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Page 6 of 15 Sabol et al. J Cancer Metastasis Treat 2021;7:20 https://dx.doi.org/10.20517/2394-4722.2021.35

[43]
by upregulating Cyclin D1 . In addition, osteocytes change the MM Notch receptor repertoire by rapidly
[43]
increasing Notch receptor 3 expression and inducing the expression of Notch receptor 4 . Notch receptor
[58]
3 knockdown in MM cells partially inhibited osteocyte-induced MM proliferation . Importantly, injection
of MM cells with knockdown for Notch receptor 3 in mice resulted in smaller tumors . In contrast,
[58]
knockdown of Notch receptor 2 in MM cells did not impair the proliferative effects of osteocytes on MM
cells . These results suggest that osteocyte-MM interactions contribute to MM growth and are mediated by
[58]
Notch receptor 3. However, it is likely that Notch receptor 1 and/or 4 also contributes to MM-osteocyte
communication. These studies are among the first ones investigating the specific contribution of each Notch
receptor to homotypic and heterotypic communication in MM. Future studies are needed to identify the
Notch receptor-ligand binding requirements for the different interactions between MM cells and local
microenvironmental cells.

γ-Secretase inhibitors (GSIs) are a class of small-molecule inhibitors that prevent the cleavage of γ-secretase
substrates and block Notch signaling activation by precluding the cleavage of Notch receptors. GSIs are
widely used to inhibit Notch and have been employed to better understand the contribution of Notch
signaling to MM growth. Treatment with GSI decreases Notch signaling, which in turn causes MM cell
apoptosis and decreases cell proliferation in vitro [43,50,54,64] . The Notch inhibition mediated by GSIs decreases
Cyclin D1 expression, consequently increasing the portion of MM cells in the G /G phase and decreasing
0
1
those in the S-phase. In addition, pharmacological inhibition of Notch with GSIs decreases the expression of
the anti-apoptotic protein Bcl-2, and activates Bak and Bax, resulting in activation of caspases and pro-
apoptotic proteins, leading to increased MM cell death [43,50,54,64] . GSIs are also able to block the proliferative
and pro-survival effects of heterotypic Notch activation by surrounding marrow cells. For instance, GSI
blocked stromal Notch activation and decreased MM cell proliferation mediated by Jagged 2
overexpression . Similarly, GSIs fully block osteocyte-induced Notch activation and increase proliferation
[47]
[43]
in MM cells . It is important to note that, besides Notch receptors, GSIs can also inhibit the processing of
other gamma-secretase substrates, such as cell-surface receptors and proteins involved in embryonic
development, hematopoiesis, cell adhesion, and cell/cell contact. Nonetheless, the results described above
with GSIs are in line with those resulting from direct manipulation of Notch components in MM cells or
environmental cells, and thus further support a role of Notch versus other γ-secretase substrates in MM cell
proliferation.

In addition to GSI, other drugs have shown to achieve their anti-MM effects through the modulation of
Notch signaling in MM cells. The ubiquitin specific peptidase 1 inhibitor SJB3-019A increases MM cell
apoptosis by downregulating the expression of Notch receptors 1 and 2 . Further, Wang et al.
[65]
[66]
demonstrated that treatment with sophocarpine triflorohydrazone (SCA), an alkaloid acting as an inhibitor
of Notch receptor 3, decreased MM cell viability, activated apoptosis, and decreased Notch receptor 3
expression in vitro, in 2 different MM cell lines . SCA treatment increased Bax proteins, decreased Bcl-2
[66]
[66]
proteins, and elevated caspase 3 levels . Additionally, SCA treatment caused a decrease in the Notch target
genes Hes1 and Hey1 expression .
[66]
Together, the results discussed in this section highlight the contribution of Notch signals to MM
progression by promoting cell cycle progression, improving survival in MM cells, and transmitting
proliferative cues from cells in the marrow niche. However, it is important to note that MM proliferation is
regulated by multiple signaling pathways . Nonetheless, these collective findings provide the rationale for
[19]
using Notch components as anti-myeloma therapeutic targets, an active area of investigation discussed in
the “Notch components as therapeutic targets” section.