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Page 12 of 22 García-Pardo et al. J Cancer Metastasis Treat 2021;7:62 https://dx.doi.org/10.20517/2394-4722.2021.103

Function of angiogenic factors in CLL cell survival/apoptosis
VEGF
Binding of many angiogenic factors to their respective receptors in CLL cells are known to induce signaling
pathways that lead to apoptosis resistance and cell survival [20,21,34,35] . It was first demonstrated that culturing
CLL cells with VEGF for 24 h significantly decreased spontaneous and chrorambucil-induced apoptosis,
and this involved upregulation of the anti-apoptotic molecules myeloid cell leukemia-1 (Mcl-1) and X-
linked inhibitor of apoptosis protein (XIAP) via activation of the transcription factor STAT3 [134,135] . These
authors also showed that the VEGFR1 and VEGFR2 receptors are constitutively phosphorylated in CLL
cells and that tyrosine kinase inhibitors or anti-VEGF antibodies inhibited this phosphorylation and STAT3
activation, reduced Mcl-1 and XIAP levels, and induced apoptosis [134,135] . These studies substantiate the
presence of a VEGF/VEGFR autocrine pathway that supports CLL cell survival and demonstrate that
blocking this pathway results in CLL cell apoptosis. Interestingly, it was further demonstrated that this
internal autocrine VEGF survival loop particularly operates in CLL cells expressing the prognostic marker
[136]
CD38, thus introducing a different VEGF behavior in CD38+ and CD38- CLL cells .
Autocrine VEGF was also shown to mediate the survival effect of CD40 ligand (CD154), a molecule
expressed on activated T cells, monocytes, macrophages, and other components of the
[137]
microenvironment . These authors demonstrated that the anti-apoptotic effect of CD154 required the
cooperative signaling provided by both CD40 and VEGFR1, 2, resulting in NF-κB activation and
[137]
upregulation of the protein survivin . Functional cooperation of VEGF receptors and other cell surface
molecules, such as integrins, has been clearly established in the context of endothelial cells and tumor
angiogenesis . In CLL cells, we demonstrated that the α4β1 integrin is associated with VEGFR2 and
[138]
modulates VEGF functions on these cells, including the survival effect induced by exogenous VEGF .
[65]
bFGF
In initial studies, it was shown that elevated intracellular levels of bFGF correlated with CLL cell resistance
to fludarabine in vitro and that addition of bFGF delayed apoptosis in fludarabine-treated cells . Using
[44]
CLL-derived cell lines and primary CLL cells, it was also demonstrated that bFGF delayed fludarabine-
induced apoptosis by upregulating Bcl-2, at both mRNA and protein levels . Bcl-2 expression was also
[139]
shown to positively correlate with the levels of bFGF in serum in the 85 patients studied . In another
[140]
[141]
study, Romanov et al. reported that bFGF suppressed p53 activation in cultured CLL cells exposed to
ionizing radiation, mainly by upregulating the p53-inhibitory protein MDM2 (mouse double minute 2).
bFGF may thus induce CLL cell survival by several mechanisms. Moreover, the survival activity induced by
bFGF interaction with its FGFR3 receptor in CLL cells can be potentiated by the tyrosine kinase Axl, which
forms a complex with FGFR3 and modulates its signaling pathway .
[60]

Ang-2 and TSP-1
It was initially reported that Ang-2 induces CLL cell survival after 24 h but that, at longer times, Ang-2 has a
pro-apoptotic activity . This was attributed to the transient modulation of the Tie-2 receptor, as well as of
[68]
[68]
caspases and ATP content . However, another study has shown that the Ang-2/Tie-2 axis induces CLL cell
[70]
survival even after 96 h of treatment . These authors also showed that Tie-2 engagement by Ang-2
activated the PI3-K-Akt signaling pathway, and this was abolished by a Tie-2 kinase inhibitor, which, in
[70]
fact, induced apoptosis . This study therefore supports an autocrine role of Ang-2 which affects CLL cell
survival.

In contrast to the described pro-survival function of some angiogenic factors, binding of TSP-1 to its
receptor CD47 was shown to induce apoptosis in CLL cells . The mechanism involved in this action was
[73]