Interferon (IFN) therapy is today a well established treatment in many
diseases, including various malignancies. How IFN exerts antitumor
activity is not known, but several mechanisms have been suggested.
Previous studies have shown a correlation between the in vitro
susceptibility of primary malignant cells to IFN and the clinical
response of the patient to IFN therapy, supporting the idea that the
antitumor activity of IFN results from direct effects of IFN on the tumor
cells. The studies in the present thesis have dealt with the cellular
effects of IFN on both malignant and normal cells, with the focus on how
IFN modulates proliferation and apoptosis.
In addition to their well known antiviral effects, IFNs can exert
pleiotropic effects on cells, including potent cell growth inhibition of
many cell types. Studies on various tumor cell lines and normal cells, as
well as primary tumor cells, have established the antiproliferative
effect as an important contributor to the decreased number of tumor cells
commonly observed following IFN treatment. The role of apoptosis with
regard to IFN's anticellular effects has so far been poorly defined.
Using as a model system a number of hematopoietic cell lines, we showed
that IFN-[alpha] in vitro is a potent inducer of apoptotic cell death,
and that IFN-[alpha] -mediated growth arrest and apoptosis are
independent responses to IFN-[alpha]. IFN-[alpha] induces remissions in
approximately 15% of patients with multiple myeloma. We have previously
shown that IFN-[alpha] exerts a direct cytotoxic effect on myeloma cells
from some patients. Analysis of expression of the apoptosis-inhibitory
protein, Bcl-2, in pre-treatment bone-marrow samples from patients with
myeloma revealed a significant association (p=0.012) between high levels
of Bcl-2 and resistance to IFN-[alpha] therapy. These data indicate that
over-expression of Bcl-2 may be a cause for resistance to IFN-[alpha]
therapy in myeloma, and that one possible mechanism for IFN's antitumor
effect in this disease may be induction of apoptosis. In addition to
these cytoreductive effects, IFNs have also in some systems been shown to
protect malignant cells from apoptosis induced by different stimuli. In a
study of p53-induced apoptosis, IFN-[gamma], but not IFN-[alpha], was
found to protect cells from apoptosis.
In other studies, the molecular mechanism behind IFN-[alpha] induced cell
growth arrest was examined. The effect of IFN-[alpha] on expression of
members of the cyclin-dependent kinase inhibitor (CKI) families was
investigated in sensitive and resistant tumor cell lines, as well as in
normal IL-2 - stimulated T-cells. The results demonstrated that
IFN-[alpha] is a potent regulator of several CK1s, both from the INK4
family (p15) and the CIP/KIP family (p21 and p27). In sensitive tumor
cell lines, a primary response to IFN-[alpha] is induction and binding of
p21 to the Gl cyclin dependent kinases (CDKS) CDK4 and CDK2, causing
inhibition of these kinases. Secondary events include the increased
expression and accumulation of p27 in these G1 CDK complexes, rather than
loss of the Gl kinase cyclin components, as well as dephosphorylation of
the different pocket proteins. Importantly, in a resistant cell line, p21
protein was not expressed, despite high levels of p21 mRNA following
IFN-[alpha] treatment. In normal IL-2 -stimulated T-cells, IFN-[alpha]
was found to prevent entry into S-phase, correlating with profound
inhibition of IL-2 -induced changes in G 1 regulatory proteins, including
the prevention of n-litogen-induced reduction of p27 levels and
upregulation of G l cyclins and CDKS