Diacylglycerol Signaling

38 P.M. Blumberg et al.
3.14 C1 Domain Ligands as Clinical Candidates: Bryostatin 1
Although there are strong mechanistic arguments for why C1 domains represent
attractive targets for development of drugs directed at DAG receptors such as PKC,
the most powerful argument is the reality that several such drugs are already in
clinical trials. The most extensively studied of these is bryostatin 1. Bryostatin 1
was identified as part of the very productive natural products screening program of
the Pettit group, evaluating marine sources for antiproliferative activity against the
P388 leukemia cell line (Pettit 1991). Currently, bryostatin 1 either as a single agent
or in combination is the subject of 38 clinical trials which have been completed, are
in progress, or are being instituted.
Bryostatin 1 is a macrocyclic lactone, possessing 11 chiral centers. After initial
reports that bryostatin 1 functioned as a potent PKC ligand able to induce several
similar effects as did the phorbol esters (Berkow and Kraft 1985; Smith et al. 1985),
a critical observation by Kraft and coworkers was that in some other instances
bryostatin 1 failed to induce a typical phorbol ester response. Importantly, in such
instances, bryostatin 1 was paradoxically able to antagonize the response to phorbol
ester if both agents were co-applied (Kraft et al. 1986). This antagonism proved to
be the rule rather than the exception. Phorbol esters block differentiation of Friend
erythroleukemia cells in response to hexamethylene bisacetamide and bryostatin 1
reverses this block (Dell’Aquila et al. 1987). Phorbol esters induce differentiation of
HL-60 promyelocytic leukemia cells and bryostatin 1 inhibits this differentiation
(Kraft et al. 1986). Phorbol esters block cell–cell communication in primary mouse
epidermal cells and bryostatin 1 leads to restoration of cell–cell communication
(Pasti et al. 1988). Phorbol esters induce arachidonic acid release in mouse C3H
10T1/2 cells and bryostatin 1 inhibits this release (Dell’Aquila et al. 1988). Phorbol
esters induce attachment and block proliferation in U937 leukemia cells whereas
bryostatin 1 blocks attachment and restores proliferation (Ng and Guy 1992;
Asiedu et al. 1995; Grant et al. 1996; Vrana et al. 1998; Keck et al. 2009). Finally
and of particular significance, we showed that bryostatin 1 failed to function as a
tumor promoter in mouse skin and indeed inhibited tumor promotion by phorbol ester
(Hennings et al. 1987). These findings provided strong motivation for the evaluation
of bryostatin 1 treatment in those cancers for which PKC was implicated.
While bryostatin 1 provides an example of the potential of C1 domain ligands to
act as antagonists of PKC action, unfortunately the mechanism(s) responsible for
the functional antagonism exerted by bryostatin 1 remains unresolved. It is clear,
for example, that in some systems bryostatin 1 treatment induces a response similar
to that of PMA but of transient duration. This is the case, for example, for inhibition
of cell–cell communication in mouse epidermal cells (Pasti et al. 1988). Here,
bryostatin 1 induces a response at 1 h but the response is largely lost by 4 h, whereas
that by PMA is persistent. Inhibition of epidermal growth factor binding behaves
similarly in these cells (Sako et al. 1987). On the other hand, the failure of bryostatin
1 to induce arachidonic acid release in C3H10T1/2 cells was evident at 30 min, the
earliest time at which response to PMA could be observed, suggesting an absolute