Tumorigenesis by Adenovirus Type 12 E1A

Abstract

All human adenovirus serotypes are capable of transforming mammalian cells in vitro. Transformation requires only two viral oncogenes, E1A and E1B, which cause unconstrained cell proliferation by deregulating the cell cycle and blocking apoptosis, respectively. In contrast to the transformation omnipotence inherent in all 51 serotypes, only a few serotypes including adenovirus type 12 (Ad12) have the additional capability of generating tumors in immunocompetent rodents. The Ad12 E1A protein (E1A-12) is the key determinant of viral tumorigenesis. Specifically, E1A-12 downregulates Major Histocompatibility Complex (MHC) class I antigens on the cell surface by shutting off the class I gene transcription. This enables Ad12-transformed cells to evade lysis by cytotoxic T lymphocytes (CTLs). E1A-12 mediates MHC class I shutoff via regulating DNA binding and transcriptional activities of both activator NF-κB (p50/p65) and repressor COUP-TFII. On the one hand, E1A-12 prevents DNA binding of NF-κB to the class I enhancer R1 site to disable this Master Regulator of immune genes from stimulating transcription. By physically binding NF-κB, E1A-12 blocks protein kinase A from phosphorylating the critical residues on p50 and p65 that are required for NF-κB DNA binding and transactivation, respectively. On the other hand, E1A-12 induces binding of COUP-TFII repression complex to the class I enhancer R2 site. E1A-12 not only upregulates COUP-TFII expression, but also acts as a physical component of the repression complex to recruit or stabilize the histone deacetylases HDAC1 and HDAC8. Histone deacetylation by HDAC in the class I enhancer region causes chromatin condensation and transcription repression of MHC class I. These two E1A-12-mediated complementary mechanisms that involve DNA binding regulation of both NF-κB and COUP-TFII/HDAC, provide a “FAIL-SAFE” strategy to ensure persistent diminution of surface MHC class I antigens under variable physiological conditions. In addition, E1A-12 mediates another compelling mechanism that involves evasion of natural killer (NK) cells, which normally lyse cells upon loss of surface MHC class I antigens. Importantly, E1A-12 downregulates surface expression of all NKG2D activating ligands including RAE1-α, -β, -γ, -ε, MULT1, and H60. Notably, the mRNA levels of these NKG2D ligands are also downregulated, perhaps through a mechanism similar to MHC class I shutoff, since promoters of these ligands also contain binding sites for NF-κB and COUP-TFII. Finally, E1A-12-mediated tumorigenesis requires the function encoded by the unique 20-amino acid “Spacer” between the conserved regions CR2 and CR3. This Spacer plays an important role in inducing neuronal and tumor-related genes. Significantly, neuronal gene induction by E1A-12 requires not only the relief of transcription repression by the neuron-restrictive silencer factor (NRSF), but also the stimulation of neuronal promoters by certain transactivators. The loss of NRSF repression is mediated by E1A-12 via proteasomal degradation of the repressor in the nucleus. The E1A-12 mediated induction of neuronal and tumor-related genes may be essential for cell outgrowth and invasion in Ad12 tumorigenesis.