Limitations of radiosensitization by direct telomerase inhibition to treat high-risk medulloblastoma

Medulloblastoma (MB) is the most common malignant pediatric brain tumor. Previous studies have elucidated the genomic landscape of MB leading to the recognition of four core molecular subgroups (WNT, SHH, group 3 and group 4) with distinct clinical outcomes. Group 3 has the worst prognosis of all MB. Radiotherapy (RT) remains a major component in the treatment of poor prognosis MB but is rarely curative alone and is associated with acute and long-term toxicities. A hallmark of cancer cells is their unlimited proliferative potential which correlates closely with telomere length. The vast majority of malignant tumors activate telomerase to maintain telomere length, whereas this activity is barely detectable in most normal human somatic tissues, making telomerase inhibition a rational therapeutic target in the setting of cancer recurrence and therapy resistance. We and others have previously shown that short telomeres confer sensitivity to ionizing radiation (IR) suggesting that telomerase inhibition mediated telomere shortening will improve the efficacy of RT while minimizing its side effects. Here, we investigated the efficacy of the combination of IR with IMT, a potent telomerase inhibitor, in an in vivo model of group 3 MB. Our results indicate that although IMT inhibited MB telomerase activity resulting in telomere shortening and delayed tumor growth, the combination with IR did not prevent tumor recurrence and did not improve survival compared to the treatment with IR alone. Together, these findings suggest that the radiosensitization by direct telomerase inhibition is not an effective approach to treat high-risk pediatric brain tumors.

Induced Telomere Damage to Treat Telomerase Expressing Therapy-Resistant Pediatric Brain Tumors

Brain tumors remain the leading cause of cancer-related deaths in children and often are associated with long-term sequelae among survivors of current therapies. Hence, there is an urgent need to identify actionable targets and to develop more effective therapies. Telomerase and telomeres play important roles in cancer, representing attractive therapeutic targets to treat children with poor-prognosis brain tumors such as diffuse intrinsic pontine glioma (DIPG), high-grade glioma (HGG), and high-risk medulloblastoma. We have previously shown that DIPG, HGG, and medulloblastoma frequently express telomerase activity. Here, we show that the telomerase-dependent incorporation of 6-thio-2'deoxyguanosine (6-thio-dG), a telomerase substrate precursor analogue, into telomeres leads to telomere dysfunction-induced foci (TIF) along with extensive genomic DNA damage, cell growth inhibition, and cell death of primary stem-like cells derived from patients with DIPG, HGG, and medulloblastoma. Importantly, the effect of 6-thio-dG is persistent even after drug withdrawal. Treatment with 6-thio-dG elicits a sequential activation of ATR and ATM pathways and induces G₂-M arrest. In vivo treatment of mice bearing medulloblastoma xenografts with 6-thio-dG delays tumor growth and increases in-tumor TIFs and apoptosis. Furthermore, 6-thio-dG crosses the blood-brain barrier and specifically targets tumor cells in an orthotopic mouse model of DIPG. Together, our findings suggest that 6-thio-dG is a promising novel approach to treat therapy-resistant telomerase-positive pediatric brain tumors. Mol Cancer Ther; 17(7); 1504-14. ©2018 AACR.

Prognostic significance of telomere maintenance mechanisms in pediatric high-grade gliomas

Children with high-grade glioma, including diffuse intrinsic pontine glioma (DIPG), have a poor prognosis despite multimodal therapy. Identifying novel therapeutic targets is critical to improve their outcome. We evaluated prognostic roles of telomere maintenance mechanisms in children with HGG, including DIPG. A multi-institutional retrospective study was conducted involving 50 flash-frozen HGG (35 non-brainstem; 15 DIPG) tumors from 45 children (30 non-brainstem; 15 DIPG). Telomerase activity, expression of hTERT mRNA (encoding telomerase catalytic component) and TERC (telomerase RNA template) and alternative lengthening of telomeres (ALT) mechanism were assayed. Cox Proportional Hazard regression analyses assessed association of clinical and pathological variables, TERC and hTERT levels, telomerase activity, and ALT use with progression-free or overall survival (OS). High TERC and hTERT expression was detected in 13/28 non-brainstem HGG samples as compared to non-neoplastic controls. High TERC and hTERT expression was identified in 13/15 and 11/15 DIPG samples, respectively, compared to controls. Evidence of ALT was noted in 3/11 DIPG and 10/19 non-brainstem HGG specimens. ALT and telomerase use were identified in 4/19 non-brainstem HGG and 2/11 DIPG specimens. In multivariable analyses, increased TERC and hTERT levels were associated with worse OS in patients with non-brainstem HGG, after controlling for tumor grade or resection extent. Children with HGG and DIPG, have increased hTERT and TERC expression. In children with non-brainstem HGG, increased TERC and hTERT expression levels are associated with a worse OS, making telomerase a promising potential therapeutic target in pediatric HGG.

Curcumin analogue GO-Y030 inhibits STAT3 activity and cell growth in breast and pancreatic carcinomas

Curcumin has numerous anti-carcinogenic properties, but low bioavailability prevents its use in chemotherapeutic applications. One strategy for circumventing this problem has been the creation of synthetic analogues. We tested the efficacy of an analogue known as GO-Y030 in human breast and pancreatic cancer cells. We compared the impact of curcumin and GO-Y030 on the breast cancer cell line MDA-MB-231 and pancreatic cancer cell lines, PANC-1, HPAC and BXPC-3. Both compounds reduced cell viability and induced apoptosis, but GO-Y030 was substantially more potent. We also demonstrated that GO-Y030 was capable of interfering with STAT3, a persistently activated transcription factor in many cancer types. GO-Y030 inhibited STAT3 phosphorylation and transcriptional activity whereas comparable dosages of curcumin had little or no effect. These results indicate that GO-Y030 is a potent inhibitor of cell viability and STAT3 activation, and may thus have potential as a therapeutic agent for cancers expressing high levels of activated STAT3.

MI-63: a novel small-molecule inhibitor targets MDM2 and induces apoptosis in embryonal and alveolar rhabdomyosarcoma cells with wild-type p53

Background:

Interruption of the role of p53s as a tumour suppressor by MDM2 may be one of the mechanisms by which cancer cells evade current therapy. Blocking the inhibition of wild-type p53 by MDM2 in cancer cells should reactivate p53's tumour suppressor functions and enhance current cancer treatments. MI-63 is a novel non-peptide small molecule that has shown strong binding affinity (K_i=3 nM) for MDM2; however, its effects on paediatric cancer cells and the specific mechanism of tumour suppressor reactivation have not been evaluated.

Methods:

Rhabdomyosarcoma (RMS), the most common childhood soft tissue sarcoma, expresses either wild-type or mutant p53 protein. We examined the inhibitory effects of MI-63 in embryonal RMS (ERMS) and alveolar RMS (ARMS) cell lines expressing wild-type or mutated p53.

Results:

Treatment with MI-63 reduced cell viability by 13.4% and by <1%, respectively, at 72 h in both RH36 and RH18 cell lines expressing wild-type p53. In contrast, RH30 and RD2 cells expressing p53 mutants are resistant to MI-63 treatment. An increased expression of p53, p21^WAF1, and Bax protein was observed after treatment with MI-63 in RMS cells with wild-type p53, and apoptosis was confirmed by cleaved PARP and caspase-3 expression. However, RD2 and RH30 RMS cells, as well as human normal skeletal muscle cells, showed a minimal increase in p53 signalling and no induction of cleaved PARP and caspase-3. MI-63 was compared with Nutlin-3, a known MDM2 inhibitor, and was found to be more potent in the inhibition of cell proliferation/viability. Further, synergy was observed when MI-63 was used in combination with doxorubicin.

Conclusion:

These results indicate that MI-63 is a potent therapeutic agent for RMS cells expressing wild-type p53 protein.

LLL-3 inhibits STAT3 activity, suppresses glioblastoma cell growth and prolongs survival in a mouse glioblastoma model

Persistent activation of the signal transducer and activator of transcription 3 (STAT3) signalling has been linked to oncogenesis and the development of chemotherapy resistance in glioblastoma and other cancers. Inhibition of the STAT3 pathway thus represents an attractive therapeutic approach for cancer. In this study, we investigated the inhibitory effects of a small molecule compound known as LLL-3, which is a structural analogue of the earlier reported STAT3 inhibitor, STA-21, on the cell viability of human glioblastoma cells, U87, U373, and U251 expressing constitutively activated STAT3. We also investigated the inhibitory effects of LLL-3 on U87 glioblastoma cell growth in a mouse tumour model as well as the impact it had on the survival time of the treated mice. We observed that LLL-3 inhibited STAT3-dependent transcriptional and DNA binding activities. LLL-3 also inhibited viability of U87, U373, and U251 glioblastoma cells as well as induced apoptosis of these glioblastoma cell lines as evidenced by increased poly (ADP-ribose) polymerase (PARP) and caspase-3 cleavages. Furthermore, the U87 glioblastoma tumour-bearing mice treated with LLL-3 exhibited prolonged survival relative to vehicle-treated mice (28.5 vs 16 days) and had smaller intracranial tumours and no evidence of contralateral invasion. These results suggest that LLL-3 may be a potential therapeutic agent in the treatment of glioblastoma with constitutive STAT3 activation.

Enhanced inflammatory response to acute ozone exposure in rats during pregnancy and lactation

Experimental evidence from several studies suggests that pregnant animals and women are more susceptible to oxidants than nonpregnant controls. In the study reported here, we sought to determine whether pregnant rats are more sensitive than age-matched virgin females to the inflammatory effects of ozone, a gaseous oxidant of considerable environmental significance. Rats at several stages of pregnancy and lactation, as well as age-matched virgin females, were exposed to 1 ppm ozone for 6 hr. Controls were sham-exposed to pure air for an identical period of time. Bronchoalveolar lavage was performed 24 hr after the beginning of exposure, and components of the lavage fluid considered to be indicators of inflammation were used to assess the severity of pulmonary inflammation. The results of this experiment showed that significantly enhanced sensitivity to ozone-induced pulmonary inflammation develops during pregnancy, is maintained during lactation, and disappears following lactation. Implicit in this pattern of differential sensitivity in rats is the possibility of a similar pattern of inflammatory response in analogous groups of humans as well as the potential for applicability to other oxidative pollutants.

Age-dependence of responses to acute ozone exposure in rats

Previous work from this laboratory demonstrated that neonatal rats and postweanling rabbits are more sensitive to ozone-induced stimulation of pulmonary arachidonic acid (AA) metabolism than are young adults (Fundam. Appl. Toxicol. 15, 779.) In the study reported here, we have extended our initial investigation to include the influence of animal age on temporal aspects of pulmonary AA metabolism and several other responses to brief exposures to 1 ppm ozone. Rats of discrete ages ranging from 13 days to 16 weeks were exposed to 1 ppm ozone or to air for 2, 4, or 6 hr. Immediately following exposure the lungs were lavaged with six consecutive volumes of phosphate-buffered saline and the acellular fluid from the first lavage volume recovered was analyzed for its content of prostaglandin E2 (PGE2), protein, and lactate dehydrogenase. Leukocytes recovered by lavage were quantitated and characterized by viability and percentage of polymorphonuclear (PMN) cells. Several lines of evidence verified that PGE2 was produced by the lung as a consequence of ozone exposure and that its concentration in the fluid from the first lavage was a reasonably good index of pulmonary AA metabolism to prostanoids. We also demonstrated that the lavage process itself stimulates the lung, resulting in increased AA metabolism to prostanoids that were recovered in the second and following lavage volumes. The time course of PGE2 production by the ozone-exposed lung varied considerably with animal age. Neonatal rats 13 days of age were the most sensitive to ozone stimulation. At 2 hr of exposure, PGE2 concentration in the first lung lavage of these animals peaked at values approximately two orders of magnitude above controls and then decreased sharply with continued exposure. Adults and older neonates (18 days of age) were much less responsive to 2-hr exposures; however, continued exposure of these rats for up to 6 hr resulted in increasing PGE2 concentration in the first lung lavage. Other responses showed various degree of age dependence. The percentage of lavaged leukocytes that were nonviable (i.e., trypan blue-positive) showed a strong inverse correlation with animal age. In 13-day-old rats that were exposed for 6 hr, the percentage of dead leukocytes reached nearly 50%. In addition, sheets or clumps of dead cells that were judged to be epithelial cells were lavaged from these animals. Conversely, 16-week-old adult males exposed to ozone for 6 hr showed little evidence of damage to cells of the respiratory tract.(ABSTRACT TRUNCATED AT 400 WORDS)

Age-dependent effect of ozone on pulmonary eicosanoid metabolism in rabbits and rats

Acute exposures to ozone have previously been shown to cause quantitative changes in the spectrum of arachidonic acid (AA) metabolites in lung lavage fluid. Since age appears to be an important variable in the toxicity of inhaled ozone, we investigated its effect on ozone-induced changes in pulmonary eicosanoid metabolism. Rats and rabbits ranging in age from neonates to young adults were exposed either to air or to 1 ppm ozone for 2 hr. Lung lavage fluid was collected within 1 hr following exposure and analyzed for its content of selected eicosanoids. In both species, there was a pronounced effect of age on ozone-induced pulmonary eicosanoid metabolism. Ozone-exposed animals at the youngest ages examined had severalfold greater amounts of two products of the cyclooxygenase pathway, prostaglandin E2 (PGE2) and prostaglandin F2 alpha (PGF2 alpha), than did age-matched controls. This effect lessened and eventually disappeared as the animals grew toward adulthood. In rabbits, ozone also induced increases in 6-keto-prostaglandin F1 alpha and thromboxane B2, but these changes were of lesser magnitude and evident only in the youngest rabbits exposed. There was no observed effect of ozone on lung lavage content of leukothriene B4. Indices of nonspecific pulmonary damage, i.e., protein concentration in lung lavage fluid and total number and viability of lavaged lung cells, were affected by ozone exposure, but not in an age-dependent manner that correlated with changes in pulmonary eicosanoid metabolism. In vitro ozone exposure of lung macrophages from naive rabbits of the same age range as those exposed in vivo demonstrated that ozone is capable of stimulating the elaboration of PGF2 alpha and especially PGE2. However, the increase in lavage fluid PGE2 and PGF2 alpha caused by ozone inhalation could not be attributed to macrophage metabolism conclusively since elaboration of PGE2 and PGF2 alpha by cultured macrophages was not enhanced by prior in vivo ozone exposure. In an ancillary study it was shown that 15-hydroxyprostaglandin dehydrogenase (PGDH) activity in rabbit lung homogenates was not affected by prior exposure to ozone, indicating that the increase in lung lavage fluid eicosanoids that occurred in these animals could not be explained by inhibition of PGDH.