The histamine H4 receptor is a potent inhibitor of adhesion-dependent degranulation in human neutrophils

The histamine H4 receptor regulates the inflammatory response. However, it is not known whether this receptor has a functional role in human neutrophils. We found that fMLP (1 μM), but not histamine (0.1-1 μM), induced Mac-1-dependent adhesion, polarization, and degranulation (release of lactoferrin). A pretreatment of neutrophils with histamine (0.001-1 μM) or JNJ 28610244 (0.1-10 μM), a specific H4 receptor agonist, led to inhibition of degranulation. Total inhibition of degranulation was obtained with 0.1 μM histamine and 10 μM JNJ 28610244. Furthermore, such inhibition by histamine of degranulation was reversed by JNJ 7777120 and JNJ 28307474, two selective H4 receptor antagonists. However, neither histamine nor the H4 receptor agonist JNJ 28610244 prevented fMLP-induced, Mac-1-dependent adhesion, indicating that the H4 receptor may block signals emanating from Mac-1-controlling degranulation. Likewise, engagement of the H4 receptor by the selective agonist JNJ 28610244 blocked Mac-1-dependent activation of p38 MAPK, the kinase that controls neutrophil degranulation. We also show expression of the H4 receptor at the mRNA level in ultrapure human neutrophils and myeloid leukemia PLB-985 cells. We concluded that engagement of this receptor by selective H4 receptor agonists may represent a good, therapeutic approach to accelerate resolution of inflammation.

Sequential treatment with flavopiridol synergistically enhances pyrrolo-1,5-benzoxazepine-induced apoptosis in human chronic myeloid leukaemia cells including those resistant to imatinib treatment

The Bcr-Abl kinase inhibitor, imatinib mesylate, is the front line treatment for chronic myeloid leukaemia (CML), but the emergence of imatinib resistance has led to the search for alternative drug treatments and the examination of combination therapies to overcome imatinib resistance. The pro-apoptotic PBOX compounds are a recently developed novel series of microtubule targeting agents (MTAs) that depolymerise tubulin. Recent data demonstrating enhanced MTA-induced tumour cell apoptosis upon combination with the cyclin dependent kinase (CDK)-1 inhibitor flavopiridol prompted us to examine whether this compound could similarly enhance the effect of the PBOX compounds. We thus characterised the apoptotic and cell cycle events associated with combination therapy of the PBOX compounds and flavopiridol and results showed a sequence dependent, synergistic enhancement of apoptosis in CML cells including those expressing the imatinib-resistant T315I mutant. Flavopiridol reduced the number of polyploid cells formed in response to PBOX treatment but only to a small extent, suggesting that inhibition of endoreplication was unlikely to play a major role in the mechanism by which flavopiridol synergistically enhanced PBOX-induced apoptosis. The addition of flavopiridol following PBOX-6 treatment did however result in an accelerated exit from the G2/M transition accompanied by an enhanced downregulation and deactivation of the CDK1/cyclin B1 complex and an enhanced degradation of the inhibitor of apoptosis protein (IAP) survivin. In conclusion, results from this study highlight the potential of these novel series of PBOX compounds, alone or in sequential combination with flavopiridol, as an effective therapy against CML.