Multiple myeloma (MM) is an incurable hematologic malignancy characterized by the clonal expansion of malignant plasma cells within the bone marrow. Activator Protein-1 (AP-1) transcription factors (TFs), comprised of the JUN, FOS, ATF and MAF multigene families, are implicated in a plethora of physiologic processes and tumorigenesis including plasma cell differentiation and MM pathogenesis. Depending on the genetic background, the tumor stage, and cues of the tumor microenvironment, specific dimeric AP-1 complexes are formed. For example, AP-1 complexes containing Fra-1, Fra-2 and B-ATF play central roles in the transcriptional control of B cell development and plasma cell differentiation, while dysregulation of AP-1 family members c-Maf, c-Jun, and JunB is associated with MM cell proliferation, survival, drug resistance, bone marrow angiogenesis, and bone disease. The present review article summarizes our up-to-date knowledge on the role of AP-1 family members in plasma cell differentiation and MM pathophysiology. Moreover, it discusses novel, rationally derived approaches to therapeutically target AP-1 TFs, including protein-protein and protein-DNA binding inhibitors, epigenetic modifiers and natural products.
1. Introduction
First described in the 1980′s [1][2][3][4][5], members of the Activator Protein-1 (AP-1) transcription factor (TF) family contain the characteristic basic leucine zipper (bZIP) domain, which enables dimer formation via a stretch of hydrophobic leucines, and facilitates DNA interaction via positively charged amino acids. AP-1 family members include the JUN (c-Jun, JunB and JunD), FOS (c-Fos, FosB, Fra-1 and Fra-2), ATF (ATF2, ATF3/LRF1, ATF4, ATF5, ATF6B, ATF7, B-ATF, B-ATF2, B-ATF3, JDP1 and JDP2) and MAF (MafA, MafB, c-Maf, Nrl and MafF/G/K) multigene subfamilies [6]. While Jun proteins heterodimerize or homodimerize with members of their own subfamily, Fos proteins must heterodimerize. Depending on their composition, AP-1 TFs bind to the TPA-response element (TRE) [5′-TGA (C/G) TCA-3′] and, with lower affinity, to the cAMP response element (CRE) [5′-TGA CG TCA-3′], which is almost identical to TRE. Specifically, Jun: Jun dimers and Jun: Fos dimers preferentially bind to TRE and CRE, but also to variant sequences within the DNA; ATF-containing Jun: ATF and ATF: ATF dimers preferentially bind CRE; and MAF-containing dimers bind either to MAF-recognition element (MARE) I [5′-TGC TGA (C/G) TCA GCA-3′] or to MARE II [5′-TGC TGA CG TCA GCA-3′], extensions of TRE and CRE sequences [1]. In addition, AP-1 dimers interact with non-bZIP proteins including CBP/p300, p65/NFκB and Rb. AP-1 activity is induced by a multitude of intrinsic and extrinsic stimuli and environmental insults including cytokines, growth factors, direct cell-cell and cell-extracellular-matrix interactions, hormones, phorbol esters, UV radiation as well as viral and bacterial infections. It is predominantly regulated via MAPK-, PI3K- and NFκB- dependent transcription, but also via post-translational phosphorylation, mRNA turnover and protein stability [1][7][8]. Ultimately, these events determine specific transcriptional programs.
Accounting for ~10% of hematologic malignancies, Multiple Myeloma (MM) is characterized by the clonal expansion of malignant plasma cells (PCs) within the bone marrow (BM) and the abnormal increase of monoclonal paraprotein, leading to specific end-organ damage, including hypercalcemia, renal failure, anemia and lytic bone lesions (CRAB criteria)
[9]. The development of MM is initiated from a pre-malignant, asymptomatic stage called Monoclonal Gammopathy of Undetermined Significance (MGUS), and a more advanced pre-malignant, asymptomatic stage called Smoldering MM (SMM), due to cytogenetic alterations in post-germinal center (GC) PCs. During the evolution of MGUS or SMM into MM and ultimately PC leukemia (PCL), additional genetic aberrations as well as the supportive BM microenvironment play pivotal roles
[10][11]. The incidence of MGUS is >3% of the population over the age of 50, with a progression rate of 1% per year to MM; whereas SMM transforms to MM at a rate of ~10% per year during the first five years after diagnosis. As primary genetic events, approximately 40% of MM patients harbor trisomies of chromosomes, ~30% have immunoglobulin (Ig) heavy chain (IgH) translocations and ~15% have both trisomies and IgH translocations. The IgH locus is located on chromosome 14q32, and the translocations and genes affected include t(4;14)(p16;q32) (
FGFR3 and
MMSET), t(6;14)(p21;q32) (
CCND3), t(11;14)(q13;q32) (
CCND1), t(14;16)(q32;q23) (
c-MAF) and t(14;20)(q32;q11) (
MAFB). Secondary genetic events include gains and deletions of chromosomes, global hypomethylation, mutations and secondary translocations t(8;14)(q24;q32) (
MYC). Specifically, high-risk MM is characterized by the presence of gain of chromosome 1q, deletion of chromosome 17p (del(17p)), t(4;14), t(14;16), t(14;20) or p53 mutations
[9][12]. Despite therapeutic advances including the introduction of ImmunoModulatory Drugs (IMiDs), proteasome inhibitors (PIs), monoclonal antibodies and most recently selinexor, a Selective Inhibitor of Nuclear Export (SINE) that binds and inactivates exportin-1 (XPO1)
[13], the B Cell Maturation Antigen (BCMA) targeting antibody-drug-conjugate (ADC) belantamab-mafodotin
[14], and BCMA-directed CAR-T cells
[9], the management of MM remains challenging, mainly due to the development of drug resistance. Therefore, the identification of novel therapeutic targets and the development of derived anti-MM treatment strategies are urgently needed.
Our increasing knowledge of B cell differentiation and resultant generation of normal PCs have been fundamental to understand how these processes are deranged in MM cells
[15]. PCs that undergo IgH switch recombination home to the BM, where they occupy special survival niches, and become long-lived PCs
[16]. Besides their central role in many, if not all, physiologic processes, including PC differentiation, deregulation of AP-1 TFs has been implicated in solid and hematologic malignancies, including MM
[17]. Deregulation of TFs contributes to MM pathogenesis through: (1) direct TF modifications (e.g., mutations); (2) intrinsic genetic alterations or extrinsic stimuli within the BM microenvironment that trigger signaling pathway-mediated TF activation or inhibition; (3) epigenetic changes in DNA methylation, histone modifications and non-coding RNAs; and (4) TF dependency on prolonged oncogene activity (“oncogenic addiction”)
[11][18][19][20][21][22].
The present review article will comprehensively summarize our up-to-date knowledge on the critical role of AP-1 TFs in PC differentiation and MM pathophysiology. Moreover, we will discuss novel, rationally derived strategies to therapeutically target AP-1 TFs, including protein-protein and protein-DNA binding inhibitors, epigenetic modifiers and natural products.
2. AP-1 in Plasma Cell Biology
AP-1 TFs play a critical role in PC formation and function. Specific functions of selected AP-1 TF family members during PC differentiation will be discussed below ( Table 1 and Figure 1). For details, please refer to the original article (10.3390/cancers13102326) [23].
3. AP-1 in Multiple Myeloma
Besides acting as critical regulators in PC differentiation, AP-1 TFs are emerging as “master regulators” of aberrant gene expression programs in MM. Below we will discuss functions of AP-1 TFs that have specifically been associated with MM pathogenesis during recent years, c-Maf and MafB, c-Jun, JunB, in particular. Whether Fra-1, Fra-2, B-ATF and other AP-1 family members are deregulated in MM cells is currently unknown and subject of our own and others’ ongoing research efforts (Table 1 and Figure 1). For details, please refer to the original article (10.3390/cancers13102326) [23].
Table 1.
Function of AP-1 in plasma cell biology and multiple myeloma pathophysiology.
66 |
] |
[ |
67 | ] |
Leucine zipper peptide
(Superzipper) |
Leucine zipper dimerization domains
of both c-Jun and c-Fos |
[68] |
Inhibition of protein-
DNA binding |
T-5224 |
bZIP domain of c-Fos/AP-1 -DNA
complex |
[69][70] |
MLN944 (XR5944) |
TRE |
[71] |
SR11302 |
TRE |
[72][73] |
Dominant negative peptide A-Fos |
bZIP domain of c-Jun |
[74] |
Regulation of epigenetic events |
Valproic acid (VPA)
Vorinostat (SAHA)
Trichostatin A (TSA)
LBH589 |
HDAC
(Transcriptional suppression of c-Jun
and Fra-1 expression) |
[75] |
TC-E 5003 (TC-E) |
PRMT
(Suppression of c-Jun expression
and nuclear translocation) |
[76] |
Natural products |
Curcumin |
Suppression of c-Fos and c-Jun
expression and their binding to DNA |
[77] |
Resveratrol |
Suppression of c-Fos and c-Jun
expression and AP-1 activity |
[78] |
Veratramine |
TRE |
[79] |
AP-1 Member |
Activity |
Mechanism |
References |
Plasma cell biology |
Fra-1 |
Suppresses B cell differentiation into PCs and decreases Ig production |
Inhibition of | Prdm1 | /Blimp-1 expression by preventing binding of c-Fos to the promoter |
[24][25][26] |
Fra-2 |
Enhances B cell proliferation and
differentiation at multiple stages |
Transcriptional induction of FOXO-1 and IRF-4 expression, and their downstream targets Ikaros, IL7Ra, Rag1/2 and Aiolos |
[27] |
B-ATF |
Essential for GC formation
and effective CSR |
Downstream of FOXO-1, modulating the expression of | Aicda | /AID and GLTs from the Ig locus of B cells in the GC |
[28][29] |
Regulates B cell activation
and GC response |
Binding of B-ATF containing AP-1 complexes and IRF-4 to the AICE motif of target genes |
[30][31] |
Multiple myeloma |
c-Maf
MafB |
Overexpressed in MM |
Chromosomal translocation t(14;16), t(14;20)
MMSET/MEK/ERK/AP-1 signaling sequelae |
[11][18][32] |
Promote MM cell proliferation,
migration and invasion, survival,
adhesion and pathological
interactions with BMSC |
Regulation of cyclin D2, ARK5, DEPTOR, and integrin β7 expression |
[33][34][35] |
Confer resistance to PIs bortezomib and carfilzomib |
Abrogation of GSK3β-mediated
proteasomal degradation of c-Maf and MafB |
[36][37] |
c-Jun |
Lower expression in primary MM cells compared to normal PCs |
Unknown |
[38] |
Upregulated in MM cells by
adaphostin or bortezomib
Inhibits proliferation and induces
apoptosis |
Caspase-mediated c-Abl cleavage
Upregulation of EGR-1
Upregulation of p53 |
[39][40][41][42] |
JunB |
BMSC- and IL-6- triggered upregulation in MM cells |
MEK/MAPK- and NFκB- dependent |
[43] |
Promotes MM cell proliferation |
Cell cycle regulation |
Protects MM cells against
dexamethasone- and bortezomib- induced cell death |
Inhibition of apoptotic pathways |
Promotes MM BM angiogenesis |
Transcriptional regulation of angiogenic factors VEGF, VEGFB and IGF1 |
[44] |
Bone metabolism |
c-Fos |
Regulates OC differentiation
(Block in OC differentiation in mice
lacking c-Fos) |
Induced by RANKL and M-CSF
Transcriptional regulation of Fra-1 and NFATc1 |
[45][46][47][48] |
Fra-1 |
Regulates OB activity
and bone matrix formation
(Mice overexpressing Fra-1 develop
osteosclerosis) |
Regulation of bone matrix component production by OBs (osteocalcin, collagen1α2, and matrix Gla protein) |
[49][50] |
Fra-2 |
Regulates OB differentiation
(Fra-2-overexpressing mice are
osteosclerotic) |
Transcriptional regulation of osteocalcin and collagen1α2 |
[51] |
Controls OC survival and size
(Increased size and numbers of OCs in Fra-2-deficient mice) |
Transcriptional induction of LIF via
Fra-2: c-Jun heterodimers
Modulation of LIF/LIF-receptor/PHD2/HIF1α
signaling sequelae |
[52] |
JunB |
Regulates OB proliferation
and differentiation
(Mice lacking JunB are osteopenic) |
Cyclin D1 and cyclin A expression,
and collagen1α2, osteocalcin and
bone sialoprotein production |
[53] |
Regulates OC proliferation
and differentiation |
Dimerization partner of c-Fos (?) |