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Tyrosine phosphorylation is an early step in lipopolysaccharide (LPS) stimulated monocytes and macrophages that appears to play a key role in signal transduction. We have demonstrated that LPS purified from Actinobacillus actinomycetemcomitans also increases protein tyrosine phosphorylation in human gingival fibroblasts (HGF). This effect was elicited rapidly after LPS stimulation at concentrations that stimulate anti-bacterial responses in human gingival fibroblasts. Two main proteins, with an apparent molecular weight of 44 and 42 kDa, were phosphorylated after LPS stimulation of the human gingival fibroblasts. The phosphorylation was detected after 5 to 15 min and reached the maximum at 30 min of treatment. The increase in tyrosine phosphorylation was apparent following stimulation with LPS at 10 ng/ml and the response was dose dependent up to 10 microg/ml. Pretreatment with the tyrosine kinase inhibitors, herbimycin A and genistein inhibited the LPS-stimulated phosphorylation of p44 and p42 MAP kinases in a dose dependent manner. Pretreatment of human gingival fibroblasts with antibodies anti-CD14 or anti-TLR-4 but not anti-TLR-2 inhibited the LPS-induced tyrosine phosphorylation of p44 and p42. Additionally, LPS-induced p44 and p42 phosphorylation was inhibited by polymyxin treatment. These findings demonstrate that LPS from A. actinomycetemcomintans increases rapidly p44 and p42 phosphorylation (ERK 1 and ERK 2, respectively) in human gingival fibroblasts. Our data also suggest that CD14 and TLR-4 receptors are involved in the LPS effects in human gingival fibroblasts.