Members of the
steroid receptor coactivator (SRC) family are overexpressed in numerous types of
cancers. In particular,
steroid receptor coactivator 3 (SRC-3) has been recognized as a critical coactivator associated with
tumor initiation, progression, recurrence,
metastasis, and chemoresistance where it interacts with multiple
nuclear receptors and other
transcription factors to enhance their transcriptional activities and facilitate cross-talk between pathways that stimulate
cancer progression. Because of its central role as an integrator of growth signaling pathways, development of small molecule inhibitors (SMIs) against SRCs have the potential to simultaneously disrupt multiple signal transduction networks and
transcription factors involved in
tumor progression. Here, high-throughput screening was performed to identify compounds able to inhibit the intrinsic transcriptional activities of the three members of the SRC family.
Verrucarin A was identified as a SMI that can selectively promote the degradation of the SRC-3
protein, while affecting SRC-1 and SRC-2 to a lesser extent and having no impact on CARM-1 and p300
protein levels.
Verrucarin A was cytotoxic toward multiple types of
cancer cells at low nanomolar concentrations, but not toward normal liver cells. Moreover,
verrucarin A was able to inhibit expression of the SRC-3 target genes MMP2 and MMP13 and attenuated
cancer cell migration. We found that
verrucarin A effectively sensitized
cancer cells to treatment with other anti-
cancer drugs. Binding studies revealed that
verrucarin A does not bind directly to SRC-3, suggesting that it inhibits SRC-3 through its interaction with an upstream effector. In conclusion, unlike other SRC SMIs characterized by our laboratory that directly bind to SRCs,
verrucarin A is a potent and selective SMI that blocks SRC-3 function through an indirect mechanism.