A traditional Chinese medicine (TCM) formula (SL) comprising Sophorae Flos and Lonicerae
Japonicae Flos was used for treating melanoma in ancient China. We have previously
shown that an ethanolic extract of SL (SLE) possesses anti-melanoma effects and suppresses
STAT3 signaling in vitro and in vivo. STAT3 has been linked to the development of
melanoma immunosuppressive microenvironment. In this work, we investigated whether
SLE inhibits melanoma growth by reprogramming the tumor microenvironment in mouse
and co-culture cell models. In B16F10 melanoma-bearing mice, we found that intragastric
administration of SLE (1.2 g/kg) dramatically inhibited tumor growth. This observation
was associated with the downregulation of protein levels of phospho-STAT3 (Tyr 705)
and STAT3-regulated immunosuppressive cytokines, and mRNA levels of STAT3-targeted
genes involved in tumor growth and immune evasion. We also observed increased Th,
Tc and dendritic cells in the melanomas and spleens in SLE-treated mice compared to
that in control mice. In a co-culture system composed of B16F10 cells and mouse primary
splenic lymphocytes, it was found that SLE not only inhibited STAT3 activation in
B16F10 cells, but also downregulated mRNA levels of STAT3-targeted genes in the splenic
lymphocytes. In this co-culture setting, SLE decreased the levels of STAT3-regulated
immunosuppressive cytokines, increased the percentages of Th, Tc and dendritic cells
as well. Furthermore, effects of SLE on STAT3 phosphorylation, cytokine levels and
immune cell subtype percentages were significantly weaker in the B16STAT3C cells (stable
cells harboring a constitutively active STAT3 variant STAT3C)/splenic lymphocytes
co-culture system than in the B16V cells (cells stably transfected with the empty
vector)/splenic lymphocytes co-culture system, indicating that STAT3 over-activation
diminishes SLE's effects. In summary, our findings indicate that reprograming the
immune microenvironment, partially mediated by inhibiting STAT3 signaling, contributes
to the anti-melanoma mechanisms of SLE. This study provides further pharmacological
groundwork for developing SLE as a modern agent for melanoma prevention/treatment,
and supports the notion that reprograming immunosuppressive microenvironment is a
viable anti-melanoma strategy.