Cellular senescence suppresses cancer by irreversibly arresting cell proliferation. Senescent cells acquire a pro-inflammatory senescence-associated secretory phenotype. Many genotoxic chemotherapies target proliferating cells non-specifically, often with adverse reactions. In accord with prior work, we show that several chemotherapeutic drugs induce senescence of primary murine and human cells. Using a transgenic mouse that permits tracking and eliminating senescent cells, we show that therapy-induced senescent (TIS) cells persist and contribute to local and systemic inflammation. Eliminating TIS cells reduced several short- and long-term effects of the drugs, including bone marrow suppression, cardiac dysfunction, cancer recurrence and physical activity and strength. Consistent with our findings in mice, the risk of chemotherapy-induced fatigue was significantly greater in humans with increased expression of a senescence marker in T-cells prior to chemotherapy. These findings suggest that senescent cells can cause certain chemotherapy side effects, providing a new target to reduce the toxicity of anti-cancer treatments.

Rho GTPases (20 human members) comprise a major branch of the Ras superfamily of small GTPases, and aberrant Rho GTPase function has been implicated in oncogenesis and other human diseases. Although many of our current concepts of Rho GTPases are based on the three classical members (RhoA, Rac1, and Cdc42), recent studies have revealed the diversity of biological functions mediated by other family members. A key basis for the functional diversity of Rho GTPases is their association with distinct subcellular compartments, which is dictated in part by three posttranslational modifications signaled by their carboxyl-terminal CAAX (where C represents cysteine, A is an aliphatic amino acid, and X is a terminal amino acid) tetrapeptide motifs. CAAX motifs are substrates for the prenyltransferase-catalyzed addition of either farnesyl or geranylgeranyl isoprenoid lipids, Rce1-catalyzed endoproteolytic cleavage of the AAX amino acids, and Icmt-catalyzed carboxyl methylation of the isoprenylcysteine. We utilized pharmacologic, biochemical, and genetic approaches to determine the sequence requirements and roles of CAAX signal modifications in dictating the subcellular locations and functions of the Rho GTPase family. Although the classical Rho GTPases are modified by geranylgeranylation, we found that a majority of the other Rho GTPases are substrates for farnesyltransferase. We found that the membrane association and/or function of Rho GTPases are differentially dependent on Rce1-and Icmt-mediated modifications. Our results further delineate the sequence requirements for prenyltransferase specificity and functional roles for protein prenylation in Rho GTPase function. We conclude that a majority of Rho GTPases are targets for pharmacologic inhibitors of farnesyltransferase, Rce1, and Icmt.Rho proteins are members of the Ras superfamily of small GTPases and function as GDP/GTP-regulated switches (1, 2). Much of our current understanding of the biochemistry and biology of the Rho family has come from the extensive evaluation of three classical members, RhoA, Rac1, and Cdc42 (3).Similar to Ras, Rho GDP/GTP cycling is regulated by guanine nucleotide exchange factors that promote the formation of the active GTP-bound form (4) and GTPase-activating proteins that catalyze the intrinsic GTPase activity and promote the formation of inactive GDP-bound Rho (5). Active, GTP-bound Rho GTPases bind preferentially to downstream effectors, stimulating diverse cytoplasmic signaling cascades that control actin reorganization and regulate cell shape, polarity, motility, adhesion, and membrane trafficking (6). As such, it is thought that activated Rho proteins contribute to cancer progression by influencing the ability of cells to migrate and thus to invade and metastasize. In addition to these alterations in cellular function, aberrant activation of Rho proteins has also been shown to contribute to other cancer phenotypes by promoting cell growth, proliferation, survival, and angiogenesis (7). Therefore, defining pharmacologic approaches fo...

Ras proteins function as signaling hubs that are activated by convergent signaling pathways initiated by extracellular stimuli. Activated Ras in turn regulates a diversity of downstream cytoplasmic signaling cascades. Ras proteins are founding members of a large superfamily of small GTPases that have significant sequence and biochemical similarities. Recent observations have established a complex signaling interplay between Ras and other members of the family. A key biochemical mechanism facilitating this crosstalk involves guanine nucleotide exchange factors (GEFs), which serve as regulators and effectors, as well as signaling integrators, of Ras signaling.

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Autoinhibition of the Rho guanine nucleotide exchange factor ASEF is relieved by interaction with the APC tumor suppressor. Here we show that binding of the armadillo repeats of APC to a 'core APC-binding' (CAB) motif within ASEF, or truncation of the SH3 domain of ASEF, relieves autoinhibition, allowing the specific activation of CDC42. Structural determination of autoinhibited ASEF reveals that the SH3 domain forms an extensive interface with the catalytic DH and PH domains to obstruct binding and activation of CDC42, and the CAB motif is positioned adjacent to the SH3 domain to facilitate activation by APC. In colorectal cancer cell lines, full-length, but not truncated, APC activates CDC42 in an ASEF-dependent manner to suppress anchorage-independent growth. We therefore propose a model in which ASEF acts as a tumor suppressor when activated by APC and inactivation of ASEF by mutation or APC truncation promotes tumorigenesis.The adenomatous polyposis coli (APC) protein is a negative regulator of the Wnt signaling pathway and promotes the phosphorylation and degradation of β-catenin 1 . The majority of colorectal cancers (CRCs) harbor C-terminally truncated forms of APC that retain the Nterminal coiled-coil domain and the highly conserved armadillo repeat region (APC Arm ) 2 and are unable to regulate the degradation of β-catenin 3 . Recent studies suggest that APC regulates cytoskeletal dynamics to influence cellular migration, cell division, polarization and adhesion, and it seems increasingly likely that deregulated cytoskeletal dynamics, together with enhancement of transcription by β-catenin, potentiate tumor formation and progression upon APC truncation. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptAPC may regulate cytoskeletal networks by binding to microtubules and to proteins implicated in reorganization of the actin cytoskeleton, such as the Rho effectors mDia and IQGAP1 and the Rho guanine nucleotide exchange factor (RhoGEF) 'APC-stimulated guanine nucleotide exchange factor' (ASEF, also called ARHGEF4) 4 . ASEF is a Dbl-family GEF that contains an Src-homology-3 (SH3) domain followed by the Dbl-homology (DH) and pleckstrinhomology (PH) domains characteristic of Dbl-family GEFs that specifically activate members of the Rho family of GTPases. DH and PH domains in Dbl proteins catalyze the exchange of GDP for GTP in Rho GTPases, allowing them to signal to downstream effectors 5 . ASEF is homologous to the Dbl proteins collybistin (also called PEM2) and SPATA13 (also called ASEF2), both of which have been shown to activate specifically CDC42 and not other Rhofamily GTPases 6,7 .Previous data suggest that ASEF exists in an autoinhibited form and is activated upon binding of APC Arm to the region termed the APC-binding region (ABR) 8 . Binding of APC Arm to the ABR stimulates activation of the Rho GTPase RAC1 by ASEF. Furthermore, truncation of the ABR is sufficient to relieve autoinhibition, rendering ASEF constitutively active. In addition, truncated, but not wild-...