G9a/GLP targeting in MM promotes autophagy-associated apoptosis and boosts proteasome inhibitor–mediated cell death

Abstract

Multiple myeloma (MM) is an (epi)genetic highly heterogeneous plasma cell malignancy that remains mostly incurable. Deregulated expression and/or genetic defects in epigenetic-modifying enzymes contribute to high-risk disease and MM progression. Overexpression of the histone methyltransferase G9a was reported in several cancers, including MM, correlating with disease progression, metastasis, and poor prognosis. However, the exact role of G9a and its interaction partner G9a-like protein (GLP) in MM biology and the underlying mechanisms of action remain poorly understood. Here, we report that high G9a RNA levels are associated with a worse disease outcome in newly diagnosed and relapsed MM patients. G9a/GLP targeting using the specific G9a/GLP inhibitors BIX01294 and UNC0638 induces a G1-phase arrest and apoptosis in MM cell lines and reduces primary MM cell viability. Mechanistic studies revealed that G9a/GLP targeting promotes autophagy-associated apoptosis by inactivating the mTOR/4EBP1 pathway and reducing c-MYC levels. Moreover, genes deregulated by G9a/GLP targeting are associated with repressive histone marks. G9a/GLP targeting sensitizes MM cells to the proteasome inhibitors (PIs) bortezomib and carfilzomib, by (further) reducing mTOR signaling and c-MYC levels and activating p-38 and SAPK/JNK signaling. Therapeutic treatment of 5TGM1 mice with BIX01294 delayed in vivo MM tumor growth, and cotreatment with bortezomib resulted in a further reduction in tumor burden and a significantly prolonged survival. In conclusion, we provide evidence that the histone methyltransferases G9a/GLP support MM cell growth and survival by blocking basal autophagy and sustaining high c-MYC levels. G9a/GLP targeting represents a promising strategy to improve PI-based treatment in patients with high G9a/GLP levels.

Pyrroline-5-Carboxylate Reductase 1: a novel target for sensitizing multiple myeloma cells to bortezomib by inhibition of PRAS40-mediated protein synthesis

“…Inhibition of p-p70, p-S6, p-4EBP1 and p-eIF4E has been show to impair MM progression and induce cell death [ 46 , 48 ]. For instance, our research group has previously shown that G9a/GLP targeting in myeloma inhibits p-mTOR, followed by a decrease in p-4EBP1 and p-eIF4E, leading to MM apoptosis [ 49 ]. However, the PRAS40-mediated protein synthesis pathway as a whole has not been studied yet in MM.…”

De novo methylation of histone H3K23 by the methyltransferases EHMT1/GLP and EHMT2/G9a

“…This work could also shed light on combinatorial post-translational modi cation states involving these newly identi ed targets in vivo, as well as provide new insights into mechanisms of PTM cross-talk and how oncohistones can impart changes in the epigenome [42], [43]. More interestingly, in vivo perturbations in EHMT1/GLP and EHMT2/G9a have been implicated in many cellular processes including autophagy [44], DNA methylation, [35], [45], hypoxia [46], tumor suppression [47]- [50], chromatin remodeling [20], and synaptic plasticity [51] to name a few, and homozygous loss of EHMT1/GLP results in embryonic lethality in mice [52]. Additionally, both enzymes have been the focus of cancer therapies in recent years [48], [53]- [57].…”

De novo methylation of histone H3K23 by the methyltransferases EHMT1/GLP and EHMT2/G9a

“…More interestingly, in vivo perturbations in EHMT1/GLP and EHMT2/G9a have been implicated in many cellular processes including autophagy [ 43 ], DNA methylation, [ 35 , 44 ], hypoxia [ 45 ], tumor suppression [ 46 – 49 ], chromatin remodeling [ 20 ], and synaptic plasticity [ 50 ] to name a few, and homozygous loss of EHMT1/GLP results in embryonic lethality in mice [ 51 ]. Additionally, both enzymes have been the focus of cancer therapies in recent years [ 44 , 47 , 49 , 52 – 54 ].…”