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Since high grade serous ovarian cancer is one of the deadliest cancers in women,
efforts to establish new therapies are of great interest. Undoubtedly, the reversion of the
suppressive and protumorigenic functions of tumor-associated immune cells would be a
significant therapeutic approach.
The interactive network of tumor-associated immune cells and metastatic tumor
cells, especially macrophages, is determined to a large extend by the secretome of the
peritoneal fluid, which occurs at advanced stages as a malignancy-associated effusion,
termed ascites. Published data have shown a correlation of high concentrations of IL-10,
IL-6 or TGFb in ascites with a poor prognosis.
In this thesis, high concentrations of polyunsaturated fatty acids, such as linoleic
acid or arachidonic acid, were identified in the ascites as natural agonists of the lipid
sensor PPARb/d, a member of the nuclear receptor superfamily. In tumor-associated
macrophages, high concentrations of these fatty acids, which are stored in intracellular
lipid droplets, result in the constitutive overexpression of PPARb/d specific target genes.
Consistent with this finding, these cells were found to be refractory to synthetic PPARb/d
agonists in vitro but repressible by inhibitory PPARb/d ligands. Expression of ANGPTL4,
one of the major target genes of PPARb/d, with functions in metastasis, is associated
with a reduced relapse free survival of the patients, underscoring the potential clinical
significance of our results.
In human macrophages from healthy donors, we identified two fundamentally
different mechanisms of agonist-induced transcriptional regulation of PPARb/d target
genes. On the one hand, there is the canonical, cell-type-independent induction of
different target genes of lipid and glucose metabolism (including ANGPTL4) by specific
synthetic agonists. On the other hand, a macrophage-specific inverse regulation, which
does not require direct PPARb/d chromatin binding and mainly affects the regulation of
immune functions, could be identified. PPARb/d agonists mainly lead to the repression
of proinflammatory genes, which might be relevant in view of the predominantly antiinflammatory
effect of the ascites. However, antiinflammatory genes are also repressed
by the same ligands. This suggests the induction of a specific, hitherto not described
polarisation state of macrophages that is regulated by PPARb/d.
In order to gain a better understanding of the protumorigenic macrophages within
the tumor microenvironment, the regulation of IL-12, a central cytokine responsible for
the proinflammatory functions of macrophages, was investigated in detail. IL-12 is not
expressed in TAMs, and is not inducible by Interferon-g (IFNg) and lipopolysaccharide in ascites-treated macrophages from healthy donors. However, the observed reversibility
of ascites-mediated suppression by subsequent ascites withdrawal or
IFNg supplementation is potentially interesting from a therapeutical view.
One of the major roles of IL-10 is to repress transcription of IL12B, which encodes
for the limiting subunit of the IL-12 heterodimer. IL-10 is known to impinge on nuclear
translocation of the NF-kB subunits c-REL and RELA/p65. Since IL12B has been
described as an NF-kB target gene, we hypothesized that NF-kB signaling is an
important target of ascites-mediated suppression of IL12B induction via IL-10. This
hypothesis could not be confirmed. The ascites-mediated suppression of IL12B indeed
coincided with a markedly reduced translocation of c-REL and p65/RELA in primary
human macrophages in vitro. Surprisingly, however, chromatin binding by these factors
to a newly identified upstream regulatory binding site was largely unaltered. These
findings suggest that besides a possible role for NF-kB, other regulatory mechanisms
play an essential role in the suppression of IL12B transcription. This conclusion is
supported by the finding that another c-REL target gene, CXCL10, is not repressed by