The deubiquitinating enzyme USP37 enhances CHK1 activity to promote the cellular response to replication stress.

The deubiquitinating enzyme USP37 is known to contribute to timely onset of S phase and progression of mitosis. However, it is not clear if USP37 is required beyond S-phase entry despite expression and activity of USP37 peaking within S phase. We have utilized flow cytometry and microscopy to analyze populations of replicating cells labeled with thymidine analogs and monitored mitotic entry in synchronized cells to determine that USP37-depleted cells exhibited altered S-phase kinetics. Further analysis revealed that cells depleted of USP37 harbored increased levels of the replication stress and DNA damage markers γH2AX and 53BP1 in response to perturbed replication. Depletion of USP37 also reduced cellular proliferation and led to increased sensitivity to agents that induce replication stress. Underlying the increased sensitivity, we found that the checkpoint kinase 1 is destabilized in the absence of USP37, attenuating its function. We further demonstrated that USP37 deubiquitinates checkpoint kinase 1, promoting its stability. Together, our results establish that USP37 is required beyond S-phase entry to promote the efficiency and fidelity of replication. These data further define the role of USP37 in the regulation of cell proliferation and contribute to an evolving understanding of USP37 as a multifaceted regulator of genome stability.

The mitotic kinesin KIF11 is a driver of invasion, proliferation, and self-renewal in glioblastoma.

The proliferative and invasive nature of malignant cancers drives lethality. In glioblastoma, these two processes are presumed mutually exclusive and hence termed "go or grow." We identified a molecular target that shuttles between these disparate cellular processes-the molecular motor KIF11. Inhibition of KIF11 with a highly specific small-molecule inhibitor stopped the growth of the more treatment-resistant glioblastoma tumor-initiating cells (TICs, or cancer stem cells) as well as non-TICs and impeded tumor initiation and self-renewal of the TIC population. Targeting KIF11 also hit the other arm of the "go or grow" cell fate decision by reducing glioma cell invasion. Administration of a KIF11 inhibitor to mice bearing orthotopic glioblastoma prolonged their survival. In its role as a shared molecular regulator of cell growth and motility across intratumoral heterogeneity, KIF11 is a compelling therapeutic target for glioblastoma.

Coordinated regulation of p31(Comet) and Mad2 expression is required for cellular proliferation.

p31(Comet) is a well-known interacting partner of the spindle assembly checkpoint (SAC) effector molecule Mad2. At the molecular level it is well established that p31(Comet) promotes efficient mitotic exit, specifically the metaphase-anaphase transition, by antagonizing Mad2 function. However, there is little knowledge of how p31(Comet) is regulated or the physiological importance of controlling p31(Comet). Here, we show that the Rb-E2F pathway regulates p31(Comet) expression. In multiple tumor types (including breast and lung) p31(Comet) expression is increased along with Mad2. Expression of this antagonist-target pair is coordinated in cells and correlated in cancer. Moreover, a narrow range of p31(Comet):Mad2 ratios is compatible with cellular viability. Our data suggest that coordinate regulation is important for the outgrowth of oncogenic cell populations. Our findings suggest that altered p31(Comet):Mad2 expression ratios may provide new insight into altered SAC function and the generation of chromosomal instability in tumors.

Skp1-Cul1-F-box ubiquitin ligase (SCF(ßTrCP))-mediated destruction of the ubiquitin-specific protease USP37 during G2-phase promotes mitotic entry.

Ubiquitin-mediated proteolysis is a key regulatory process in cell cycle progression. The Skp1-Cul1-F-box (SCF) and anaphase-promoting complex (APC) ubiquitin ligases target numerous components of the cell cycle machinery for destruction. Throughout the cell cycle, these ligases cooperate to maintain precise levels of key regulatory proteins, and indirectly, each other. Recently, we have identified the deubiquitinase USP37 as a regulator of the cell cycle. USP37 expression is cell cycle-regulated, being expressed in late G(1) and ubiquitinated by APC(Cdh1) in early G(1). Here we report that in addition to destruction at G(1), a major fraction of USP37 is degraded at the G(2)/M transition, prior to APC substrates and similar to SCF(ßTrCP) substrates. Consistent with this hypothesis, USP37 interacts with components of the SCF in a ßTrCP-dependent manner. Interaction with ßTrCP and subsequent degradation is phosphorylation-dependent and is mediated by the Polo-like kinase (Plk1). USP37 is stabilized in G(2) by depletion of ßTrCP as well as chemical or genetic manipulation of Plk1. Similarly, mutation of the phospho-sites abolishes ßTrCP binding and renders USP37 resistant to Plk1 activity. Expression of this mutant hinders the G(2)/M transition. Our data demonstrate that tight regulation of USP37 levels is required for proper cell cycle progression.