01. Gene therapy Boulikas.pdf - Gene therapy & Molecular Biology

molecules in the nucleus (Lowe and Ruley, 1993);
increased levels of p53 are then believed to upregulate the
bax gene.
The transcription factor E2F was originally identified
as an activator of the adenovirus E2 gene and is implicated
in the regulation of DNA replication (Shirodkar et al.,
1992). Following infection of cells with adenovirus, the
DNA binding activity of E2F increases and as a
consequence transcription of the E2 gene of adenovirus
increases (Kovesdi et al., 1987). These changes in E2F are
mediated by E1A protein of adenovirus. RB forms specific
complexes with E2F keeping E2F in a form unable to
upregulate its target regulatory sequences. E2F can form
specific complexes also with cyclin A during S-phase in
NIH 3T3 cells (Mudryj et al., 1991). Both types of
complexes, E2F-RB and E2F-cyclin A, can be dissociated
by the adenovirus E1A protein (Chellappan et al., 1991;
Bagchi et al., 1990; reviewed by White, 1998 this volume)
but also by phosphorylation of RB at G1/S causing release
of E2F and stimulation in transcription of genes required
Gene Therapy and Molecular Biology Vol 1, page 9
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for DNA replication (myc, DHFR). These events
contribute to the uncontrolled proliferation of adenovirustransformed
cells (Mudryj et al., 1990, 1991). Release of
E2F from RB induced by E1A is critical for
transformation of cells by E1A (for references see Hiebert
et al, 1995).
C. Strategies of adenoviruses to enter the
cell
In order to enter the host cell the adenovirus first
attaches with a high affinity to a cell surface receptor,
whose nature still remains elusive, using the head domains
of the protruding viral fibers; the fibronectin-binding
integrin on the cell surface then associates with the penton
base protein on the adenovirus triggering endocytosis of
the virus particle via coated pits and coated vesicles
(Svensson and Persson, 1984; Greber et al, 1996). The
third step in adenovirus entry into the host cell includes