Cholera toxin treatment of the human
T cell lymphoma Jurkat resulted in inhibition of signalling via the
T cell antigen receptor complex (
TcR/CD3-complex).
Cholera toxin specifically
ADP-ribosylated the alpha-subunit of the stimulatory
G-protein of the
adenylate cyclase (Gs alpha), no other
proteins were modified in the intact cells. ADP-ribosylation of Gs alpha and its subsequent activation led to an increase of the
cyclic AMP level and in addition, to a drastic reduction of the cell-surface density of the
TcR/CD3-complex. Recently, we demonstrated that the effect of
cholera toxin at the receptor level is not due to an increased cAMP level (4). As inhibition of signalling is also not cAMP-mediated (8), we examined whether the modulation of the
TcR/CD3-complex could be the reason for the interruption of the signalling cascade. Analyzing the time courses of the multiple
cholera toxin effects in Jurkat cells at 37 degrees C, the following sequence was found: ADP-ribosylation of Gs alpha--increase of
cyclic AMP level--inhibition of signalling via the
TcR/CD3-complex--decrease of cell-surface density of the
TcR/CD3-complex. Treatment of Jurkat cells at 20 degrees C with
cholera toxin resulted in an increase of
cyclic AMP and inhibition of signal transduction, while no decrease of
TcR/CD3-complex density could be observed. These data imply that receptor loss from the cell-surface is not causative for the inhibition of signalling. More likely, activation of Gs uncouples signal transduction in Jurkat cells via the TcR, which by a so far unknown mechanism is followed by a loss of the receptor from the cell surface.