STAT3 and MAPK signaling maintain overexpression of heat shock proteins 90α and β in multiple myeloma cells, which critically contribute to tumor-cell survival

Abstract

IntroductionMultiple myeloma (MM) is a common hematologic disorder in which expansion of a malignant plasma-cell (PC) clone in the bone marrow (BM) leads to osteolytic bone destruction, impaired hematopoiesis, and renal failure. 1 Despite therapeutic progress that has been achieved through the optimization of chemotherapeutic regimens through the introduction of novel drugs, such as bortezomib and lenalidomide, most MM patients currently succumb to their disease. 2 Clinically apparent MM is assumed to evolve through a multistep transformation process leading to oncogenic deregulation of signaling pathways. Acquired genetic alterations and the microenvironment are supposed to critically support the establishment and expansion of the malignant PC clone in the BM. 3,4 In particular, the interaction between MM cells and BM stromal cells (BMSCs) has been shown to protect the former from apoptosis. 5 Additionally, osteoclasts (OCs) and endothelial cells (ECs) have recently been reported to support tumor growth. [6][7][8] The interleukin-6 receptor/signal transducer and activator of transcription 3 (IL-6R/STAT3), the Ras/mitogen-activated protein kinase (Ras/MAPK), and the phosphatidylinositol 3-kinase/Akt (PI3K/Akt) pathways are well-characterized BM microenvironment (BMM)-triggered signal transduction cascades that sustain the viability of MM cells. 9,10 It has therefore been suggested that these pathways might serve as therapeutic targets. However, we have previously demonstrated that MM cells cultured in the presence of cells from the BMM are protected from apoptosis induced by a single pathway blockade. 5 Specifically, in the presence of BMSCs, disruption of either the MAPK/extracellular signal-regulated kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) module or of the IL-6R/STAT3 pathway is not sufficient to induce apoptosis of MM cells, whereas combined targeting of both pathways induces MM-cell death. 11 This indicates that both pathways cooperate to maintain MM-cell growth and survival. To investigate the mechanism that might distinguish the fatal effects of combined pathway blockade from the benign consequences of any single pathway disruption, we performed gene chip analysis and found genes encoding for heat shock proteins 90 (Hsp90s) to feature prominently among down-regulated genes. We therefore analyzed the role of Hsp90s in MM in greater detail.The ATP-dependent chaperone Hsp90 comprises 2 homologous proteins (Hsp90␣ and Hsp90␤) that are encoded by separate genes. Hsp90 accounts for the maturation and functional stability of a plethora of polypeptides termed Hsp90 client proteins. 12,13 Hsp90s are overexpressed in many cancers, and it is presumed that they are required to sustain aberrant signaling in malignant cells. 14 Several components of tumor-cell-associated growth and survival pathways have been found to be Hsp90 clients, and Hsp90s are thus thought to sustain functional expression of oncoproteins while enabling the transformed cell to tolerate the imbalanced signaling this might ...

Molecular Imaging of the Efficacy of Heat Shock Protein 90 Inhibitors in Living Subjects

“…Different classes of small molecule inhibitors have been developed to inhibit Hsp90 (a and h), but the mechanisms by which these inhibitors disrupt Hsp90/p23 interactions, as well as the role of each Hsp90 isoform (a and h) in determining sensitivity to these inhibitors, have not been noninvasively and longitudinally examined in living subjects. Because both Hsp90a and Hsp90h are expressed in cancer cells and their expression was compensatory (38), the individual role(s) of Hsp90a/p23 and Hsp90h/p23 in determining sensitivity and selectivity to Hsp90 inhibitors cannot be easily deciphered.…”

“…Because FL requires ATP for its enzymatic activity (43), SFL-PFAC may not be suitable for evaluating the efficacy of Hsp90 inhibitors (and other Feasibility of using SRL-PFAC for development of isoformselective Hsp90 inhibitors. Given (a) the Hsp90 chaperone system is highly up-regulated in multiple cancers (37), (b) both Hsp90a and Hsp90h are expressed in cancer cells and may play distinct roles in determining drug responses, and (c) the expression of Hsp90a and Hsp90h can be compensatory (38), Hsp90 inhibitors aimed at disruption of Hsp90a/p23 and Hsp90h/p23 interactions specifically or both interactions are being developed through indirect imaging of Hsp90a/p23 and Hsp90h/p23 interactions in conjunction with high-throughput screening. The lead compounds from the cell culture screen will be validated in living mice using xenografts, orthotopic and transgenic cancer models that express the split reporters.…”

“…The up-regulation of the Hsp90 chaperone system is thought to allow cancer cells to sustain expression of client proteins that are overexpressed or mutated in cancer (6,37,38). Different classes of small molecule inhibitors have been developed to inhibit Hsp90 (a and h), but the mechanisms by which these inhibitors disrupt Hsp90/p23 interactions, as well as the role of each Hsp90 isoform (a and h) in determining sensitivity to these inhibitors, have not been noninvasively and longitudinally examined in living subjects.…”

RARα2 expression is associated with disease progression and plays a crucial role in efficacy of ATRA treatment in myeloma

“…It is well-documented that JAK/STAT3 and MEK/ERK signaling pathways play a crucial role in myeloma tumor cell survival, tumor development, and progression. [33][34][35][36] Thus, these results strongly suggest that STAT3 and MEK/ERK signaling pathways are the target signaling pathways associated with high RAR␣2-induced MM cell growth and survival. …”

Novel HSP90 inhibitors, NVP-AUY922 and NVP-BEP800, radiosensitise tumour cells through cell-cycle impairment, increased DNA damage and repair protraction

“…Several key Hsp90 clients are involved in the processes characteristic to the malignant phenotype, such as invasion, angiogenesis and metastasis (Neckers, 2002). Hsp90 clients also contribute to the pathways leading to the induction of mitogen-activated protein kinases (MAPK) and nuclear factorkappa B (NF-kB) (Sato et al, 2003;Mitsiades et al, 2006;Chatterjee et al, 2007). Moreover, Hsp90 stabilises Raf-1, Akt and ErbB2 proteins (Schulte et al, 1995;Sato et al, 2000;Bull et al, 2004), which are known to be associated with protection against radiation-induced cell death (Pirollo et al, 1997;Gupta et al, 2001;Tanno et al, 2004).…”