17β-hydroxywortmannin: A potent inhibitor of bone resorption and phosphatidylinositol-3-kinase

Recent progress in the synthesis of the furanosteroid family of natural products

This review focuses on an overview of recent advances in the synthesis of furanosteroids and illustrates their applications in medicinal chemistry over the period of 2005–present.

17-Hydroxy Wortmannin Restores TRAIL's Response by Ameliorating Increased Beclin 1 Level and Autophagy Function in TRAIL-Resistant Colon Cancer Cells

Targeting of extrinsic apoptosis pathway by TNF-related apoptosis-inducing ligand (TRAIL) is an attractive approach for cancer therapy. However, two TRAIL drug candidates failed in clinical trials due to lack of efficacy. We identified 17-hydroxy wortmannin (17-HW) in a drug repurposing screen that resensitized TRAIL's response in the resistant colon cancer cells. The deficiency of caspase-8 in drug-resistant cells along with defects in apoptotic cell death was corrected by 17-HW, an inhibitor of PIK3C3-beclin 1 (BECN1) complex and autophagy activity. Further study found that BECN1 significantly increased in the TRAIL-resistant cells, resulting in increased autophagosome formation and enhanced autophagy flux. The extracellular domain (ECD) of BECN1 directly bound to the caspase-8 catalytic subunit (p10), leading to sequestration of caspase-8 in the autophagosome and its subsequent degradation. Inhibition of BECN1 restored the caspase-8 level and TRAIL's apoptotic response in the resistant colon cancer cells. An analysis of 120 colon cancer patient tissues revealed a correlation of a subgroup of patients (30.8%, 37/120) who have high BECN1 level and low caspase-8 level with a poor survival rate. Our study demonstrates that the increased BECN1 accompanied by enhanced autophagy activity is responsible for the TRAIL resistance, and a combination of TRAIL with a PIK3C3-BECN1 inhibitor is a promising therapeutic approach for the treatment of colon cancer.

Pharbinilic acid, an allogibberic acid from morning glory (Pharbitis nil)

Pharbinilic acid (1), the first naturally occurring allogibberic acid, was isolated from ethanol extracts of morning glory (Pharbitis nil) seeds. Its absolute configuration was determined by NOESY NMR and ECD experiments. Compound 1 showed weak cytotoxicity against A549, SK-OV-3, SK-MEL-2, and HCT-15 cells and weakly inhibited nitric oxide production in lipopolysaccharide-activated BV-2 microglia cells.

Chemistry and biology of wortmannin

Recent synthetic and biological studies of the viridin class of steroidal furans have revealed multiple opportunities for fundamental discoveries as well as advanced drug design. Wortmannin is a potent enzyme inhibitor that binds to the ATP site of important regulatory kinases such as PI-3 kinase and Polo-like kinase. The natural product shares a unique mechanism-based biological activation pathway with other viridins. Furthermore, while there have been several encouraging approaches toward the total synthesis of these compounds, there is still ample room for improvements in synthetic strategies and tactics, and the development of structurally simplified analogs that exert more specific biological effects and are devoid of toxicity issues that have thwarted the clinical development of the parent compounds.

Integrins and signaling in osteoclast function

Integrins are heterodimeric adhesion receptors that mediate cell-matrix and cell-cell interactions. Osteoclasts highly express the alphavbeta3 integrin, which binds to a variety of extracellular matrix proteins including vitronectin, osteopontin and bone sialoprotein. RGD-containing peptides, RGD-mimetics and alphavbeta3 blocking antibodies inhibit bone resorption in vitro and in vivo, suggesting that this integrin plays an important role in osteoclast function. RGD-containing peptides were shown to raise cytosolic calcium in osteoclasts. Furthermore, several signaling and adaptor molecules were found to be involved in alphavbeta3 integrin-dependent signaling pathways, including phosphatidylinositol 3-kinase, c-Src, PYK2 and p130(cas). In addition, cytoskeletal molecules such as paxillin, vinculin, gelsolin and F-actin are recruited to adhesion contacts upon integrin activation. Many of these molecules signaling and cytoskeletal localize to the sealing zone of actively resorbing osteoclasts, suggesting that they play a role in linking the adhesion of osteoclasts to the bone matrix with the cytoskeletal organization and the polarization and activation of these cells for bone resorption.

Wortmannin inhibits spreading and chemotaxis of rat osteoclasts in vitro

Wortmannin (WT) and 17beta-hydroxywortmannin (HWT), which are inhibitors of phosphatidylinositol-3(OH)-kinase (PI3K), have been shown previously to inhibit bone resorption in vitro and in vivo, possibly by interfering with formation of the osteoclast ruffled border. Since migration of osteoclasts also plays an important role in the process of bone resorption, we investigated the effects of these inhibitors on osteoclast morphology and motility. Both HWT and WT caused a sustained decrease in the planar area of osteoclasts in vitro (half maximal effect at 25 and 165 nM, respectively), with the effect of HWT on cell area more readily reversible than WT. These agents also caused accumulation of intracellular vesicles. Time-lapse video microscopy was used to record the migration of osteoclasts in response to macrophage colony-stimulating factor (M-CSF) or vehicle, flowing passively from a micropipette positioned 200-400 microm from the cell. M-CSF caused directed migration of osteoclasts, indicating chemotaxis (over 3 h osteoclasts migrated 96 +/- 14 microm in response to M-CSF vs. 11 +/- 2 microm in control experiments). Both WT (100 or 500 nM) and LY294002 (100 microM), a specific PI3K inhibitor structurally unrelated to WT, significantly inhibited osteoclast chemotaxis in response to M-CSF. Taken together, these effects of WT, HWT, and LY294002 are consistent with an important role for PI3K in regulating cytoskeletal function in osteoclasts. The inhibitory effects of WT and HWT on bone resorption may be due, in part, to impairment of osteoclast motility.

Phosphatidylinositol 3-kinase association with the osteoclast cytoskeleton, and its involvement in osteoclast attachment and spreading

Osteoclast activation involves attachment to the mineralized bone matrix and reorganization of the cytoskeleton, leading to polarization of the cell. Signaling molecules, PI3-kinase, rho A, and pp60c-src, were shown to be essential for osteoclastic bone resorption. In this study we have focused on the involvement of these signaling molecules in the early event of osteoclast activation: attachment, spreading, and organization of the cytoskeleton. Highly purified osteoclasts were fractionated into Triton X-100-soluble or cytosolic and Triton X-100-insoluble or cytoskeletal fractions, and the distribution of above-mentioned signaling molecules between the two fractions was examined. PI3-kinase, rho A, and pp60c-src all showed translocation to the cytoskeletal fraction upon osteoclast attachment to plastic. However, PI3-kinase and rho A, but not pp60c-src, showed further translocation of 2.4- and 3.2-fold, respectively, upon attachment of osteoclasts to bone. PI3-kinase translocation to the cytoskeleton was inhibited by either cytochalasin B or colchicine. Furthermore, treatment of osteoclasts with the PI3-kinase inhibitor wortmannin decreased its translocation, suggesting that PI3-kinase activity was needed for its translocation. Moreover, wortmannin inhibited osteoclast attachment to both bone and plastic and caused drastic changes in osteoclast morphology resulting in rounding of the cells, disappearance of F-actin structures or podosomes, and appearance of punctate or vesicular structures inside the cells. Osteoblastic MB1.8 cells and IC-21 macrophages did not show additional translocation of PI3-kinase or rho A upon attachment to bone or changes in attachment or morphology in response to wortmannin. Finally, PI3-kinase coimmunoprecipitated with alpha v beta 3 integrin from osteoclasts.

Studies on the mechanism of phosphatidylinositol 3-kinase inhibition by wortmannin and related analogs

Wortmannin, a fungal metabolite, was identified as a potent inhibitor (IC50 = 4.2 nM) of phosphatidylinositol 3-kinase (PI 3-kinase). Due to the importance of PI 3-kinase in several intracellular signaling pathways, structure-activities studies on wortmannin analogs were performed in an effort to understand the structural requirements necessary for PI 3-kinase inhibition. Since wortmannin is an irreversible inhibitor of PI 3-kinase, it was postulated that covalent attachment at the electrophilic C-21 site was a possible mode of action for PI 3-kinase inhibition. We have prepared various wortmannin analogs which address the possibility of this mechanism. Of particular interest are compounds which affect the C-21 position of wortaminnin either sterically or electronically. Our results support the conclusion that nucleophilic addition by the kinase onto the C-21 position of wortmannin is required for inhibition of PI 3-kinase by wortmannin analogs. Additionally, we have prepared several D-ring analogs of wortmannin, and their activities are reported herein. We conclude that the wortmannin D ring is an important recognition site since modifications have such a dramatic effect on inhibitor potency. Finally, the identification of 17beta-hydroxywortmannin represents the first reported subnanomolar inhibitor of PI 3-kinase. These studies, along with in vivo antitumor experiments, suggest that the mechanism of PI 3-kinase inhibition correlates to the associated toxicity observed with wortmannin-based inhibitors of PI 3-kinase.