## 8 Proteasome Inhibitors and Osteoblast Functions
### Bortezomib
Notably, this upregulation is transcriptionally dependent on the activation of ER stress signaling pathways IRE1\(\alpha\)/XBP1s induced by bortezomib. The same group demonstrated that both IRE1\(\alpha\)/XBP1s and PERK-ATF4 stress signaling are activated during PI-induced OB differentiation. Moreover, XBP1s overexpression triggered the expression of OB differentiation-related genes as well as the induction of ATF4, whereas its inhibition abolished new bone formation in mice after bortezomib administration [80]. These findings strongly support the notion that the signaling cascade of IRE1\(\alpha\)/XBP1s is a key mediator of the PIs-induced bone formation. In addition to this, it has been demonstrated that bortezomib, and its analog izaxomib, promote OB differentiation of osteogenic cells by enhancing Osterix protein levels [81]. An interesting study proposed by [PERSON] et al. examined the potential bone-protective effects of bortezomib against radiation damage in a mouse focal radiation model [82]. Intriguingly, bortezomib preserves bone architecture and strength after radiation, as well as OB number and mineralizing surface compared with non-irradiated controls. To further understand the underlying cellular mechanism, the authors established a fluorescent reporter mouse model to visualize mesenchymal lineage cells within BM. Of note, bortezomib administration reduces the percentage of apoptotic OB and increases DNA repair in OB in irradiated bones. The same protective effect was seen on osteocytes [82]. As reviewed by others, clinical studies have demonstrated that bortezomib and carfilizomib administration is associated with increased levels of markers of bone formation, such as bone ALP, OC and serum PTH, and reduced levels of bone resorption markers, such as Dkk-1, in MM patients [7; 36; 83; 84; 85; 86; 87]. Histomorphometric and imaging studies supported this biomarker evidence, demonstrating that PIs have beneficial effects on bone health in MM patients [7]. In breast cancers, bortezomib reduced the growth of osteolytic lesions in mice intrivially inoculated with breast cancer cell lines [88]. Moreover, bortezomib administration increased trabecular bone formation in both tumor-containing tibias and in distal femurs of non-tumor bearing mice. In order to assess the ability of bortezomib to protect bone from subsequent osteolysis, the authors evaluated bortezomib treatment for a period of time before tumor cell inoculation. They found that bortezomib administration before tumor inoculation provides protection from osteolysis, suggesting that bortezomib may protect the skeleton in advanced breast cancer [88]. The analysis of gene expression of breast cancer cell lines treated with bortezomib revealed that bortezomib decreases the expression of genes promoting tumor growth such as _RUNX2_, _MMP-9_, and _VEGF_[88]. It has been also demonstrated that bortezomib prevents bone metastasis of prostate cancer by inhibiting the E3 ubiquitin protein ligases, WWP1, Smurf1, and Smurf2, which are known to be upregulated in metastatic patients compared with those without bone metastases [89]. Moreover, by using a murine intratibial injection model, [PERSON] et al. showed that bortezomib is able to inhibit the growth of osteolytic lesions but not their initial formation [90].