Daily oral supplementation of Hochu-Ekki-To prevents osteoclastic activation and bone loss in ovariectomized mice.

Bone remodeling is sophisticatedly regulated by two different cell types: bone-resorbing osteoclasts and bone-forming osteoblasts. Hochu-Ekki-To, a Japanese traditional herbal medicine, is commonly used for the treatment of chronic diseases or frailty after an illness; however, its effects on metabolic bone diseases such as osteoporosis are not well known. We herein report that daily oral Hochu-Ekki-To administration significantly inhibits osteoclast activation as well as the reduction in bone volume in ovariectomized mice. Our results suggest that supplementation with Hochu-Ekki-To might be beneficial for the prophylaxis and treatment of metabolic bone diseases associated with abnormal osteoclast activation.

mTOR regulates skeletogenesis through canonical and noncanonical pathways.

The mechanistic/mammalian target of rapamycin (mTOR) regulates various cellular processes, in part through incorporation into distinct protein complexes. The mTOR complex 1 (mTORC1) contains the Raptor subunit, while mTORC2 specifically contains the Rictor subunit. Mouse genetic studies, including ours, have revealed a critical role for mTOR in skeletogenesis through its expression in undifferentiated mesenchymal cells. In addition, we have recently revealed that mTORC1 expression in chondrocytes is crucial for skeletogenesis. Recent work indicates that mTOR regulates cellular functions, depending on the context, through both complex-dependent (canonical pathway) and complex-independent roles (noncanonical pathway). Here, we determined that mTOR regulates skeletal development through the noncanonical pathway, as well as the canonical pathway, in a cell-type and context-specific manner. Inactivation of Mtor in undifferentiated mesenchymal cells or chondrocytes led to either severe hypoplasia in appendicular skeletons or a severe and generalized chondrodysplasia, respectively. Moreover, Rictor deletion in undifferentiated mesenchymal cells or chondrocytes led to mineralization defects in some skeletal components. Finally, we revealed that simultaneous deletion of Raptor and Rictor in undifferentiated mesenchymal cells recapitulated the appendicular skeletal phenotypes of Mtor deficiency, whereas chondrocyte-specific Raptor and Rictor double-mutants exhibited milder hypoplasia of appendicular and axial skeletons than those seen upon Mtor deletion. These findings indicate that mTOR regulates skeletal development mainly through the canonical pathway in undifferentiated mesenchymal cells, but at least in part through the noncanonical pathway in chondrocytes.

mTORC1 Overactivation Leads to Abnormalities in Skeletal Development.

The mechanistic/mammalian target of rapamycin complex-1 (mTORC1) integrates multiple signaling pathways and regulates various cellular processes. Tuberous sclerosis complex 1 (Tsc1) and complex 2 (Tsc2) are critical negative regulators of mTORC1. Mouse genetic studies, including ours, have revealed that inactivation of mTORC1 in undifferentiated mesenchymal cells and chondrocytes leads to severe skeletal abnormalities, indicating a pivotal role for mTORC1 in skeletogenesis. Here, we show that hyperactivation of mTORC1 influences skeletal development through its expression in undifferentiated mesenchymal cells at the embryonic stage. Inactivation of Tsc1 in undifferentiated mesenchymal cells by paired-related homeobox 1 (Prx1)-Cre-mediated recombination led to skeletal abnormalities in appendicular skeletons. In contrast, Tsc1 deletion in chondrocytes using collagen type II α1 (Col2a1)-Cre or in osteoprogenitors using Osterix (Osx)-Cre did not result in skeletal defects in either appendicular or axial skeletons. These findings indicate that Tsc complex-mediated chronic overactivation of mTORC1 influences skeletal development at the embryonic stage through its expression in undifferentiated mesenchymal cells but not in chondrocytes or osteoprogenitors.

The role of CDK8 in mesenchymal stem cells in controlling osteoclastogenesis and bone homeostasis.

Bone marrow mesenchymal stem cells (MSCs) are critical regulators of postnatal bone homeostasis. Osteoporosis is characterized by bone volume and strength deterioration, partly due to MSC dysfunction. Cyclin-dependent kinase 8 (CDK8) belongs to the transcription-related CDK family. Here, CDK8 in MSCs was identified as important for bone homeostasis. CDK8 level was increased in aged MSCs along with the association with aging-related signals. Mouse genetic studies revealed that CDK8 in MSCs plays a crucial role in bone resorption and homeostasis. Mechanistically, CDK8 in MSCs extrinsically controls osteoclastogenesis through the signal transducer and transcription 1 (STAT1)-receptor activator of the nuclear factor κ Β ligand (RANKL) axis. Moreover, aged MSCs have high osteoclastogenesis-supporting activity, partly through a CDK8-dependent manner. Finally, pharmacological inhibition of CDK8 effectively repressed MSC-dependent osteoclastogenesis and prevented ovariectomy-induced osteoclastic activation and bone loss. These findings highlight that the CDK8-STAT1-RANKL axis in MSCs could play a crucial role in bone resorption and homeostasis.

CDK8 maintains stemness and tumorigenicity of glioma stem cells by regulating the c-MYC pathway.

Glioblastoma (GBM) is the most malignant form of glioma. Glioma stem cells (GSCs) contribute to the initiation, progression, and recurrence of GBM as a result of their self-renewal potential and tumorigenicity. Cyclin-dependent kinase 8 (CDK8) belongs to the transcription-related CDK family. Although CDK8 has been shown to be implicated in the malignancy of several types of cancer, its functional role and mechanism in gliomagenesis remain largely unknown. Here, we demonstrate how CDK8 plays an essential role in maintaining stemness and tumorigenicity in GSCs. The genetic inhibition of CDK8 by shRNA or CRISPR interference resulted in an abrogation of the self-renewal potential and tumorigenicity of patient-derived GSCs, which could be significantly rescued by the ectopic expression of c-MYC, a stem cell transcription factor. Moreover, we demonstrated that the pharmacological inhibition of CDK8 significantly attenuated the self-renewal potential and tumorigenicity of GSCs. CDK8 expression was significantly higher in human GBM tissues than in normal brain tissues, and its expression was positively correlated with stem cell markers including c-MYC and SOX2 in human GBM specimens. Additionally, CDK8 expression is associated with poor survival in GBM patients. Collectively, these findings highlight the importance of the CDK8-c-MYC axis in maintaining stemness and tumorigenicity in GSCs; these findings also identify the CDK8-c-MYC axis as a potential target for GSC-directed therapy.