferating, established T2ECs still require TGF-alpha, but are independent of exogenous TGF-beta. Using a TGF-beta-neutralizing antibody or expressing a dominant-negative TGF-beta receptor II, we demonstrate that T2ECs generate an autocrine loop involving TGF-beta during their establishment, which is required for sustained proliferation. Using specific inhibitors, we also show that signalling via Mek-1 is specifically required for induction and maintenance of cell proliferation driven by cooperation between the TGF-alpha and -beta receptors. These results establish a novel mechanism by which self-renewal of erythrocytic progenitors is induced and establish avian T2ECs as a new, quasi-optimal model system to study erythrocytic progenitors.     

The inv(16) cooperates with ARF haploinsufficiency to induce acute myeloid leukemia

The inv(16) is one of the most frequent chromosomal translocations associated with acute myeloid leukemia (AML) and creates a chimeric fusion protein consisting of most of the runt-related X1 co-factor, core binding factor beta fused to the smooth muscle myosin heavy chain MYH11. Expression of the ARF tumor suppressor is regulated by runt-related X1, suggesting that the inv(16) fusion protein (IFP) may repress ARF expression. We established a murine bone marrow transplant model of the inv(16) in which wild type, Arf+/-, and Arf-/- bone marrow were engineered to express the IFP. IFP expression was sufficient to induce a myelomonocytic AML even when expressed in wild type bone marrow, yet removal of only a single allele of Arf greatly accelerated the disease, indicating that Arf is haploinsufficient for the induction of AML in the presence of the inv(16).