FLLL32 inhibits STAT3 phosphorylation and gene e pression in human melanoma cell lines FLLL32 inhibited STAT3 phosphorylation at Tyr705 but not at Tyr727 in multiple human melanoma cell lines after a 24 hour treatment. Prior studies indicated FLLL32 could inhibit Jak2 kinase activity in an in vitro cell free assay. However, we did not observe an appreciable alteration in Jak2 phosphorylation even at a concentration of 8 uM, suggesting that this compound likely acted directly against the STAT3 protein. Time course studies also revealed that fulminant cell death occurred after 24 hours of continuous culture, yet e posure to FLLL32 at 2 4 uM for only 4 hours was suf ficient to reduce pSTAT3 and induce cell death.

FLLL32 did not appear to inhibit the phosphorylation of other key signaling path ways that are constitutively active in malignant cells at doses capable of inhibiting STAT3 phospho rylation after 24 hours. Consistent with reciprocal activa tion of the p38 MAPK and STAT3 pathways, FLLL32 treatment led to increased levels of total p38 MAPK pro tein once pSTAT3 decreased. Importantly, FLLL32 was capable of reducing pSTAT3 levels, cyclin D1 e pression and inducing apoptosis in primary human melanoma cell cultures derived from recurrent cutaneous melanoma tumors. Finally, treatment of basal pSTAT3 positive human melanoma cell lines with FLLL32 for 24 hours led to reduced STAT3 DNA binding as determined by gel shift assays and e pression of the STAT3 regulated genes, cyclin D1 and survivin as mea sured by immunoblot.

FLLL32 induced cell death is caspase dependent The mechanism by which FLLL32 induces apoptosis was subsequently investigated in the A375 melanoma cell line. Immunoblot Anacetrapib analysis demonstrated a concentration dependent increase in the processing of both initiator and effector caspases following a 24 hour treatment with FLLL32. Treatment of with FLLL32 also resulted in a concen tration dependent loss of mitochondrial membrane potential as measured by flow cytometry. These data support the involvement of the mitochondrial amplification loop in promoting cell death in response to this treatment. Apoptosis was caspase dependent, as cul ture with a pan caspase inhibitor inhib ited melanoma cell death as compared to culture with the Z FA FMK control compound. These data were confirmed at the 48 hour time point by flow cytometry following anne in V PI staining, and by reduced PARP cleavage by immunob lot analysis. Interestingly, reduced levels of pSTAT3 and cyclin D1 occurred following treatment of A375 cells with FLLL32 in the presence of the pan cas pase inhibitor. These data are consistent with a mechanism that places reduced pSTAT3 and its cellular targets upstream of the caspase cascade and subsequent apoptosis.