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The adenovirus E1A oncogene induces innate immune rejection of tumors by sensitizing tumor cells to apoptosis in response to injuries, such as those inflicted by macrophage-produced TNF α and NO. E1A sensitizes cells to TNF by repressing its activation of NF-κB-dependent, antiapoptotic defenses. This suggested the hypothesis that E1A blockade of the NF-κB activation response might be the central mechanism of E1A induced cellular sensitivity to other proapoptotic injuries, such as macrophage-produced NO. However, creation of E1A-positive NIH-3T3 mouse cell variants with high-level, NF-κB-dependent resistance to TNF did not coselect for resistance to apoptosis induced by either macrophage-NO or chemical-NO, as the hypothesis would predict. E1A expression did block cellular recovery from NO-induced mitochondrial injury and converted the reversible, NO-induced cytostasis response of cells to an apoptotic response. This viral oncogene-induced phenotypic conversion of the cellular injury response of mouse and human cells was mediated by an E1A-related increase in NO-induced activation of caspase-2, an apical initiator of intrinsic apoptosis. Blocking caspase-2 activation or expression eliminated the NO-induced apoptotic response of E1A-positive cells. These results define an NF-κB-independent pathway through which the E1A gene of human adenovirus sensitizes mouse and human cells to apoptosis by enhancement of caspase-2-mediated mitochondrial injury.

Vascular endothelial cells undergo albumin endocytosis using a set of albumin binding proteins. This process is important for maintaining cellular homeostasis. We showed by several criteria that the previously described 73-kDa endothelial cell surface albumin binding protein is the 75-kDa transforming growth factor (TGF)-beta receptor type II (TbetaRII). Albumin coimmunoprecipitated with TbetaRII from a membrane fraction from rat lung microvascular endothelial cells. Albumin endocytosis-negative COS-7 cells became albumin endocytosis competent when transfected with wild-type TbetaRII but not when transfected with a domain-negative kinase mutant of TbetaRII. An antibody specific for TbetaRII inhibited albumin endocytosis. A mink lung epithelial cell line, which expresses both the TGF-beta receptor type I (TbetaRI) and the TbetaRII receptor, exhibited albumin binding to the cell surface and endocytosis. In contrast, mutant L-17 and DR-26 cells lacking TbetaRI or TbetaRII, respectively, each showed a dramatic reduction in binding and endocytosis. Albumin endocytosis induced Smad2 phosphorylation and Smad4 translocation as well as increased protein expression of the inhibitory Smad, Smad7. We identified regions of significant homology between amino acid sequences of albumin and TGF-beta, suggesting a structural basis for the interaction of albumin with the TGF-beta receptors and subsequent activation of TbetaRII signaling. The observed albumin-induced internalization of TbetaRII signaling may be an important mechanism in the vessel wall for controlling TGF-beta responses in endothelial cells.

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