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Sabol et al. J Cancer Metastasis Treat 2021;7:20 https://dx.doi.org/10.20517/2394-4722.2021.35 Page 5 of 15

[45]
receptor 2 increases have been associated with translocations t(14;16)(q32;q23) and t(14;20)(q32;q11) . The
increased copy number of Notch ligands and receptors has also been linked to trisomies of various
chromosomes at which genes of the Notch pathway are located. This includes Notch1 (chr.9q34.3), Notch3
(19p13.2-p13.1), Dll3 (19q13), and Dll4 (15q14) [25,44] . De novo activation due to microenvironmental cues
may also underlie the changes in the expression of Notch components. Supporting this notion, chromatin
activation of genes at different steps of the Notch pathway, including ligands, receptor protease machinery,
and downstream targets, has recently been detected in MM cells . Moreover, preclinical studies from
[46]
different groups have shown that interactions between MM cells and microenvironmental cells, such as
stromal cells or osteocytes, can upregulate and/or change the repertoire of Notch receptors in MM cells [43,47] .
Further studies are needed to determine the expression of Notch members in MM patients and changes that
might occur as patients progress through the different stages of MM. Similarly, a better understanding of
the underlying mechanisms involved in the transcriptional regulation of Notch components is required to
determine the specific contribution of Notch dysregulation to MM disease.

Notch and multiple myeloma tumor growth
Overexpression of Notch receptor 1, Jagged 1, and Jagged 2 occurs early in MM disease [40-42] . Additionally,
dysregulation of Notch receptor 1 and Jagged 1 has been associated with progression from MGUS to
[41]
MM . These initial observations suggest that the main outcome of Notch activation in MM is increased
tumor growth. Indeed, in vitro and animal studies show that both homotypic (among MM cells) and
heterotypic (between MM cells and host cells) Notch activation increases MM cell proliferation and
decreases apoptosis in both human and murine MM cell lines and primary cells from patients [40,43,48-52] . The
increased levels of Notch ligands and receptors in MM cells facilitate physical communication with other
neighboring MM cells, leading to increased Cyclin D1 expression and Il-6 production, which in turn
stimulates proliferation and promotes survival [43,53] . The pro-survival effect of Notch is due to upregulation
of anti-apoptotic proteins like Bcl-2 and Bcl-x2, and downregulation of Bax and Bak, pro-apoptotic
proteins [54,55] . Notch signaling also appears to contribute to the migration of MM cells through the
expression of the Cxcr4/Sdf1α axis system . Yet, the specific contribution of individual Notch receptors to
[56]
MM proliferation remains unclear. For instance, Notch receptor 1 overexpression increases MM cell
proliferation, suggesting a relevant role of this receptor in MM growth . We recently found that lentiviral-
[57]
[58]
mediated inhibition of Notch receptor 3 reduces proliferation and Cyclin D1 expression in MM cells . In
contrast, genetic inhibition of Notch receptor 2 does affect the growth of MM cells . Although yet to be
[58]
determined, due to the low/undetectable levels of Notch receptor 4, it is likely that this receptor does not
contribute to regulation of MM proliferation through homotypic interactions. Less is known about the
specific contribution of Notch ligands to MM proliferation. In this regard, it has been shown that Jagged 2
regulates MM self-renewal in vitro and in vivo .
[59]

Notch communication between MM cells and local microenvironmental cells also supports MM growth.
Most of the work performed in this area has focused on the supportive role of stromal cells. Interaction of
MM cells and bone marrow stromal cells induces the expression of Notch receptor 2 and Jagged 2 in MM
cells, which results in increased expression of the Notch target genes Hes1, Hey2, and Hes5 . Stromal cells
[60]
can activate Notch in MM cells via Dll 1 and cause an upregulation of Notch receptor 2 signaling, resulting
in increased Notch transcription [61,62] . MM cells can also employ Notch ligands to activate the pathway in
stromal cells, supporting the existence of bidirectional Notch communication between these cells types. In
this regard, MM Jagged 2-mediated Notch activation stimulates Il-6, Vegf, and Igf expression in stromal
cells, which in turn promotes MM growth and progression . Osteoclast-MM communication via Notch
[42]
also appears to promote MM survival through a mechanism involving the regulation of chrondoitin
synthase 1 (Chys1) and Notch receptor 2 expression . Our group recently demonstrated that osteocytes,
[63]
the most abundant cells in bone , activate Notch signaling in MM cells and increase MM cell proliferation
[11]