Polycyclic aromatic hydrocarbons are ubiquitous
environmental pollutants formed during incomplete combustion of organic material. For example
benzo[a]pyrene (B[a]P) is a constituent and contaminant of cigarette
smoke,
automobile exhaust,
industrial waste and even food products. B[a]P is carcinogenic to rodents and humans. B[a]P induces its own metabolism, which generates different metabolites such as the highly reactive electrophilic
genotoxin and ultimal
carcinogen B[a]P-7,8-
dihydrodiol-9,10-
epoxide (
BPDE).
BPDE can bind to nucleophilic macromolecules such as
proteins and
DNA and causes mutations. Multiple defence mechanisms have evolved to protect the cell from DNA damage. Specific signalling pathways operate to detect and repair different kinds of lesions. In case, the damage is poorly removed expansion of damaged cells can be counteracted, e.g., by the inhibition of proliferation or triggering apoptosis. Examples of damage sensors and transducers are stress-activated
protein kinases (SAPKs) and the tumour suppressor
protein p53. Here, we studied the role of p53 and the
pro-apoptotic protein BAX in
BPDE-induced cell death by using wild-type- or knock-out-human colon
carcinoma cells. As reported previously, we could reconfirm a critical role of p53 in
BPDE-induced apoptosis. Furthermore, induced levels of total p53 and its transcriptional target p21 declined at higher
BPDE concentrations correlating with reduced rates of apoptosis. Interestingly, increased phosphorylation of p53 at
serine 15 remained elevated at higher
BPDE concentrations thus disconnecting p53 phosphorylation from downstream apoptosis. Hence, phosphorylation of p53 seems not only to be a more sensitive
biomarker of
BPDE exposure but might serve other functions unrelated to apoptosis. In addition, we identify BAX as a novel and essential factor to trigger the intrinsic pathway of apoptosis in response to
BPDE. Furthermore,
BPDE in parallel activates the SAPKs p38 and JNK, which are as well involved in apoptosis. Although several routes of mutual regulation of p53 and SAPK have been described, we present evidence that the SAPK pathway in response to genotoxic stress can unexpectedly operate independently of p53 and controls apoptosis by a novel mechanism possibly downstream of
caspases.