Tumor promotion is strongly associated with
inflammation and increased
polyamine levels. Our understanding of relevant mechanisms responsible for
arsenic-induced
cancer remains limited. Previous studies suggest that
arsenic targets and dysregulates stem cell populations that remain dormant in the skin until promoted to be recruited out of the bulge stem cell region, thus giving rise to skin
tumors. In this study, we explored a possible mechanism by which increased keratinocyte
polyamine biosynthesis promotes tumorsphere formation and invasiveness of
arsenic-transformed HaCaT keratinocytes (As-HaCaT). Unlike parental HaCaT cells, As-HaCaT cells were tumorigenic in athymic nude mice, and the CD45negative epithelial
tumor cells had enriched expression of
Toll-Like Receptor 4 (TLR4), CD34 and CXCR4 as did As-HaCaT tumorsphere cultures compared to As-HaCaT monolayer cultures.
Ornithine decarboxylase (ODC) overexpressing keratinocytes (Ker/ODC) release increased levels of the
alarmin high mobility group box 1 (
HMGB1). Ker/ODC
conditioned medium (CM) stimulated As-HaCaT but not parental HaCaT tumorsphere formation, and this was inhibited by
glycyrrhizin, an inhibitor of
HMGB1, and by
TAK242, an inhibitor of the
HMGB1 receptor TLR4. Compared to parental HaCaT cells, As-HaCaT cells demonstrated greater invasiveness across a
Matrigel-coated filter using either fibroblast CM or SDF-1α as
chemoattractants. Addition of Ker/ODC CM or
HMGB1 dramatically increased As-HaCaT invasiveness.
Glycyrrhizin and
TAK242 inhibited this Ker/ODC CM-stimulated invasion of As-HaCaT cells but not HaCaT cells. These results show that
polyamine-dependent release of
HMGB1 promotes the expansion of stem cell-like subpopulations in
arsenic-transformed keratinocytes while also increasing their invasiveness, suggesting that
polyamines may be a potential therapeutic target for the prevention and treatment of
arsenic-initiated
skin cancers.