Anti-apoptotic Bcl-2 family member Mcl-1 regulates cell viability and bone-resorbing activity of osteoclasts

PMAIP1 regulates autophagy in osteoblasts via the AMPK/mTOR pathway in osteoporosis

Osteoporosis (OP) is a highly prevalent disorder characterized by low bone mass that severely reduces patient quality of life. Although numerous treatments for OP have been introduced in clinic, many have side effects and high costs. Therefore, there is still an unmet need for optimal solutions. Here, raw signal analysis was used to identify potential high-risk factors for OP, and the biological functions and possible mechanisms of action (MOAs) of these factors were explored via gene set enrichment analysis (GSEA). Subsequently, molecular biological experiments were performed to verify and analyze the discovered risk factors in vitro and in vivo. PMAIP1 was identified as a potential risk factor for OP and significantly suppressed autophagy in osteoblasts via the AMPK/mTOR pathway, thereby inhibiting the proliferation and differentiation of osteoblasts. Furthermore, we constructed an ovariectomy (OVX) model of OP in rats and simultaneously applied si-PMAIP1 for in vivo interference. si-PMAIP1 upregulated the expression of LC3B and p-AMPK and downregulated the expression of p-mTOR, and these effects were reversed by the autophagy inhibitor. Micro-CT revealed that, si-PMAIP1 significantly inhibited the development of osteoporosis in OVX model rats, and this therapeutic effect was attenuated by treatment with an autophagy inhibitor. This study explored the role and mechanism of PMAIP1 in OP and demonstrated that PMAIP1 may serve as a novel target for OP treatment.

Leukemia/lymphoma-related factor (LRF) or osteoclast zinc finger protein (OCZF) overexpression promotes osteoclast survival by increasing Bcl-xl mRNA: A novel regulatory mechanism mediated by the RNA binding protein SAM68

RANKL induces NFATc1, a key transcriptional factor to induce osteoclast-specific genes such as cathepsin K, whereas transcriptional control of osteoclast survival is not fully understood. Leukemia/lymphoma-related factor (LRF) in mouse and osteoclast zinc finger protein (OCZF) in rat are zinc finger and BTB domain-containing protein (zBTB) family of transcriptional regulators, and are critical regulators of hematopoiesis. We have previously shown that differentiation and survival were enhanced in osteoclasts from OCZF-Transgenic (Tg) mice. In the present study, we show a possible mechanism of osteoclast survival regulated by LRF/OCZF and the role of OCZF overexpression in pathological bone loss. In the in vitro cultures, LRF was highly colocalized with NFATc1 in cells of early stage in osteoclastogenesis, but only LRF expression persisted after differentiation into mature osteoclasts. LRF expression was further enhanced in resorbing osteoclasts formed on dentin slices. Osteoclast survival inhibitor such as alendronate, a bisphosphonate reduced LRF expression. Micro CT evaluation revealed that femurs of OCZF-Tg mice showed significantly lower bone volume compared to that of WT mice. Furthermore, OCZF overexpression markedly promoted bone loss in ovariectomy-induced osteolytic mouse model. The expression of anti-apoptotic Bcl-xl mRNA, which is formed by alternative splicing, was enhanced in the cultures in which osteoclasts are formed from OCZF-Tg mice. In contrast, the expression of pro-apoptotic Bcl-xs mRNA was lost in the culture derived from OCZF-Tg mice. We found that the expression levels of RNA binding splicing regulator, Src substrate associated in mitosis of 68 kDa (Sam68) protein were markedly decreased in OCZF-Tg mice-derived osteoclasts. In addition, shRNA-mediated knockdown of Sam68 expression increased the expression of Bcl-xl mRNA, suggesting that SAM68 regulates the expression of Bcl-xl. These results indicate that OCZF overexpression reduces protein levels of Sam68, thereby promotes osteoclast survival, and suggest that LRF/OCZF is a promising target for regulating pathological bone loss.

Anticancer effects and the mechanism underlying 2-methoxyestradiol in human osteosarcoma in vitro and in vivo

Osteosarcoma (OS) occurs in both children and adolescents and leads to a poor prognosis. 2-methoxyestradiol (2-ME) has a strong antitumor effect and is effective against numerous types of tumor. However, 2-ME has a low level of antitumor effects in OS. The present study investigated the effects of 2-ME on the proliferation and apoptosis of human MG63 OS cells. The potential biological mechanisms by which 2-ME exerts its biological effects were also investigated in the present study. The results of the present study demonstrated that 2-ME inhibited the proliferation of OS cells in a time- and dose-dependent manner, induced G₂/M phase cell cycle arrest and early apoptosis. The expression levels of vascular endothelial growth factor (VEGF), Bcl-2 and caspase-3 were measured via western blotting and reverse transcription-quantitative PCR. As the concentration of 2-ME increased, the RNA and protein expression levels of VEGF and Bcl-2 decreased gradually, whereas the expression of caspase-3 increased gradually. In addition, tumor growth in nude mice was suppressed by 2-ME with no toxic side effects observed in the liver or kidney. Immunohistochemistry demonstrated that the expression levels of Bcl-2 and VEGF were significantly lower, and those of caspase-3 were significantly higher in test mice compared with the control group. TUNEL staining of xenograft tumors revealed that with increased 2-ME concentration, the number of apoptotic cells also gradually increased. Thus, 2-ME effectively inhibited the proliferation and induced apoptosis of MG63 OS cells in vitro and in vivo with no obvious side effects. The mechanism of the anticancer effect of 2-ME may be associated with the actions of Bcl-2, VEGF and caspase-3.

Heme oxygenase-1 attenuates the inhibitory effect of bortezomib against the APRIL-NF-κB-CCL3 signaling pathways in multiple myeloma cells: Corelated with bortezomib tolerance in multiple myeloma

Osteoclasts (OCs) play an essential role in bone destruction in patients with multiple myeloma (MM). Bortezomib can ameliorate bone destruction in patients with MM, but advanced MM often resists bortezomib. We studied the molecular mechanisms of bortezomib tolerance in MM. The expression of the MM-related genes in newly diagnosed patients with MM and normal donors were studied. C-C motif chemokine ligand 3 (CCL3) is a cytokine involved in the differentiation of OCs, and its expression is closely related to APRIL (a proliferation-inducing ligand). We found that bortezomib treatment inhibited APRIL and CCL3. But the heme oxygenase-1 (HO-1) activator hemin attenuated the inhibitory effects of bortezomib on APRIL and CCL3. We induced mononuclear cells to differentiate into OCs, and the enzyme-linked immunosorbent assay showed that the more OCs differentiated, the higher the levels CCL3 secretions detected. Animal experiments showed that hemin promoted MM cell infiltration in mice. The weight and survival rate of tumor mice were associated with HO-1 expression. Immunohistochemical staining showed that HO-1, APRIL, and CCL3 staining were positively stained in the tumor infiltrating sites. Then, MM cells were transfected with L-HO-1/si-HO-1 expression vectors and cultured with an nuclear factor (NF)-kappa B (κB) pathway inhibitor, QNZ. The results showed that HO-1 was the upstream gene of APRIL, NF-κB, and CCL3. We showed that HO-1 could attenuate the inhibitory effect of bortezomib against the APRIL-NF-κB-CCL3 signaling pathways in MM cells, and the tolerance of MM cells to bortezomib and the promotion of bone destruction are related to HO-1.

A Novel MCL1 Inhibitor Combined with Venetoclax Rescues Venetoclax-Resistant Acute Myelogenous Leukemia

Suppression of apoptosis by expression of antiapoptotic BCL2 family members is a hallmark of acute myeloblastic leukemia (AML). Induced myeloid leukemia cell differentiation protein (MCL1), an antiapoptotic BCL2 family member, is commonly upregulated in AML cells and is often a primary mode of resistance to treatment with the BCL2 inhibitor venetoclax. Here, we describe VU661013, a novel, potent, selective MCL1 inhibitor that destabilizes BIM/MCL1 association, leads to apoptosis in AML, and is active in venetoclax-resistant cells and patient-derived xenografts. In addition, VU661013 was safely combined with venetoclax for synergy in murine models of AML. Importantly, BH3 profiling of patient samples and drug-sensitivity testing ex vivo accurately predicted cellular responses to selective inhibitors of MCL1 or BCL2 and showed benefit of the combination. Taken together, these data suggest a strategy of rationally using BCL2 and MCL1 inhibitors in sequence or in combination in AML clinical trials. SIGNIFICANCE: Targeting antiapoptotic proteins in AML is a key therapeutic strategy, and MCL1 is a critical antiapoptotic oncoprotein. Armed with novel MCL1 inhibitors and the potent BCL2 inhibitor venetoclax, it may be possible to selectively induce apoptosis by combining or thoughtfully sequencing these inhibitors based on a rational evaluation of AML.See related commentary by Leber et al., p. 1511.This article is highlighted in the In This Issue feature, p. 1494.

Recombinant MDA-7/IL24 Suppresses Prostate Cancer Bone Metastasis through Downregulation of the Akt/Mcl-1 Pathway

Prostate cancer is a principal cause of cancer-associated morbidity in men. Although 5-year survival of patients with localized prostate cancer approaches 100%, survival decreases precipitously after metastasis. Bone is the preferred site for disseminated prostate cancer cell colonization, altering the equilibrium of bone homeostasis resulting in weak and fragile bones. Currently, no curative options are available for prostate cancer bone metastasis. Melanoma differentiation associated gene-7 (MDA-7)/IL24 is a well-studied cytokine established as a therapeutic in a wide array of cancers upon delivery as a gene therapy. In this study, we explored the potential anticancer properties of MDA-7/IL24 delivered as a recombinant protein. Using bone metastasis experimental models, animals treated with recombinant MDA-7/IL24 had significantly less metastatic lesions in their femurs as compared with controls. The inhibitory effects of MDA-7/IL24 on bone metastasis resulted from prostate cancer-selective killing and inhibition of osteoclast differentiation, which is necessary for bone resorption. Gain- and loss-of-function genetic approaches document that prosurvival Akt and Mcl-1 pathways are critically important in the antibone metastatic activity of MDA-7/IL24. Our previous findings showed that MDA-7/IL24 gene therapy plus Mcl-1 inhibitors cooperate synergistically. Similarly, an Mcl-1 small-molecule inhibitor synergized with MDA-7/IL24 and induced robust antibone metastatic activity. These results expand the potential applications of MDA-7/IL24 as an anticancer molecule and demonstrate that purified recombinant protein is nontoxic in preclinical animal models and has profound inhibitory effects on bone metastasis, which can be enhanced further when combined with an Mcl-1 inhibitory small molecule. Mol Cancer Ther; 17(9); 1951-60. ©2018 AACR.

Evidence for altered osteoclastogenesis in splenocyte cultures from VDR knockout mice

The indirect action of 1α,25(OH)₂-vitamin-D₃ (1,25D) on the osteoclast through stromal signalling is well established. The role of vitamin D in osteoclasts through direct 1,25D-VDR signalling is less well known. We showed previously that local 1,25D synthesis in osteoclasts modified osteoclastogenesis and osteoclastic resorptive activity. In this study, we hypothesised that osteoclasts lacking VDR expression would display an enhanced resorptive capacity due to the loss of 1,25D signalling. Splenocytes were cultured under osteoclast-differentiating conditions from mice with global deletion of the Vdr gene (VDRKO) and this was compared with age-matched wild-type littermate controls (WT). In VDRKO cultures, osteoclastogenesis was reduced, as indicated by fewer TRAP-positive multinucleated cells at all time points measured (p<0.05) compared to WT levels. However, VDRKO osteoclasts demonstrated greater resorption on a per cell basis than their WT counterparts. VDRKO cultures expressed greatly increased c-Fos mRNA compared to WT. In addition, the ratio of expression of the pro-apoptotic gene Bax to the pro-survival gene Bcl-2 was decreased in VDRKO cultures, implying that these osteoclasts may survive longer than WT osteoclasts. Our data indicate abnormal osteoclastogenesis due to the absence of Vdr expression, consistent with direct effects of vitamin D signalling being important for regulating the maturation and resorptive activities of osteoclasts.

Lutein, a carotenoid, suppresses osteoclastic bone resorption and stimulates bone formation in cultures

Lutein, a member of the xanthophyll family of carotenoids, suppressed IL-1-induced osteoclast differentiation and bone resorption. The survival of mature osteoclasts was also suppressed by lutein in cultures. When lutein was added to the cultures of osteoblasts, lutein enhanced the formation of mineralized bone nodules by elevating BMP2 expression and inhibiting sclerostin expression. Lutein may be beneficial for bone health.

Evidence for altered osteoclastogenesis in splenocyte cultures from Cyp27b1 knockout mice

The association between increased serum 25-hydroxyvitamin D (25D) and reduced osteoclastic bone resorption is well known. Previously, we have demonstrated that mechanism by which this occurs, may include the conversion of 25D to 1,25-dihydroxyvitamin D (1,25D) by osteoclasts, catalysed by the CYP27B1 enzyme. Local 1,25D synthesis in osteoclasts was shown to regulate osteoclastogenesis and moderating resorptive activity. Thus, we hypothesised that osteoclasts differentiated from mice with global deletion of the Cyp27b1 gene (Cyp27b1 KO) would display enhanced resorptive capacity due to the lack of an ameliorating effect of 1,25D. Splenocytes isolated from Cyp27b1 KO mice or their wild-type (WT) littermates between 6 and 8 weeks of age were cultured under osteoclast-forming conditions for up to 14 days. Osteoclast formation was measured by staining for the osteoclast marker tartrate resistant acid phosphatase (TRAP). Bone resorption activity was measured by plating the cells on a bone-like substrate. In Cyp27b1 KO cultures, osteoclastogenesis was reduced, as indicated by fewer TRAP-positive multinucleated cells at all time points measured (p<0.05) when compared to wild-type (WT) levels. However, Cyp27b1 KO osteoclasts demonstrated greater resorption on a per cell basis than their WT counterparts (p<0.03). In addition, the ratio of expression of the pro-apoptotic gene Bax to the pro-survival gene Bcl-2 was decreased in Cyp27b1 KO cultures, implying that these smaller osteoclasts survive longer than WT osteoclasts. Our data indicate abnormal osteoclastogenesis due to the absence of CYP27B1 expression, consistent with the notion that endogenous metabolism of 25D optimises osteoclastogenesis and ameliorates the resulting activity of mature osteoclasts.

Osteoprotegerin expressed by osteoclasts: an autoregulator of osteoclastogenesis

Osteoprotegerin (OPG) is secreted by stromal and osteoblastic lineage cells and inhibits osteoclastogenesis by preventing the interaction of receptor activator of nuclear factor-κB ligand (RANKL) with receptor activator of nuclear factor-κB (RANK). In this study, the expression of OPG in osteoclasts themselves and its biological functions during osteoclastogenesis were investigated for the first time. OPG expression in vivo in the developing rat maxilla was examined by immunofluorescence analysis. OPG expression in osteoclasts during in vitro osteoclastogenesis was determined by reverse-transcription polymerase chain-reaction (RT-PCR), Western blot, and immunofluorescence staining. We determined the function of OPG produced by osteoclasts during osteoclastogenesis by silencing the OPG gene. The effects of OPG on bone-resorbing activity and apoptosis of mature osteoclasts were examined by the assay of resorptive pit formation on calcium-phosphate-coated plate and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining, respectively. In the immunofluorescence findings, strong immunoreactivities were unexpectedly seen in multinucleated tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts around the growing and erupting tooth germs in the rat alveolar bone. In vitro, OPG expression was significantly increased during the differentiation of osteoclasts from mouse bone-marrow-derived cells treated with a combination of macrophage colony-stimulating factor (M-CSF) and RANKL. Interestingly, it was found that OPG small interfering (si)RNA treatment during osteoclastogenesis enhanced the sizes of osteoclasts, but attenuated their bone-resorbing activity. Also, the increased chromosomal DNA fragmentation and caspase-3 activity in the late phase of osteoclastogenesis were found to be decreased by treatment with OPG siRNA. Furthermore, effects of OPG siRNA treatment on osteoclastogenesis and bone-resorbing activity were recovered by the treatment of exogenous OPG. These results suggest that OPG, expressed by the osteoclasts themselves, may play an auto-regulatory role in the late phase of osteoclastogenesis through the induction of apoptosis.