Photodynamic therapy (
PDT) is not always effective as an anticancer treatment, therefore,
PDT is combined with other
anticancer agents for improved efficacy. The combination of
dasatinib and
PDT with the
silicone phthalocyanine photosensitizer Pc 4 was assessed for increased killing of SCCVII mouse
squamous cell carcinoma cells, a preclinical model of
head and neck squamous cell carcinoma, using apoptotic markers and colony formation as experimental end-points. Because each of these treatments regulates the metabolism of the
sphingolipid ceramide, their effects on
mRNA levels of
ceramide synthase, a
ceramide-producing
enzyme, and the
sphingolipid profile were determined.
PDT +
dasatinib induced an additive loss of clonogenicity. Unlike
PDT alone or
PDT +
dasatinib,
dasatinib induced
zVAD-fmk-dependent cell killing.
PDT or
dasatinib-induced
caspase-3 activation was potentiated after the combination.
PDT alone induced mitochondrial depolarization, and the effect was inhibited after the combination.
Annexin V+ and
propidium iodide+ cells remained at control levels
after treatments. In contrast to
PDT alone,
dasatinib induced upregulation of
ceramide synthase 1
mRNA, and the effect was enhanced after the combination.
Dasatinib induced a modest increase in C20:1- and
C22-ceramide but had no effect on total
ceramide levels.
PDT increased the levels of 12 individual
ceramides and total
ceramides, and the addition of
dasatinib did not affect these increases.
PDT alone decreased substantially
sphingosine levels and inhibited the activity of
acid ceramidase, an
enzyme that converts
ceramide to
sphingosine. The data suggest that
PDT-induced increases in
ceramide levels do not correlate with
ceramide synthase mRNA levels but rather with inhibition of
ceramidase. Cell killing was
zVAD-fmk-sensitive after
dasatinib but not after either
PDT or the combination and enhanced cell killing after the combination correlated with potentiated
caspase-3 activation and upregulation of
ceramide synthase 1
mRNA but not the production of
ceramide. The data imply potential significance of the combination for
cancer treatment.