Diterpene forskolin

The tricyclic substance 18A and 18B are both potential synthetic intermediates for synthesis of the biologically active diterpene forskolin. These intermediates can be prepared from the monocyclic precursors shown. Indicate the nature of the reactions involved in these transformations. 

One of the best-characterized effectors and second messenger systems is the cAMP cascade that can be either activated or inhibited by neurotransmit-ter/neuropeptide receptors, including those implicated in anxiety/stress such as CRE Receptors that activate cAMP synthesis couple with the stimulatory G protein, Gsa, and those that inhibit this second messenger couple with the inhibitory G protein, Gia, and these either stimulate or inhibit adenylyl cyclase, the effector enzyme responsible for synthesis of cAMP (Duman and Nestler 1999). There are at least nine different forms of adenylyl cyclase that have been identified by molecular cloning, each with a unique distribution in the brain. The different types of adenylyl cyclase are activated by Gsa as well as the diterpene forskolin, but are differentially regulated by Gia, the Py subunits, Ca, and by phosphorylation. This provides for fine control of adenylyl cyclase enzyme activity and regulation by other effector pathways. 

A common feature of the different adenylyl cyclases is the stimulation of their enzyme activity by the GTP-bound form of the a-subimit of the Gs-protein. Furthermore, all subtypes are stimulated by the diterpene forskolin. In addition to the central regulation by the activated a-subunit, there are other stimulatory or inhibitory influences on the different subtypes of adenylyl cyclase, in a manner characteristic for the particular subtype. The various subtypes differ in these regulatory influences, whereby none of the different subtypes have an identical pattern of regulation. 

It contains diterpene forskolin and is used for circulatory problems such as heart failure and high blood pressure, and for respiratory problems such as bronchial asthma. 

Labdanes (core C6 C6 linked to variously reduced furan or pyran moieties) include the Lamiaceae (Labiatae) diterpenes forskolin (C6 C6 pyran) (that activates adenylyl cyclase, the enzyme catalysing cyclic AMP formation from ATP) and premarrubiin (C40L G6 C6 furan-furan) that converts to the bitter non-opiate antinociceptive marrubiin (G4L C6 C6 -(CH2)2-furan). 

Adenylyl cyclase catalyses the reaction ATP— cAMP + pyrophosphate (PP,). Membrane-bound ACs are activated by hormones and NTs that act via G protein-linked receptors to generate AC-activating Gas GTP (Chapter 5). Particular AC isoforms are activated by Ca2+-calmodulin, this representing an example of cross-talk between cAMP and Ca2+ signalling pathways. As outlined in Chapter 5, Gai GTP inhibits AC and hence lowers cAMP concentration. A variety of hormones and NTs act via GPCRs to either activate or inhibit AC and in turn a variety of plant-derived compounds interfere with these processes (Chapter 5). The plant-derived diterpene forskolin and related compounds directly activate AC (Table 7.2). 

An unusual glycoside, wedeloside (96), from Wedelia asperrima (Asteraceae), inhibits mitochondrial ADP/ATP carrier protein and has the ability to protect animals against the toxic effects of aflatoxin Bi (Croteau and Johnson, 1985 Node et al., 1982). Uie diterpene forskolin (97), from Coleus barbatus and C. forskohlii (Lamiaceae), is an activator of adenylate cyclase, and is an active inhibitor of the action of brefeldin A (a fungal metabolite with pronounced effects on protein trafficking in cells) (De Sousa and Shah, 1988 Schreiber, 1992 Valdes et al., 1987 Wagner, 1988). This compound also is of interest as an antihypertensive agent (Alcaraz and Rios, 1991 Hanson, 1991). 

Seamon KB, Padgett W, Daly JW. (1981) Forskolin Unique diterpene activator of adenylate cyclase in membranes and in intact cells. Proc Natl Acad Sci USA 78 3363-3367. 

The most important manoyl oxide derivative is forskolin (9), Fig. (7), (7p-acetoxy-8, 13-epoxy-la, 6p, 9a-trihydroxylabd-14-en-ll-one) [151-153]. It belongs to the labdane series of diterpenes and was isolated from the Indian herb Coleus forskohlii (Willd.) Briq. (Labiatae). Since ancient times it has been used in Hindu and Ayurvedic traditional medicine [154]. The plant Coleus forskohlii (Willd.) Briq. has been extensively studied, and from its extracted roots a group of diterpenoids, with the basic skeleton of 11-oxo-manoyl oxide, have been isolated. The main compound, forskolin, presented remarkable chemical and biological properties [155]. Analogues of forskolin were then prepared by semisynthesis [156] or obtained by microbial transformations [157]. New analogues, more soluble than forskolin have shown activities comparable to and even higher than forskolin [158]. 

As noted, the labdanes display a broad spectrum of biochemical and pharmacological activities, suggesting that they may significantly affect the function of the immune system and inflammatory cells. The labdanes may affect critical enzymes such as adenylate cyclase, protein kinases and phospholipase A2, which are intimately involved in signal transduction and cell activation processes. Much of the information on labdane-type diterpene effects has been provided by forskolin and mainly in in vitro systems. 

Several effects of forskolin on B-lymphocytes, the cells of the immune system responsible for the production of immunoglobulins, have further been reported. This diterpene was found to inhibit cellular proliferation of B cells stimulated either by antibodies to surface immunoglobulins (anti-mu), and an antibody to CD20 antigen or 12-O-tetradecanoyl phorbol 13-acetate [219]. There was also a clear inhibition of G1 entry and DNA synthesis, and forskolin maintained its inhibitory effect even when added later after anti-mu stimulation. Additionally, no differences were found in the inhibitory effect of forskolin on neoplastic B cells, as compared to the responses of normal cells. Growth inhibition associated with an accumulation of cells in G1 was later found when cells of the B-lymphoid precursor cell line Reh were incubated with forskolin [220]. In that study, a delay of cells in G2/M prior to G1 arrest was observed, suggesting that important restriction points located in the G1 and G2 phases of the cell cycle may be controlled by forskolin (due to cAMP levels elevation). In a subsequent study [221], it was found that the arrest of Reh cells was accompanied by rapid dephosphorylation of retinoblastoma protein, which was suggested to be a prerequisite for the forskolin mediated arrest of these cells in Gl. 

Labdane type diterpenes obviously exhibit a wide spectrum of actions. Primarily forskolin exhibits (a) antihypertensive properties, lowering blood pressure in different animal species through a vasodilatory effect (b) positive inotropic effects in cardiac muscle and (c) bronchodilatory effects [155] and (d) pressure-lowering properties [244]. 

Seamon, K., and Daly, J. (1981). Forskolin A unique diterpene activator of cyclic AMP-generating systems. J. Cyclic Nucleotide Res. 7, 201-224. 

Laurenza A, Sutkowski EM and Seamon KB (1989) Forskolin a specific stimulator of adenylyl cyclase or a diterpene with multiple sites of action Trends Pharmacol Sci 10, 442-447. 

The catalytic component of the adenylate cyclase complex, i.e., the subunit which catalyzes the conversion of ATP to cAMP, has proved more difficult to purify than the other components. It has recently been purified from myocardium and brain [50-52] using affinity chromatography on forskolin linked to agarose and other chromatographic procedures. Forskolin is a diterpene which can activate the catalytic moiety directly [53]. The heart enzyme was purified approximately 60000-fold and exhibited two major peptides of MT 150000 and 42000 on NaDodS04 polyacrylamide gel electrophoresis [50]. The lower A/r peptide was probably 05, whereas the higher Mt peptide was probably the catalytic subunit. Crosslinking of the partially purified catalytic subunit with a Gs preparation, after [32P]ADP-ribosylation... 

Another important use of pure natural substances is as pharmacological or biochemical tools for the study of novel mechanisms of action, or of pathophysiological processes e.g. forskolin is a diterpene in the root of Coleus forskohlii (Lamiaceae), which activates adenylate cyclase and causes an increase in cellular c AMP levels. Phorbol esters (from family Euphorbiaceae) stimulate protein kinase C, and tetrodotoxin in Fu iuspc-cies influences the transport of sodium ions. 

Epothilones A and B are promising anticancer agents that bind to microtubules in the cell in the same way that paclitaxel (Taxol) does and are more potent.410 Paclitaxel was obtained first from the bark of the Pacific yew (Taxus brevifolia).4n Widespread extraction of the compound from this source could have wiped out the species. Fortunately, it can now be extracted from the needles of more common yews or produced in tissue culture. (See Sec II.C on tissue culture.) Forskolin is a diterpene, from the roots of Coleus forskohlii, that lowers blood pressure.412 Him-bacine, from an Australian pine tree, offers a potential treatment for Alzheimer s disease.413 Combretastatin, from the bark of the African bush willow, Combretum caffrum, cuts the flow of blood to tumors, causing 95% of the cancer cells to die in 24 h, but does not harm healthy blood vessels.414... 

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