Showing papers by "João T. Barata published in 2015"

CHK1 overexpression in T-cell acute lymphoblastic leukemia is essential for proliferation and survival by preventing excessive replication stress

Abstract: Checkpoint kinase 1 (CHK1) is a key component of the ATR (ataxia telangiectasia-mutated and Rad3-related)-dependent DNA damage response pathway that protect cells from replication stress, a cell intrinsic phenomenon enhanced by oncogenic transformation. Here, we show that CHK1 is overexpressed and hyperactivated in T-cell acute lymphoblastic leukemia (T-ALL). CHEK1 mRNA is highly abundant in patients of the proliferative T-ALL subgroup and leukemia cells exhibit constitutively elevated levels of the replication stress marker phospho-RPA32 and the DNA damage marker γH2AX. Importantly, pharmacologic inhibition of CHK1 using PF-004777736 or CHK1 short hairpin RNA-mediated silencing impairs T-ALL cell proliferation and viability. CHK1 inactivation results in the accumulation of cells with incompletely replicated DNA, ensuing DNA damage, ATM/CHK2 activation and subsequent ATM- and caspase-3-dependent apoptosis. In contrast to normal thymocytes, primary T-ALL cells are sensitive to therapeutic doses of PF-004777736, even in the presence of stromal or interleukin-7 survival signals. Moreover, CHK1 inhibition significantly delays in vivo growth of xenotransplanted T-ALL tumors. We conclude that CHK1 is critical for T-ALL proliferation and viability by downmodulating replication stress and preventing ATM/caspase-3-dependent cell death. Pharmacologic inhibition of CHK1 may be a promising therapeutic alternative for T-ALL treatment.

PI3K inhibition synergizes with glucocorticoids but antagonizes with methotrexate in T-cell acute lymphoblastic leukemia.

Abstract: The PI3K pathway is frequently hyperactivated in primary T-cell acute lymphoblastic leukemia (T-ALL) cells. Activation of the PI3K pathway has been suggested as one mechanism of glucocorticoid resistance in T-ALL, and patients harboring mutations in the PI3K negative regulator PTEN may be at increased risk of induction failure and relapse. By gene expression microarray analysis of T-ALL cells treated with the PI3K inhibitor AS605240, we identified Myc as a prominent downstream target of the PI3K pathway. A significant association was found between the AS605240 gene expression signature and that of glucocorticoid resistance and relapse in T-ALL. AS605240 showed anti-leukemic activity and strong synergism with glucocorticoids both in vitro and in a NOD/SCID xenograft model of T-ALL. In contrast, PI3K inhibition showed antagonism with methotrexate and daunorubicin, drugs that preferentially target dividing cells. This antagonistic interaction, however, could be circumvented by the use of correct drug scheduling schemes. Our data indicate the potential benefits and difficulties for the incorporation of PI3K inhibitors in T-ALL therapy.

Lymphotoxin-β receptor in microenvironmental cells promotes the development of T-cell acute lymphoblastic leukaemia with cortical/mature immunophenotype

Abstract: Lymphotoxin-mediated activation of the lymphotoxin-β receptor (LTβR; LTBR) has been implicated in cancer, but its role in T-cell acute lymphoblastic leukaemia (T-ALL) has remained elusive. Here we show that the genes encoding lymphotoxin (LT)-α and LTβ (LTA, LTB) are expressed in T-ALL patient samples, mostly of the TAL/LMO molecular subtype, and in the TEL-JAK2 transgenic mouse model of cortical/mature T-ALL (Lta, Ltb). In these mice, expression of Lta and Ltb is elevated in early stage T-ALL. Surface LTα1 β2 protein is expressed in primary mouse T-ALL cells, but only in the absence of microenvironmental LTβR interaction. Indeed, surface LT expression is suppressed in leukaemic cells contacting Ltbr-expressing but not Ltbr-deficient stromal cells, both in vitro and in vivo, thus indicating that dynamic surface LT expression in leukaemic cells depends on interaction with its receptor. Supporting the notion that LT signalling plays a role in T-ALL, inactivation of Ltbr results in a significant delay in TEL-JAK2-induced leukaemia onset. Moreover, young asymptomatic TEL-JAK2;Ltbr(-/-) mice present markedly less leukaemic thymocytes than age-matched TEL-JAK2;Ltbr(+/+) mice and interference with LTβR function at this early stage delayed T-ALL development. We conclude that LT expression by T-ALL cells activates LTβR signalling in thymic stromal cells, thus promoting leukaemogenesis.

The Bone Marrow-Mediated Protection of Myeloproliferative Neoplastic Cells to Vorinostat and Ruxolitinib Relies on the Activation of JNK and PI3K Signalling Pathways.

Abstract: The classical BCR-ABL-negative Myeloproliferative Neoplasms (MPN) are a group of heterogeneous haematological diseases characterized by constitutive JAK-STAT pathway activation. Targeted therapy with Ruxolitinib, a JAK1/2-specific inhibitor, achieves symptomatic improvement but does not eliminate the neoplastic clone. Similar effects are seen with histone deacetylase inhibitors (HDACi), albeit with poorer tolerance. Here, we show that bone marrow (BM) stromal cells (HS-5) protected MPN-derived cell lines (SET-2; HEL and UKE-1) and MPN patient-derived BM cells from the cytotoxic effects of Ruxolitinib and the HDACi Vorinostat. This protective effect was mediated, at least in part, by the secretion of soluble factors from the BM stroma. In addition, it correlated with the activation of signalling pathways important for cellular homeostasis, such as JAK-STAT, PI3K, JNK, MEK-ERK and NF-κB. Importantly, the pharmacological inhibition of JNK and PI3K pathways completely abrogated the BM protective effect on MPN cell lines and MPN patient samples. Our findings shed light on mechanisms of tumour survival and may indicate novel therapeutic approaches for the treatment of MPN.

Checkmate to CHK1 in T-cell ALL?

Abstract: DNA replication ensures accurate duplication of the original genetic information present in a cell in order for it to be properly transmitted to daughter cells. However, replication can be perturbed, for instance in rapidly dividing cancer cells, in a process referred to as replication stress (RS). Checkpoint kinase 1 (CHK1) is an essential component of the ATR-dependent DNA damage-response pathway that protect cells from RS by preventing replication fork collapse and activating homologous DNA repair. The ATR-CHK1 pathway is triggered upon exposure of single-stranded DNA that arises with the stalling of replication forks [1], and it is required to reset proper origin firing, and to promote fork stability and checkpoint activation, delaying mitosis until replication is completed and thereby avoiding mitotic catastrophe [2]. Whereas these functions point towards a tumor suppressor role for CHK1, mouse models modulating ATR-CHK1 expression and genetic evidence from human tumors suggest otherwise: Atr and Chk1 knock-out models do not display higher tumor frequency; Chk1 favors oncogene-induced transformation in mice; CHK1 is frequently overexpressed in human cancers, while loss-of-function mutations are rare [2, 3]. Moreover, CHK1 affords protection against DNA damaging agents, a fact that prompted the use of CHK1 inhibitors as chemosensitizers [4]. Similarly, tumors whose oncogenic profile fuels RS were proposed to become addicted to ATR-CHK1 response [1, 2]. In our recent study published in Oncogene [5], we hypothesized that T-cell acute lymphoblastic leukemia (T-ALL), an aggressive hematological cancer arising from T-cell precursor clonal expansion, could be one of such tumors and showed that CHK1 plays a key role in T-ALL cell maintenance. T-ALL cells tend to be highly proliferative due to a myriad of genetic lesions that culminate in cyclin-dependent kinase hyperactivation, and deregulated progression of S-phase that may impact on DNA replication [5, 6]. We found that T-ALL cells overexpressed CHK1 mRNA and protein as compared to normal hematopoietic progenitors. This was accompanied by aberrantly high CHK1 kinase activity, likely triggered by high basal levels of RS [5]. Experimental inactivation of CHK1, by a CHK1 selective inhibitor (PF-00477736) or by gene silencing, demonstrated that CHK1 is essential to control the accumulation of RS and to prevent apoptosis of T-ALL cells that appear to enter mitosis without having concluded DNA replication. Furthermore, accumulation of DNA damage in the context of CHK1 loss induced the activation of the ATM-CHK2 DNA double-strand break (DSB) response pathway, likely due to DSB formation upon the collapse of stalled replication forks. T-ALL apoptosis upon CHK1 inactivation was in the first instance dependent on ATM and caspase-3, since ATM inhibition prevented caspase-3 cleavage and rescued T-ALL cell viability despite sustained elevated amounts of RS markers [5]. Following the demonstration that T-ALL cells were eliminated using a CHK1 small molecule inhibitor as single agent, we showed that this effect was leukemia-specific, since normal T-cell progenitors were not sensitive to the low doses of PF-00477736 that killed primary T-ALL patient cells. Moreover, the in vitro anti-leukemia effect of PF-00477736 was not prevented by microenvironment pro-survival factors, and the potential clinical value of CHK1 inhibition was further demonstrated by the fact that PF-00477736 limited the growth of xenografted T-ALL tumors [5]. Interestingly, our preliminary analyses indicated that T-ALL cells expressing higher CHK1 levels appeared more sensitive to CHK1 pharmacological inhibition, suggesting that CHK1 expression could be a suitable drug response marker in T-ALL patients. As clinical trials against ATR-CHK1 pathway may be envisaged, this issue warrants extended T-ALL patient analysis. T-ALL constitutes only a fraction of all ALL cases, but it associates with high-risk. Therapeutic options with less detrimental side-effects and/or effective upon relapse are most desired. Our findings defining CHK1 as a ‘subverted’ tumor suppressor that stands in T-ALL as a major guardian of leukemia cell survival, thereby formally acting as an oncogene, reinforce a new way of viewing the mechanisms of cancer progression [2] and may set the ground for anti-leukemia breakthrough approaches. In this context, it is important to understand the mechanisms of CHK1 upregulation in T-ALL. We thoroughly documented CHK1 transcript overexpression in primary T-ALL [5]. However, how this occurs remains undetermined. Maybe transcription factors known to activate CHK1, such as E2F (downstream of G1/S-phase CDK activity) or MYC (downstream of NOTCH1), are involved in CHK1 overexpression in T-ALL. Or maybe as yet unidentified CHK1 regulatory elements are mutated or epigenetically altered. Curiously, in contrast to our findings, Chk1 mRNA downregulation was documented in a murine T-ALL model [7]. A more integrative view of the role of CHK1 in T-ALL is therefore required. We believe CHK1 downregulation may occur at T-ALL initiation, driving genomic instability secondary to an increase in RS. As the pro-proliferative oncogenic program establishes and RS rises, leukemic cells are naturally selected for their ability to upregulate CHK1 as a means to maintain RS levels compatible with cell viability.