Mutation in the BRAF gene (BRAFV600E) exists in nearly 70% of human
melanomas. Targeted
therapy against BRAFV600E
kinase using a recently identified RAF-selective inhibitor,
PLX4032, has been successful in early clinical trials. However, in patients with the normal BRAF allele (wild-type),
PLX4032 is protumorigenic. This conundrum identifies the unmet need for novel therapeutic agents to target BRAFV600E
kinase that are not counterproductive. We have identified
gossypin, a pentahydroxy
flavone, as a potent antimelanoma agent.
Gossypin inhibited human
melanoma cell proliferation, in vitro, in
melanoma cell lines that harbor both BRAFV600E
kinase and
cyclin-dependent kinase 4 (CDK4) as well as in cells with BRAF wild-type allele.
Gossypin inhibited
kinase activities of BRAFV600E and CDK4, in vitro, possibly through direct binding of
gossypin with these
kinases, as confirmed by molecular docking studies. For cells harboring the BRAFV600E,
gossypin inhibited cell proliferation through abrogation of the
MEK-ERK-
cyclin D1 pathway and in cells with BRAF wild-type allele, through attenuation of the
retinoblastoma-
cyclin D1 pathway. Furthermore,
gossypin significantly inhibited
melanoma growth in an organotypic three-dimensional skin culture mimicking human skin.
Gossypin (10 and 100 mg/kg) treatment for 10 days in human
melanoma (A375) cell xenograft
tumors harboring BRAFV600E significantly reduced
tumor volume through induction of apoptosis and increased survival rate in mice, and the effect was significantly superior to that of
PLX4032 (10 mg/kg) or
roscovitine 10 mg/kg. In summary, this study identified
gossypin as a novel agent with dual inhibitory effects for BRAFV600E
kinase and CDK4 for treatment of
melanoma.