Caffeine and Apoptosis - Caffeine: Chemistry, Analysis, Function and Effects - page 386

It appears that the ATR-Chk1 pathway may represent the main biologically
relevant target of caffeine in normal keratinocytes (Heffernan et al 2009).
siRNA of ATR, but not that of the related kinase ATM, augmented UVB-
induced apoptosis, mimicking the effect of caffeine in these cells. Importantly,
caffeine did not further enhance UVB-induced apoptosis following siRNA of
ATR in normal keratinocytes, indicating that caffeine's effects are epistatic to
those of ATR-pathway inhibition. Furthermore, treatment of normal
keratinocytes with PF610666, a novel Chk1 kinase inhibitor that appears to
be more selective than UCN-01, the archetypal Chk1 inhibitor, similarly
potentiated UVB-induced apoptosis (Blasina et al 2008).
Caffeine also increases apoptosis in tumors long after all UVB treatment has
ceased (Lu et al 2011). In the absence of caffeine, UVB-induced tumors often
had islands of phospho-Chk1 (Ser317)-staining cells that were not present in
non-tumor areas of the epidermis. Treatment of mice with topical caffeine
significantly diminished phospho-Chk1 (Ser317) staining and increased the
number of mitotic cells that expressed cyclin B1 and caspase 3 in tumors,
consistent with caffeine-induced lethal mitosis selectively in tumors. It was
proposed that as compared with adjacent uninvolved skin, UVB-induced skin
tumors have elevated activation of and dependence on the ATR-Chk1 pathway
long after UVB exposure has ceased, and that caffeine can induce apoptosis
selectively in tumors by inhibiting this pathway and promoting lethal mitosis.

d n 0 t 2 n g | 2

Figure 21.2

p53-independent apoptosis induced by caffeine: ATR mechanism. UVB
causes DNA damage, resulting in activation of ataxia-telangiectasia-
mutated-Rad3 related (ATR) kinase. ATR-dependent activation leads
to Chk1 phosphorylation and to an inactivation of Cdc25c phosphatase
by increased phosphorylation of Cdc25C. Cdc25C phosphorylation has
been shown to increase both its turnover and cytoplasmic retention.
Consequently, activation of the Cdc2/cyclin B1 complex is prevented
and mitosis entry is inhibited. Caffeine may abrogate the G 2 checkpoint
and enhance apoptosis inhibiting ATR-mediated phosphorylation of
Chk1.