Sinularamides A-G, Terpenoid-Derived Spermidine and Spermine Conjugates with Casitas B-Lineage Lymphoma Proto-Oncogene B (Cbl-b) Inhibitory Activities from a Sinularia sp. Soft Coral

An extract of a Sinularia sp. soft coral showed inhibitory activity against the E3-ubiquitin ligase casitas B-lineage lymphoma proto-oncogene B (Cbl-b). Subsequent bioassay-guided separation of the extract provided a series of terpenoid-derived spermidine and spermine amides that were named sinularamides A-G (1-7). Compounds 1-7 represent new natural products; however, sinularamide A (1) was previously reported as a synthetic end product. The structures of sinularamides A-G (1-7) were elucidated by analysis of spectroscopic and spectrometric data from NMR, IR, and HRESIMS experiments and by comparison with literature data. All of the isolated compounds showed Cbl-b inhibitory activities with IC₅₀ values that ranged from approximately 6.5 to 33 μM.

Suberitamides A-C, Aryl Alkaloids from a Pseudosuberites sp. Marine Sponge that Inhibit Cbl-b Ubiquitin Ligase Activity

Three new aryl alkaloids named suberitamides A-C (1-3), were isolated from an extract of the marine sponge Pseudosuberites sp. collected along the coast of North Carolina. Their planar structures were established by extensive nuclear magnetic resonance (NMR) and mass spectrometry (MS) analysis. To assign the challenging relative configuration of the saturated five-membered ring in suberitamide A (1), a simple and efficient NMR protocol was applied that is based on the analysis of 2- and 3-bond 1H-13C spin-spin coupling constants using a PIP (pure in-phase) HSQMBC (heteronuclear single quantum multiple bond correlation) IPAP (in-phase and anti-phase) experiment. Suberitamides A (1) and B (2) inhibited Cbl-b, an E3 ubiquitin ligase that is an important modulator of immune cell function, with IC50 values of approximately 11 μM.

Casitas B-Lineage Lymphoma RING Domain Inhibitors Protect Mice against High-Fat Diet-Induced Obesity and Insulin Resistance

The casitas b-lineage lymphoma (c-Cbl) is an important adaptor protein with an intrinsic E3 ubiquitin ligase activity that interacts with E2 proteins such as UbCH7. c-Cbl plays a vital role in regulating receptor tyrosine kinase signaling. c-Cbl involves in whole-body energy homeostasis, which makes it a potential target for the treatment of type 2 diabetes and obesity. In the present study, we have designed two parental peptides and 55 modified peptides based on the structure of UbCH7 loop L1 and L2. Thirteen of the modified peptides showed increased inhibitory activity in a fluorescence polarization-based assay. In the in vivo proof of study principle, mice treated with peptides 10, 34, 49 and 51 were protected against high-fat diet-induced obesity and insulin resistant. These inhibitors may potentially lead to new therapeutic alternatives for obesity and type 2 diabetes.

N-myristoylated ubiquitin ligase Cbl-b inhibitor prevents on glucocorticoid-induced atrophy in mouse skeletal muscle

A DGpYMP peptide mimetic of tyrosine(608)-phosphorylated insulin receptor substrate-1 (IRS-1), named Cblin, was previously shown to significantly inhibit Cbl-b-mediated IRS-1 ubiquitination. In the present study, we developed N-myristoylated Cblin and investigated whether it was effective in preventing glucocorticoid-induced muscle atrophy. Using HEK293 cells overexpressing Cbl-b, IRS-1 and ubiquitin, we showed that the 50% inhibitory concentrations of Cbl-b-mediated IRS-1 ubiquitination by N-myristoylated Cblin and Cblin were 30 and 120 μM, respectively. Regarding the DEX-induced atrophy of C2C12 myotubes, N-myristoylated Cblin was more effective than Cblin for inhibiting the DEX-induced decreases in C2C12 myotube diameter and IRS-1 degradation. The inhibitory efficacy of N-myristoylated Cblin on IRS-1 ubiquitination in C2C12 myotubes was approximately fourfold larger than that of Cblin. Furthermore, N-myristoylation increased the incorporation of Cblin into HEK293 cells approximately 10-folds. Finally, we demonstrated that N-myristoylated Cblin prevented the wet weight loss, IRS-1 degradation, and MAFbx/atrogin-1 and MuRF-1 expression in gastrocnemius muscle of DEX-treated mice approximately fourfold more effectively than Cblin. Taken together, these results suggest that N-myristoylated Cblin prevents DEX-induced skeletal muscle atrophy in vitro and in vivo, and that N-myristoylated Cblin more effectively prevents muscle atrophy than unmodified Cblin.

Adoptive transfer of siRNA Cblb-silenced CD8+ T lymphocytes augments tumor vaccine efficacy in a B16 melanoma model

The ubiquitin ligase Cbl-b is an established regulator of T cell immune response thresholds. We recently showed that adoptive cell transfer (ACT) of cblb(-/-) CD8(+) T cells enhances dendritic cell (DC) immunization-mediated anti-tumor effects in immune-competent recipients. However, translation of cblb targeting to clinically applicable concepts requires that inhibition of cblb activity be transient and reversible. Here we provide experimental evidence that inhibition of cblb using chemically synthesized siRNA has such potential. Silencing cblb expression by ex vivo siRNA transfection of polyclonal CD8(+) T cells prior to ACT increased T cell tumor infiltration, significantly delayed tumor outgrowth, and increased survival rates of tumor-bearing mice. As shown by ex vivo recall assays, cblb silencing resulted in significant augmentation of intratumoral T cell cytokine response. ACT of cblb-silenced polyclonal CD8(+) T cells combined with DC-based tumor vaccines predominantly mediated anti-tumor immune responses, whereas no signs of autoimmunity could be detected. Importantly, CBLB silencing in human CD8(+) T cells mirrored the effects observed for cblb-silenced and cblb-deficient murine T cells. Our data validate the concept of enhanced anti-tumor immunity by repetitive ACT of ex vivo cblb siRNA-silenced hyper-reactive CD8(+) T cells as add-on adjuvant therapy to augment the efficacy of existing cancer immunotherapy regimens in clinical practice.

Distinct roles of fibroblast growth factor receptor 1 and 2 in regulating cell survival and epithelial-mesenchymal transition

Two related receptor tyrosine kinases (RTKs), fibroblast growth factor receptor 1 and 2 (FGFR1 and FGFR2), exert distinct effects during carcinogenesis. To examine FGFR1 and FGFR2 signaling in polarized epithelia, we have developed an in vitro three-dimensional HC11 mouse mammary epithelial cell culture model combined with a chemically inducible FGFR (iFGFR) dimerization system. Although activation of both RTKs led to reinitiation of cell proliferation and loss of cell polarity, only iFGFR1 activation induced cell survival and epithelial to mesenchymal transition. In contrast, iFGFR2 activation induced cell apoptosis even in the cells in direct contact with the extracellular matrix. Activation of iFGFR2, but not iFGFR1, led to rapid receptor down-regulation and transient activation of downstream signaling, which were partially rescued by Cbl small interfering RNA knockdown or the proteasome inhibitor lactacystin. Importantly, inhibition of proteasome activity in iFGFR2-activated structures led to epithelial to mesenchymal transition and invasive phenotypes resembling those observed after iFGFR1 activation. These studies demonstrate, for the first time, that the duration of downstream signaling determines the distinct phenotypes mediated by very homologous RTKs in three-dimensional cultures.

Spred-2 steady-state levels are regulated by phosphorylation and Cbl-mediated ubiquitination

Spred proteins modulate growth factor receptor signaling by inhibiting the Ras-MAPK cascade. Here, we show that Spred-1, Spred-2, and Spred-3 are ubiquitinated in HEK293T cells stimulated with epidermal growth factor (EGF) or pervanadate. Spred-2 tyrosines Y228 and/or Y231 in the Kit binding domain were identified as putative phosphorylation site(s) critical for Spred-2 ubiquitination. Depletion of Cbl and Cbl-b E3 ubiquitin ligases by RNA interference, or overexpression of a Cbl dominant inhibitory mutant (Cbl-N), inhibited Spred-2 ubiquitination, while conversely, wild type Cbl enhanced Spred-2 ubiquitination. Interaction of Spred-2 with Cbl-N was detectable by co-immunoprecipitation and required the Cbl SH2 domain and Spred-2 Y228 and Y231 residues. Studies on endogenous Spred-2 in ME4405 melanoma cells showed that pervanadate induced Spred-2 ubiquitination and a marked reduction in Spred-2 steady-state levels that was partially blocked by the proteasomal inhibitor, MG-132. These results suggest a role for Spred-2 tyrosine phosphorylation and ubiquitination in controlling Spred-2 expression levels.

Abl tyrosine kinase regulates endocytosis of the epidermal growth factor receptor

Signal attenuation from ligand-activated epidermal growth factor receptor (EGFR) is mediated in part by receptor endocytosis and trafficking to the lysosomal degradative compartment. Uncoupling the activated EGFR from endocytosis and degradation has emerged as a mechanism for oncogenic activation of the EGFR. The Abl nonreceptor tyrosine kinase is activated by ligand-stimulated EGFR, but the role of Abl in EGFR signaling has not been defined. Here we uncovered a novel role for the activated Abl kinase in the regulation of EGFR endocytosis. We show that activated Abl impairs EGFR internalization. Moreover, we show that activated Abl phosphorylates the EGFR primarily on tyrosine 1173, and that mutation of this site to phenylalanine restores ligand-dependent endocytosis of the EGFR in the presence of activated Abl. Furthermore, we show that activated Abl allows the ligand-activated EGFR to escape Cbl-dependent down-regulation by inhibiting the accumulation of Cbl at the plasma membrane in response to epidermal growth factor stimulation and disrupting the formation of the EGFR.Cbl complex without affecting Cbl protein stability. These findings reveal a novel role for Abl in promoting increased cell-surface expression of the EGFR and suggest that Abl/EGFR signaling may cooperate in human tumors.

Cbl escapes Cdc42-mediated inhibition by downregulation of the adaptor molecule βPix

The Pix/Cool proteins are involved in the regulation of cell morphology by binding to small Rho GTPases and kinases of the Pak family. Recently, it has been shown that betaPix/Cool-1 associates with the ubiquitin ligase Cbl, which appears to be a critical step in Cdc42-mediated inhibition of epidermal-growth-factor-receptor (EGFR) ubiquitylation and downregulation. Here we show that the SH3 domain of betaPix specifically interacts with a proline-arginine motif (PxxxPR) present within the ubiquitin ligase Cbl and Pak1 kinase. Owing to targeting of the same sequence, Cbl and Pak1 compete for binding to betaPix. In this complex, Cbl mediates ubiquitylation and subsequent degradation of betaPix. Our findings reveal a double feedback loop in which the Cdc42/betaPix complex blocks Cbl's ability to downregulate EGFR, while Cbl in turn promotes degradation of betaPix in order to escape this inhibition. Such a relationship provides a mechanism to fine-tune the kinetics of RTK endocytosis and degradation depending on the pool of active Cdc42 and the duration of EGFR signaling.