Structure of the Cand1-Cul1-Roc1 Complex Reveals Regulatory Mechanisms for the Assembly of the Multisubunit Cullin-Dependent Ubiquitin Ligases

AbstractThe SCF ubiquitin ligase complex regulates diverse cellular functions by ubiquitinating numerous protein substrates. Cand1, a 120 kDa HEAT repeat protein, forms a tight complex with the Cul1-Roc1 SCF catalytic core, inhibiting the assembly of the multisubunit E3 complex. The crystal structure of the Cand1-Cul1-Roc1 complex shows that Cand1 adopts a highly sinuous superhelical structure, clamping around the elongated SCF scaffold protein Cul1. At one end, a Cand1 β hairpin protrusion partially occupies the adaptor binding site on Cul1, inhibiting its interactions with the Skp1 adaptor and the substrate-recruiting F box protein subunits. At the other end, two Cand1 HEAT repeats pack against a conserved Cul1 surface cleft and bury a Cul1 lysine residue, whose modification by the ubiquitin-like protein, Nedd8, is able to block Cand1-Cul1 association. Together with biochemical evidence, these structural results elucidate the mechanisms by which Cand1 and Nedd8 regulate the assembly-disassembly cycles of SCF and other cullin-dependent E3 complexes

Screening for pre-clinical disability in different residential settings

<p>Abstract</p> <p>Background</p> <p>Preventing disability and offering effective interventions to older people during early decline in function is most likely to be effective if those most at risk of progressive disablement are able to be identified. Similarly the ability to easily identify a group with similar functional profile from disparate sectors of the community is of significant benefit to researchers. This study aimed to (1) describe the use of a pre-clinical disability screening tool to select a functionally comparable group of older men and women with early functional limitation from different settings, and (2) explore factors associated with function and disability.</p> <p>Methods</p> <p>Self-reported function and disability measured with the Late-Life Function and Disability Instrument along with a range of physical performance measurements were compared across residential settings and gender in a sample of 471 trial participants identified as pre-clinically disabled after being screened with the Fried pre-clinical disability tool. Factors that might lie on the pathway to progressive disablement were identified using multiple linear regression analysis.</p> <p>Results</p> <p>We found that a sample population, screened for pre-clinical disability, had a functional status and disability profile reflecting early functional limitation, regardless of residential setting or gender. Statistical models identified a range of factors associated with function and disability which explained a greater degree of the variation in function, than disability.</p> <p>Conclusions</p> <p>We selected a group of people with a comparable function and disability profile, consistent with the pre-clinical stage of disability, from a sample of older Australian men and women from different residential settings using the Fried pre-clinical disability screening tool. The results suggest that the screening tool can be used with greater confidence for research, clinical and population health purposes. Further research is required to examine the validity of the tool. These findings offer insight into the type of impairment factors characterising early functional loss that could be addressed through disability prevention initiatives.</p> <p>Trial Registration</p> <p>ACTRN01206000431527</p

A mechanistic insight into a proteasome-independent constitutive inhibitor kappaBalpha (IkappaBalpha) degradation and nuclear factor kappaB (NF-kappaB) activation pathway in WEHI-231 B-cells.

Inducible activation of the transcription factor NF-kappaB (nuclear factor kappaB) is classically mediated by proteasomal degradation of its associated inhibitors, IkappaBalpha (inhibitory kappaBalpha) and IkappaBbeta. However, certain B-lymphocytes maintain constitutively nuclear NF-kappaB activity (a p50-c-Rel heterodimer) which is resistant to inhibition by proteasome inhibitors. This activity in the WEHI-231 B-cell line is associated with continual and preferential degradation of IkappaBalpha, which is also unaffected by proteasome inhibitors. Pharmacological studies indicated that there was a correlation between inhibition of IkappaBalpha degradation and constitutive p50-c-Rel activity. Domain analysis of IkappaBalpha by deletion mutagenesis demonstrated that an N-terminal 36-amino-acid sequence of IkappaBalpha represented an instability determinant for constitutive degradation. Moreover, domain grafting studies indicated that this sequence was sufficient to cause IkappaBbeta, but not chloramphenicol acetyltransferase, to be rapidly degraded in WEHI-231 B-cells. However, this sequence was insufficient to target IkappaBbeta to the non-proteasome degradation pathway, suggesting that there was an additional cis-element(s) in IkappaBalpha that was required for complete targeting. Nevertheless, the NF-kappaB pool associated with IkappaBbeta now became constitutively active by virtue of IkappaBbeta instability in these cells. These findings further support the notion that IkappaB instability governs the maintenance of constitutive p50-c-Rel activity in certain B-cells via a unique degradation pathway

Regulation of Constitutive p50/c-Rel Activity via Proteasome Inhibitor-Resistant IκBα Degradation in B Cells

Constitutive NF-κB activity has emerged as an important cell survival component of physiological and pathological processes, including B-cell development. In B cells, constitutive NF-κB activity includes p50/c-Rel and p52/RelB heterodimers, both of which are critical for proper B-cell development. We previously reported that WEHI-231 B cells maintain constitutive p50/c-Rel activity via selective degradation of IκBα that is mediated by a proteasome inhibitor-resistant, now termed PIR, pathway. Here, we examined the mechanisms of PIR degradation by comparing it to the canonical pathway that involves IκB kinase-dependent phosphorylation and β-TrCP-dependent ubiquitylation of the N-terminal signal response domain of IκBα. We found a distinct consensus sequence within this domain of IκBα for PIR degradation. Chimeric analyses of IκBα and IκBβ further revealed that the ankyrin repeats of IκBα, but not IκBβ, contained information necessary for PIR degradation, thereby explaining IκBα selectivity for the PIR pathway. Moreover, we found that PIR degradation of IκBα and constitutive p50/c-Rel activity in primary murine B cells were maintained in a manner different from B-cell-activating-factor-dependent p52/RelB regulation. Thus, our findings suggest that nonconventional PIR degradation of IκBα may play a physiological role in the development of B cells in vivo

A Potent and Selective ULK1 Inhibitor Suppresses Autophagy and Sensitizes Cancer Cells to Nutrient Stress

Summary: In response to stress, cancer cells generate nutrients and energy through a cellular recycling process called autophagy, which can promote survival and tumor progression. Accordingly, autophagy inhibition has emerged as a potential cancer treatment strategy. Inhibitors targeting ULK1, an essential and early autophagy regulator, have provided proof of concept for targeting this kinase to inhibit autophagy; however, these are limited individually in their potency, selectivity, or cellular activity. In this study, we report two small molecule ULK1 inhibitors, ULK-100 and ULK-101, and establish superior potency and selectivity over a noteworthy published inhibitor. Moreover, we show that ULK-101 suppresses autophagy induction and autophagic flux in response to different stimuli. Finally, we use ULK-101 to demonstrate that ULK1 inhibition sensitizes KRAS mutant lung cancer cells to nutrient stress. ULK-101 represents a powerful molecular tool to study the role of autophagy in cancer cells and to evaluate the therapeutic potential of autophagy inhibition. : Therapeutics; Functional Aspects of Cell Biology; Cancer Subject Areas: Therapeutics, Functional Aspects of Cell Biology, Cance

Unique functions of CHK1 and WEE1 underlie synergistic anti-tumor activity upon pharmacologic inhibition

<p>Abstract</p> <p>Background</p> <p>Inhibition of kinases involved in the DNA damage response sensitizes cells to genotoxic agents by abrogating checkpoint-induced cell cycle arrest. CHK1 and WEE1 act in a pathway upstream of CDK1 to inhibit cell cycle progression in response to damaged DNA. Therapeutic targeting of either CHK1 or WEE1, in combination with chemotherapy, is under clinical evaluation. These studies examine the overlap and potential for synergy when CHK1 and WEE1 are inhibited in cancer cell models.</p> <p>Methods</p> <p>Small molecules MK-8776 and MK-1775 were used to selectively and potently inhibit CHK1 and WEE1, respectively.</p> <p>Results</p> <p>In vitro, the combination of MK-8776 and MK-1775 induces up to 50-fold more DNA damage than either MK-8776 or MK-1775 alone at a fixed concentration. This requires aberrant cyclin-dependent kinase activity but does not appear to be dependent on p53 status alone. Furthermore, DNA damage takes place primarily in S-phase cells, implying disrupted DNA replication. When dosed together, the combination of MK-8776 and MK-1775 induced more intense and more durable DNA damage as well as anti-tumor efficacy than either MK-8776 or MK-1775 dosed alone. DNA damage induced by the combination was detected in up to 40% of cells in a treated xenograft tumor model.</p> <p>Conclusions</p> <p>These results highlight the roles of WEE1 and CHK1 in maintaining genomic integrity. Importantly, the strong synergy observed upon inhibition of both kinases suggests unique yet complimentary anti-tumor effects of WEE1 and CHK1 inhibition. This demonstration of DNA double strand breaks in the absence of a DNA damaging chemotherapeutic provides preclinical rationale for combining WEE1 and CHK1 inhibitors as a cancer treatment regimen.</p