Selective Inhibition of PAR4 (Protease-Activated Receptor 4)-Mediated Platelet Activation by a Synthetic Nonanticoagulant Heparin Analog.

Objective- PAR4 (protease-activated receptor 4), one of the thrombin receptors in human platelets, has emerged as a promising target for the treatment of arterial thrombotic disease. Previous studies implied that thrombin exosite II, known as a binding site for heparin, may be involved in thrombin-induced PAR4 activation. In the present study, a heparin octasaccharide analog containing the thrombin exosite II-binding domain of heparin was chemically synthesized and investigated for anti-PAR4 effect. Approach and Results- PAR4-mediated platelet aggregation was examined using either thrombin in the presence of a PAR1 antagonist or γ-thrombin, which selectively activates PAR4. SCH-28 specifically inhibits PAR4-mediated platelet aggregation, as well as the signaling events downstream of PAR4 in response to thrombin. Moreover, SCH-28 prevents thrombin-induced ß-arrestin recruitment to PAR4 but not PAR1 in Chinese Hamster Ovary-K1 cells using a commercial enzymatic complementation assay. Compared with heparin, SCH-28 is more potent in inhibiting PAR4-mediated platelet aggregation but has no significant anticoagulant activity. In an in vitro thrombosis model, SCH-28 reduces thrombus formation under whole blood arterial flow conditions. Conclusions- SCH-28, a synthetic small-molecular and nonanticoagulant heparin analog, inhibits thrombin-induced PAR4 activation by interfering with thrombin exosite II, a mechanism of action distinct from other PAR4 inhibitors that target the receptor. The characteristics of SCH-28 provide a new strategy for targeting PAR4 with the potential for the treatment of arterial thrombosis.

Different effects of 4ß-hydroxywithanolide E and withaferin A, two withanolides from Solanaceae plants, on the Akt signaling pathway in human breast cancer cells.

BACKGROUND: Triple-negative breast cancer (TNBC) represents a clinical challenge because it lacks sensitivity to hormone therapy or other available molecule-targeted agents. In addition, TNBC frequently exhibits over-activation of the PI3K/Akt survival pathway that can contribute to chemotherapy resistance. 4ß-Hydroxywithanolide E (4-HW) and withaferin A (WA) are two withanolides from Solanaceae plants that exhibit promising anticancer activity in vitro and in vivo. PURPOSE: The aim of this study is to investigate and compare the effects of 4-HW and WA on TNBC cells and underling mechanisms. STUDY DESIGN/METHODS: The anticancer effects of 4-HW and WA were evaluated by cell viability, cell cycle arrest, and apoptosis assays. PI3K/Akt signaling and the expression of survivin, Bcl-2 family proteins and cyclin-dependent kinase inhibitors were evaluated by Western blot. The role of PI3K/Akt signaling in the withanolides-induced anticancer effects was examined by using a PI3K inhibitor and overexpression of a constitutively active form of Akt. RESULTS: In TNBC MDA-MB-231 cells, 4-HW and WA displayed different kinetic effect on cell availability. Cell cycle analysis revealed that 4-HW induced the G1-phase arrest while WA caused the G2/M-phase block. Both withanolides induced apoptosis, but WA also caused necrosis. 4-HW inhibited the PI3K/Akt pathway and survivin expression as well as up-regulated the cyclin-dependent kinase inhibitors p21 and p27. In contrast, WA is a more potent inhibitor of Hsp90 and elicited Akt activation at low doses but inhibited Akt signaling at higher doses by depleting the Akt protein. The PI3K inhibitor LY294002 mimicked the effects of 4-HW and potentiated the cytotoxic activity of WA. In contrast, overexpressing a constitutively active form of myristoylated Akt rescue cancer cells from 4-HW-induced cell death. CONCLUSION: The withanolides 4-HW and WA potently inhibit the viability of TNBC cells through induction of cell cycle arrest and apoptosis/necrosis. The PI3K/Akt pathway plays distinct roles in cancer cells respond to 4-HW and WA. These results suggest the potential applications of the withanolides for the treatment of TNBC.