Overexpression of
steroid receptor coactivator (SRC)-1 and SRC-3 is associated with
cancer initiation,
metastasis, advanced disease, and resistance to
chemotherapy. In most of these cases, SRC-1 and SRC-3 have been shown to promote
tumor cell growth by activating
nuclear receptor and multiple
growth factor signaling cascades that lead to uncontrolled
tumor cell growth. Up until now, most targeted chemotherapeutic drugs have been designed largely to block a single pathway at a time, but
cancers frequently acquire resistance by switching to alternative
growth factor pathways. We reason that the development of chemotherapeutic agents against SRC coactivators that sit at the
nexus of multiple cell growth signaling networks and transcriptional factors should be particularly effective
therapeutics. To substantiate this hypothesis, we report the discovery of 2,2'-bis-(Formyl-1,6,7-trihydroxy-5-isopropyl-3-methylnaphthalene (
gossypol) as a small molecule inhibitor of coactivator SRC-1 and SRC-3. Our data indicate that
gossypol binds directly to SRC-3 in its receptor interacting domain. In MCF-7
breast cancer cells,
gossypol selectively reduces the cellular
protein concentrations of SRC-1 and SRC-3 without generally altering overall
protein expression patterns, SRC-2, or other coactivators, such as p300 and
coactivator-associated arginine methyltransferase 1.
Gossypol reduces the concentration of SRC-3 in prostate, lung, and
liver cancer cell lines.
Gossypol inhibits cell viability in the same
cancer cell lines where it promotes SRC-3 down-regulation. Additionally,
gossypol sensitizes lung and
breast cancer cell lines to the inhibitory effects of other chemotherapeutic agents. Importantly,
gossypol is selectively cytotoxic to
cancer cells, whereas normal cell viability is not affected. This data establish the proof-of-principle that, as a class, SRC-1 and SRC-3 coactivators are accessible chemotherapeutic targets. Given their function as integrators of multiple cell growth signaling systems, SRC-1/SRC-3 small molecule inhibitors comprise a new class of drugs that have potential as novel chemotherapeutics able to defeat aspects of acquired
cancer cell resistance mechanisms.