Studying the negative regulatory factors of the Propionibacterium acnes-induced signaling pathways in in vitro cultured immortalized keratinocytes

Propionibacterium acnes (P. acnes) is a commensal bacterium, but it can also activate different pathogen recognition receptors (e.g Toll-like receptors; TLRs), and thus induce innate immune and inflammatory events in human epidermal keratinocytes. These molecular pathways are well characterized, but little is known about the endogen negative regulatory mechanism that may control these events, and counteracts the TLR-induced signal transduction pathways. In order to identify and analyze endogen factors playing a key role in the attenuation of the P. acnes–induced TLR activation, we studied the mRNA and protein expression of selected negative regulators of these signaling events (SIGIRR, TOLLIP, TNFAIP3, TNIP1) in a human immortalized keratinocyte cell line (HPV-KER) in response to the bacterial treatment by real time RT-PCR and western blotting. Our results show that all the investigated negative regulators were expressed in HPV-KER cells. Moreover, the TNFAIP3 and TNIP1 mRNA expressions significantly, and dose dependently increased in response to the bacterium. Next, we studied the effect of various P. acnes strains (889, 6609) belonging to different phylogenetic groups within the species, but no major differences have been observed in the induced expression changes. By monitoring these factors at the protein level by western blotting we found increased TNFAIP3 and decreased SIGIRR expressions following the bacterial treatment, and these events also appeared to be dose dependent. Our study suggests that in our in vitro model system P. acnes causes the dose-dependent activation of downstream TLR signaling processes. Specialized, endogen negative regulators do exist in these cells which may control the bacterial-induced molecular events, and thus can be important for the maintenance of epidermal homeostasis