Although
platelet-activating factor (PAF) is a well-known acute inflammatory mediator, little is known regarding the role of PAF in chronic
inflammation.
Phorbol esters are known to stimulate PAF production. Moreover, the ability of repeated applications of
phorbol esters to induce a sustained inflammatory response is crucial to their tumorigenic activity. We therefore examined whether PAF acts as a mediator of
phorbol ester-induced
inflammation and
tumorigenesis. While PAF receptor knockout mice (PAFR(-/-)) showed an expected but modest reduction in the acute inflammatory response to
phorbol 12-myristate 13-acetate (PMA), these mice exhibited a surprising increase in
inflammation following chronic PMA application. This increased
inflammation was documented by a number of findings that included: increased skin thickness, increased
myeloperoxidase activity and expression and increased expression of known inflammatory mediators. Interestingly, vehicle-treated PAFR(-/-) mice also exhibited modest increases in levels of inflammatory markers. This suggests that the
platelet activating factor receptor (PAFR) acts to suppress chronic
inflammation in response to other stimuli, such as barrier disruption. The idea that chronic PAFR activation is anti-inflammatory was documented by repetitive topical PAFR agonist administration that resulted in reduced
myeloperoxidase activity in skin. We next utilized a 7,12-dimethylbenz(a)
anthracene/PMA
carcinogenesis protocol to demonstrate that PAFR(-/-) mice exhibit significantly increased
tumor formation and malignant progression compared with wild-type control mice. These studies provide evidence for two important, unexpected and possibly interrelated pathological roles for the PAFR: first, the PAFR acts to suppress PMA-induced chronic
inflammation; secondly, the PAFR acts to suppress neoplastic development in response to chemical
carcinogens.