NOTCH2 Hajdu-Cheney Mutations Escape SCF-Dependent Proteolysis to Promote Osteoporosis.

Hajdu-Cheney syndrome (HCS), a rare autosomal disorder caused by heterozygous mutations in NOTCH2, is clinically characterized by acro-osteolysis, severe osteoporosis, short stature, neurological symptoms, cardiovascular defects, and polycystic kidneys. Recent studies identified that aberrant NOTCH2 signaling and consequent osteoclast hyperactivity are closely associated with the bone-related disorder pathogenesis, but the exact molecular mechanisms remain unclear. Here, we demonstrate that sustained osteoclast activity is largely due to accumulation of NOTCH2 carrying a truncated C terminus that escapes FBW7-mediated ubiquitination and degradation.

The SCFβ-TRCP E3 ubiquitin ligase complex targets Lipin1 for ubiquitination and degradation to promote hepatic lipogenesis.

The SCF E3 ubiquitin ligase complex plays pivotal roles in normal cellular physiology and in pathophysiological conditions. Identification of β-transducin repeat-containing protein (β-TRCP) substrates is therefore critical to understand SCF biology and function. We used a β-TRCP-phosphodegron motif-specific antibody in a β-TRCP substrate screen coupled with tandem mass spectrometry and identified multiple β-TRCP substrates.

Nutrient-induced FNIP degradation by SCFβ-TRCP regulates FLCN complex localization and promotes renal cancer progression.

Folliculin-interacting protein 1 and 2 (FNIP1 and FNIP2) play critical roles in preventing renal malignancy through their association with the tumor suppressor FLCN. Mutations in FLCN are associated with Birt-Hogg-Dubé (BHD) syndrome, a rare disorder with increased risk of renal cancer. Recent studies indicated that FNIP1/FNIP2 double knockout mice display enlarged polycystic kidneys and renal carcinoma, which phenocopies FLCN knockout mice, suggesting that these two proteins function together to suppress renal cancer.

[NF-κB signaling pathways and the future perspectives of bone disease therapy using selective inhibitors of NF-κB].

The transcriptional factor nuclear factor κB(NF-κB)regulates the expression of a wide variety of genes that are involved in immune and inflammatory responses, proliferation, and tumorigenesis. NF-κB consists of five members, such as p65(RelA), RelB, c-Rel, p50/p105(NF-κB1), and p52/p100(NF-κB2). There are two distinct NF-κB activation pathways, termed the classical and alternative NF-κB signaling pathways.

The novel IκB kinase β inhibitor IMD-0560 prevents bone invasion by oral squamous cell carcinoma.

Oral squamous cell carcinoma (OSCC) cells display significantly augmented nuclear factor-κB (NF-κB) activity, and inhibiting this activity suppresses malignant tumor characteristics. Thus, we evaluated the effect of IMD-0560, a novel inhibitor of IκB kinase (IKK) β that is under assessment in a clinical trial of rheumatoid arthritis, on bone invasion by the mouse OSCC cell line SCCVII. We examined the inhibitory effects of IMD-0560 on NF-κB activity and tumor invasion using human OSCC cell lines and SCCVII cells in vitro.

Inhibition of BMP2-induced bone formation by the p65 subunit of NF-κB via an interaction with Smad4.

Bone morphogenic proteins (BMPs) stimulate bone formation in vivo and osteoblast differentiation in vitro via a Smad signaling pathway. Recent findings revealed that the activation of nuclear factor-κB (NF-κB) inhibits BMP-induced osteoblast differentiation. Here, we show that NF-κB inhibits BMP signaling by directly targeting the Smad pathway.

SCFβ-TRCP regulates osteoclastogenesis via promoting CYLD ubiquitination.

CYLD negatively regulates the NF-κB signaling pathway and osteoclast differentiation largely through antagonizing TNF receptor-associated factor (TRAF)-mediated K63-linkage polyubiquitination in osteoclast precursor cells. CYLD activity is controlled by IκB kinase (IKK), but the molecular mechanism(s) governing CYLD protein stability remains largely undefined. Here, we report that SCFβ-TRCP regulates the ubiquitination and degradation of CYLD, a process dependent on prior phosphorylation of CYLD at Ser432/Ser436 by IKK.

RelB-induced expression of Cot, an MAP3K family member, rescues RANKL-induced osteoclastogenesis in alymphoplasia mice by promoting NF-κB2 processing by IKKα.

The alternative nuclear factor-κB (NF-κB) pathway, mainly the RelB-p52 heterodimer, plays important roles in bone metabolism through an unknown mechanism. We have previously reported that alymphoplasia (aly/aly) mice, which lack active NF-κB-inducing kinase (NIK), show mild osteopetrosis due to the inhibition of osteoclastogenesis. p100 retains RelB in the cytoplasm and inhibits RANKL-induced osteoclastogenesis in aly/aly cells.

SCF-mediated Cdh1 degradation defines a negative feedback system that coordinates cell-cycle progression.

Proper cell-cycle transitions are driven by waves of ubiquitin-dependent degradation of key regulators by the anaphase-promoting complex (APC) and Skp1-Cullin1-F-box (SCF) E3 ubiquitin ligase complexes. But precisely how APC and SCF activities are coordinated to regulate cell-cycle progression remains largely unclear. We previously showed that APC/Cdh1 earmarks the SCF component Skp2 for degradation.

Regulation of APC(Cdh1) E3 ligase activity by the Fbw7/cyclin E signaling axis contributes to the tumor suppressor function of Fbw7.

Fbw7 and Cdh1 are substrate-recognition subunits of the SCF- and APC-type E3 ubiquitin ligases, respectively. There is emerging evidence suggesting that both Fbw7 and Cdh1 function as tumor suppressors by targeting oncoproteins for destruction. Loss of Fbw7, but not Cdh1, is frequently observed in various human tumors.

p130Cas, Crk-associated substrate, plays important roles in osteoclastic bone resorption.

p130Cas, Crk-associated substrate (Cas), is an adaptor/scaffold protein that plays a central role in actin cytoskeletal reorganization. We previously reported that p130Cas is not tyrosine-phosphorylated in osteoclasts derived from Src-deficient mice, which are congenitally osteopetrotic, suggesting that p130Cas serves as a downstream molecule of c-Src and is involved in osteoclastic bone resorption. However, the physiological role of p130Cas in osteoclasts has not yet been confirmed because the p130Cas-deficient mice displayed embryonic lethality.

Disruption of NF-κB1 prevents bone loss caused by mechanical unloading.

Mechanical unloading, such as in a microgravity environment in space or during bed rest (for patients who require prolonged bed rest), leads to a decrease in bone mass because of the suppression of bone formation and the stimulation of bone resorption. To address the challenges presented by a prolonged stay in space and the forthcoming era of a super-aged society, it will be important to prevent the bone loss caused by prolonged mechanical unloading. Nuclear factor κB (NF-κB) transcription factors are activated by mechanical loading and inflammatory cytokines.

DNA damage-induced activation of ATM promotes β-TRCP-mediated Mdm2 ubiquitination and destruction.

The Mdm2 oncoprotein promotes p53 ubiquitination and destruction. Yet, exact molecular mechanisms of Mdm2 destruction itself, under DNA damaging conditions, remain unclear. Recently, we identified SCFβ-TRCP as a novel E3 ligase that targets Mdm2 for ubiquitination and destruction in a Casein Kinase Iδ (CKIδ)-dependent manner.

Acetylation-dependent regulation of Skp2 function.

Aberrant Skp2 signaling has been implicated as a driving event in tumorigenesis. Although the underlying molecular mechanisms remain elusive, cytoplasmic Skp2 correlates with more aggressive forms of breast and prostate cancers. Here, we report that Skp2 is acetylated by p300 at K68 and K71, which is a process that can be antagonized by the SIRT3 deacetylase.

SCF(β-TRCP) suppresses angiogenesis and thyroid cancer cell migration by promoting ubiquitination and destruction of VEGF receptor 2.

The incidence of human papillary thyroid cancer (PTC) is increasing and an aggressive subtype of this disease is resistant to treatment with vascular endothelial growth factor receptor 2 (VEGFR2) inhibitor. VEGFR2 promotes angiogenesis by triggering endothelial cell proliferation and migration. However, the molecular mechanisms governing VEGFR2 stability in vivo remain unknown.

SCF(Fbw7) modulates the NFkB signaling pathway by targeting NFkB2 for ubiquitination and destruction.

The NFkB/Rel family of proteins play critical roles in a variety of cellular processes. Thus, their physiological activation is tightly controlled. Recently, the NFkB2/p100 precursor has been characterized as the fourth IkB type of suppressor for NFkB.

Tumor suppressor functions of FBW7 in cancer development and progression.

FBW7 (F-box and WD repeat domain-containing 7) has been characterized as an onco-suppressor protein in human cancers. Recent studies have also shown that FBW7 exerts its anti-tumor function primarily by promoting the degradation of various oncoproteins, through which FBW7 regulates cellular proliferation, differentiation and causes genetic instability. In this review, we will discuss the role of FBW7 downstream substrates and how dysregulation of Fbw7-mediated proteolysis of these substrates contributes to tumorigenesis.

The Fbw7 and betaTRCP E3 ubiquitin ligases and their roles in tumorigenesis.

The Ubiquitin Proteasome System (UPS) is a major regulator of protein abundance in the cell. The UPS influences the functions of multiple biological processes by targeting key regulators for destruction. E3 ubiquitin ligases are a vital component of the UPS machinery, working with E1 and E2 enzymes to bind substrates and facilitate the transfer of ubiquitin molecules onto the target protein.

The current and future therapies of bone regeneration to repair bone defects.

Bone defects often result from tumor resection, congenital malformation, trauma, fractures, surgery, or periodontitis in dentistry. Although dental implants serve as an effective treatment to recover mouth function from tooth defects, many patients do not have the adequate bone volume to build an implant. The gold standard for the reconstruction of large bone defects is the use of autogenous bone grafts.

Accumulation of p100, a precursor of NF-κB2, enhances osteoblastic differentiation in vitro and bone formation in vivo in aly/aly mice.

We previously reported that alymphoplasia (aly/aly) mice, which have a natural loss-of-function mutation in the Nik gene, which encodes a kinase essential for the processing of p100 to p52 in the alternative nuclear factor-κB (NF-κB) pathway, show mild osteopetrosis with an increase in several parameters of bone formation: bone formation rate, mineral apposition rate, and osteoblast number. We therefore investigated the molecular mechanisms triggered by the alternative NF-κB pathway in the regulation of osteoblast differentiation using primary osteoblasts (POB) prepared from aly/aly mice. Alkaline phosphatase (ALP) activity and mineralization induced by the presence of β-glycerophosphate and ascorbic acid were enhanced in POB from aly/aly compared with wild-type (WT) mice.

Skp2 is a promising therapeutic target in breast cancer.

Breast cancer is the most common type of cancer among American women, and remains the second leading cause of cancer-related death for female in the United States. It has been known that several signaling pathways and various factors play critical roles in the development and progression of breast cancer, such as estrogen receptor, Notch, PTEN, human epidermal growth factor receptor 2, PI3K/Akt, BRCA1, and BRCA2. Emerging evidence has shown that the F-box protein S-phase kinase associated protein 2 (Skp2) also plays an important role in the pathogenesis of breast cancer.

Emerging roles of the FBW7 tumour suppressor in stem cell differentiation.

FBW7 is a ubiquitin E3 ligase substrate adaptor that targets many important oncoproteins-such as Notch, c-Myc, cyclin E and c-Jun-for ubiquitin-dependent proteolysis. By doing so, it plays crucial roles in many cellular processes, including cell cycle progression, cell growth, cellular metabolism, differentiation and apoptosis. Loss of FBW7 has been observed in many types of human cancer, and its role as a tumour suppressor was confirmed by genetic ablation of FBW7 in mice, which leads to the induction of tumorigenesis.

Cdh1 regulates osteoblast function through an APC/C-independent modulation of Smurf1.

The APC/Cdh1 E3 ubiquitin ligase plays an essential role in both mitotic exit and G1/S transition by targeting key cell-cycle regulators for destruction. There is mounting evidence indicating that Cdh1 has other functions in addition to cell-cycle regulation. However, it remains unclear whether these additional functions depend on its E3 ligase activity.

mTOR drives its own activation via SCF(βTrCP)-dependent degradation of the mTOR inhibitor DEPTOR.

The activities of both mTORC1 and mTORC2 are negatively regulated by their endogenous inhibitor, DEPTOR. As such, the abundance of DEPTOR is a critical determinant in the activity status of the mTOR network. DEPTOR stability is governed by the 26S-proteasome through a largely unknown mechanism.

The two faces of FBW7 in cancer drug resistance.

Chemotherapy is an important therapeutic approach for cancer treatment. However, drug resistance is an obstacle that often impairs the successful use of chemotherapies. Therefore, overcoming drug resistance would lead to better therapeutic outcomes for cancer patients.