Pseudolarix Acid B, a New Tubulin-Binding Agent, Inhibits Angiogenesis by Interacting with a Novel Binding Site on Tubulin

Abstract

Tubulin-binding agents have received considerable interest as potential tumor-selective angiogenesis-targeting drugs. Herein, we report that pseudolarix acid B (PAB), isolated from the traditional Chinese medicinal plant Pseudolarix kaempferi Gordon, is a tubulin-binding agent. We further demonstrate that PAB significantly and dose-dependently inhibits proliferation, migration, and tube formation by human microvessel enthothelial cells. It is noteworthy that PAB eliminated newly formed endothelial tubes and microvessels both in vitro and in vivo. In addition, PAB dramatically arrested the cell cycle at G 2 /M phase. PAB also induced endothelial cell retraction, intercellular gap formation, and promoted actin stress fiber formation in conjunction with disruption of the tubulin and actin cytoskeletons. All of these effects occurred at noncytotoxic concentrations of PAB. We found that these effects of PAB are attributable to depolymerization of tubulin by direct interaction with a distinct binding site on tubulin compared with those of colchicine and vinblastine. Taken together, these findings show that PAB is a candidate antiangiogenic agent for use in cancer therapy, and they provide proof of principle for targeting this novel binding site on tubulin as a new strategy for treating cancer.

Newly discovered angiogenesis inhibitors and their mechanisms of action

“…Pseudolaric acid B inhibits angiogenesis by reducing the stability of HIF-1α and thereby downregulating the VEGF-VEGFR axis [15,16,18] . However, there has been no direct evidence indicating any association between its antiangiogenic activity and its inhibition of microtubulin [23,24] . By contrast, MFTZ-1 does not affect either the degradation of HIF-1α protein or the level of HIF-1α mRNA.…”

“…Pseudolaric acid B has been demonstrated to both elicit potent anticancer effects by depolymerizing microtubulin [22,23] and circumvent tumor multidrug resistance [22] . Detailed structure-activity studies have revealed that the components of pseudolaric acid B that are essential to its anticancer activity include a hydrophobic group (-CO 2 Me or -Me) at C-7, a Δ 7 double bond, an acyloxy (OAc) at C-4,3 and a side chain with a conjugated double bond and a hydrophilic terminal group [33] .…”

“…Pseudolaric acid B targets microtubulin and causes its depolymerization [22,23] . Pseudolaric acid B inhibits angiogenesis by reducing the stability of HIF-1α and thereby downregulating the VEGF-VEGFR axis [15,16,18] .…”

“…In particular, through drug discovery guided by therapeutic experiences from long-established TCM practices, inhibitors of angiogenesis could be obtained that have new chemical structures and unique mechanisms of action; examples of these sorts of inhibitors include pseudolaric acid B, triptolide, 11,11'-dideoxyverticillin and shiraiachrome A ( Table 1). The original medicinal materials producing these compounds have long been used to either treat various angiogenesisrelated diseases, such as rheumatoid arthritis, microbial skin diseases and psoriasis [7,21,23] , or cause the early termination of pregnancies [23] . (2) Marine-derived compounds could be another important source of angiogenesis inhibitors, and certain compounds with specific types of chemical structures, such as saccharides, should be subjected to particular scrutiny; as demonstrated by JG3 [27] and GLP [30] , these compounds may exhibit unique modes of antiangiogenic activity.…”

Discovery of structurally diverse and bioactive compounds from plant resources in China

“…PAB also showed strong cytotoxic activity. In our collaboration study, this compound was found for the first time to have strong antiangiogenic activity with a unique mode of antitumor action [118][119][120] . A structural modification of PAB was thus conducted, and more than 40 derivatives were prepared in the first round.…”