The Landscape of Signaling Pathways and Proteasome Inhibitors Combinations in Multiple Myeloma

Abstract

Multiple myeloma is a malignancy of terminally differentiated plasma cells, characterized by an extreme genetic heterogeneity that poses great challenges for its successful treatment. Due to antibody overproduction, MM cells depend on the precise regulation of the protein degradation systems. Despite the success of PIs in MM treatment, resistance and adverse toxic effects such as peripheral neuropathy and cardiotoxicity could arise. To this end, the use of rational combinatorial treatments might allow lowering the dose of inhibitors and therefore, minimize their side-effects. Even though the suppression of different cellular pathways in combination with proteasome inhibitors have shown remarkable anti-myeloma activities in preclinical models, many of these promising combinations often failed in clinical trials. Substantial progress has been made by the simultaneous targeting of proteasome and different aspects of MM-associated immune dysfunctions. Moreover, targeting deranged metabolic hubs could represent a new avenue to identify effective therapeutic combinations with PIs. Finally, epigenetic drugs targeting either DNA methylation, histone modifiers/readers, or chromatin remodelers are showing pleiotropic anti-myeloma effects alone and in combination with PIs. We envisage that the positive outcome of patients will probably depend on the availability of more effective drug combinations and treatment of early MM stages. Therefore, the identification of sensitive targets and aberrant signaling pathways is instrumental for the development of new personalized therapies for MM patients.

Keywords: combinatorial treatment; drug resistance; multiple myeloma; proteasome inhibitors; synthetic lethality.

Figure 1

Cellular processes affected by proteasome inhibition. Druggable targets within these pathways are a reservoir of synthetic lethal partners to proteasome inhibitors (PI).

Figure 2

Inhibition of proteasome inhibitor (PI) and PI3K/AKT/mTOR pathway results in synergistic cellular death. The PI3K/AKT/mTOR pathway is a central signaling hub in eukaryotic cells and it is connected to the ubiquitin–proteasome system (UPS) by balancing amino acid homeostasis (amino acid pool). Intensive feedback loops between mTORC1, mTORC2, and AKT pose a challenge to the successful inhibition of this pathway. Rapalogs target the mTORC1 complex; the mTORC1/mTORC2 dual inhibitor pp242 (torkinib) is more effective in combination with PI. The PI3K inhibitors copanlisib and TGR-1202 demonstrated synergistic cytotoxicity with PI. Several AKT inhibitors (perifosine, TAS-117, nelfinavir, montelukast) were found to synergize with (PI) by affecting endoplasmic reticulum (ER) stress, ERK, or c-Myc.

Figure 3

Dual inhibition of cellular stress and proteasome components. Ubiquitin-proteasome system (UPS), endoplasmic reticulum (ER) stress, and autophagy are precisely regulated and connected processes. Simultaneous blockage of targets within these pathways and proteasome leads to apoptosis. Proteasome inhibitors (PI) itself induces ER stress, thus targeting ER stress (verapamil, WRR139, KW-2478, and VER15508) in combination with PI leads to increased cellular death. As compensation to proteasome inhibition, cells turn on autophagy. Drugs affecting autophagy (metformin, bafilomycin A, chloroqouine, hydroxychloroquine) are synthetic lethal with PI. Inhibition of the pro-survival protein BCL2 (venetoclax) leads to increased apoptosis in combination with PI.

Figure 4

Proteasome inhibitors and metabolic pathways. Enhancement of glucose and glutamine metabolism is a hallmark of cancer cells, including multiple myeloma (MM). Targeting deranged cellular metabolism at various levels enhances the cytotoxicity of proteasome inhibitors in MM. Inhibition of the glucose transporter GLUT1 (STF-31) or the non-pharmacological inhibition of HIF1α and LDHA enzymes result in synergistic cell death with proteasome inhibitors (PI). The tricarboxylic acid (TCA) cycle enzyme IDH2 is synthetic lethal to PI through NAMPT/SIRT3/IDH2 pathway (AGI-6780, FK866). Degradation of glutamine can be targeted with Asparaginase (ASNase) or Glutaminase inhibitor (CB-832), affecting glutamate-dependent metabolites of TCA cycle. The cellular energy sensor AMPK is synthetically lethal to PI. OXPHOS inhibition by the mitochondrial protease ClpP (ONC201) leads to cellular stress and upregulation of transcriptional factor ATF4.

Figure 5

The molecular mechanisms underlying the effects of proteasome inhibitors (PIs) combination with epigenetic inhibitors. A plethora of anti-myeloma effects are observed by targeting epigenetic reprogrammers in combination with proteasome inhibition. HDAC6 specific inhibitors (MPT0G413, Tubacin, ACY-1215, WT161) lead to increased α-tubulin acetylation and to the inhibition of aggresomal pathways, thus activating unfolded protein response (UPR) and autophagy pathways. This mechanism is emphasized by proteasome inhibition. MPT0G413 can also affect MM cell growth, survival and adhesion to bone marrow stromal cells(BMSCs), through the inhibition of adhesion molecules and cytokines expression. The fusion molecule EDO-S101 acts as HDAC inhibitor as well as alkylating agent. EDO-S101 promotes polyUb-proteins accumulation, p21 expression, and induces DNA damage. Multiple pathways are responsible for the synergy between EDO-S101 and proteasome inhibitors, such as UPR hyper-activation, induction of autophagy, inhibition of cell cycle via upregulation of p21, and reduction of c-MYC expression. The BET inhibitor JQ1 in combination with carfilzomib enhances CHOP/EBPα-dependent Bim and Mcl-1 transcription, thus triggering ER stress and apoptosis. The DNA hypomethylating agent decitabine (DAC) demonstrated synergistic anti-MM effects in combinations with bortezomib. It is speculated that DAC inhibits β-catenin activity by promoting the expression of Wnt antagonists (DKK-1 and sFRP3). The EZH1/EZH2 inhibitor UNC1999 enhances the cytotoxicity of PI through a cooperative repression of MYC transcription.