Endotoxin activation of mitogen-activated protein kinase in THP-1 cells; diminished activation following endotoxin desensitization.

The signal transduction events occurring in monocytes in response to endotoxin (LPS) stimulation are incompletely delineated, although pertussis toxin (PT)-sensitive G proteins and the mitogen-activated protein kinase (MAPK) cascade have been implicated. Cellular desensitization in response to 18-h pre-exposure to 1 microgram/mL LPS alters signal transduction pathways of cellular activation and decreases production of certain inflammatory mediators such as thromboxane (Tx)B2, the stable metabolite of TxA2. We hypothesized that LPS stimulation of the human monocyte cell line THP-1 occurs via MAPK activation, and that LPS desensitization, induced by pre-exposure to LPS, is associated with altered signaling through the MAPK cascade. Involvement of a specific MAPK, ERK, in LPS-stimulated TxB2 production was further tested using a specific MAPK cascade inhibitor, PD98059 (PD). PD inhibited LPS and phorbol myristate acetate (PMA)-stimulated ERK activation as demonstrated by immunoblots using anti-activated ERK antibodies. PD significantly inhibited LPS and PMA-stimulated TxB2 synthesis to non-detectable levels, suggesting an involvement of MAPK in LPS-stimulated activation. Because PT-sensitive G proteins mediate LPS-stimulated signal transduction, their role in MAPK activation was tested. Pretreatment with PT inhibited basal and LPS-stimulated, but not PMA-stimulated ERK activation. Activation of ERK after LPS desensitization was also assessed. LPS pre-exposure resulted in a profound decrease in LPS-stimulated activation of ERK, but did not affect PMA activation of ERK. These data implicate the involvement of the MAPK cascade in LPS-stimulated activation of THP-1 cells and suggest coupling of Gi proteins and MAPKs in LPS-stimulated events. LPS desensitization is associated with decreased MAPK activation, but does not impair MAPK activation by PMA. Thus, LPS desensitization appears to selectively alter signal transduction upstream of ERK.