We previously demonstrated that stimulation of human T-lymphocytes with calcium 
ionophores induced the phosphorylation and enzymatic activation of ERK2. We now 
report on the mechanism by which calcium-ionophore-induced activation of ERK1 
and 2 occurs in these cells. The activation of ERK1 and 2 by increases in 
intracellular calcium was inhibited by calmidazolium suggesting the involvement 
of calmodulin in this response. To further elucidate the mechanism by which 
calcium-induced ERK activation occurs, we used the CaM-kinase inhibitor KN-93 
and an inactive analog of KN-93 (KN-92). KN-93, but not KN-92, blocked 
ionomycin-induced activation of ERK1 and 2 in human T lymphocytes. We previously 
demonstrated that stimulation of T lymphocytes with ionomycin or A23187 resulted 
in a CaM-kinase-dependent shift in the mobility of p56(Lck). To determine if 
p56(Lck) was involved in calcium-induced ERK activation, we stimulated the 
p56(Lck) negative Jurkat cell derivatives, J.CaM1.6 and J.CaM1/Rep3, with 
ionomycin. In these p56(Lck) negative cell lines, activation of ERK1 and 2 in 
response to ionomycin was only minimally detected. When J.CaM1 cells were 
reconstituted with p56(Lck), ionomycin induced ERK1 and 2 activation. Treatment 
of Jurkat cells with PP2, an inhibitor of p56(Lck), inhibited calcium-induced, 
but not PMA-induced, ERK1 and 2 activation. Treatment of Jurkat cells with the 
MEK inhibitor PD98059 blocked ionomycin-induced ERK activation, but not the 
shift in the mobility of p56(Lck). Our data suggests that increases in 
intracellular calcium induce the activation of ERK1 and 2 in human T lymphocytes 
via sequential activation of CaM-kinase and phosphorylation of p56(Lck).