Effects of mitogen-activated protein kinases on nuclear protein import.

ERK-2 MAP kinase activation induces inhibitory effects on nuclear protein import in vascular smooth muscle cells. The mechanism and characteristics of this effect of ERK-2 were investigated. An unusual dose-dependent effect of ERK-2 on nuclear protein import was identified. At higher concentrations (1 microg/mL) of ERK-2, nuclear protein import was stimulated, whereas lower concentrations (0.04 microg/mL) inhibited import. Intermediate concentrations exerted intermediate effects. The stimulatory and inhibitory effects at the 2 different ERK-2 concentrations were observed in both conventional, permeabilized cell assays of nuclear protein import and with in situ microinjection of smooth muscle cells. The biphasic effects of ERK-2 on import were also found for the other 2 members of the MAPK family, p38 and JNK. RanGAP was identified by structural analysis as a candidate target protein responsible for mediating the effects of ERK-2. After pretreatment with high concentrations of ERK-2, RanGAP activity was significantly increased by approximately 50%. In contrast, low concentrations of ERK-2 significantly attenuated RanGAP activity. These results demonstrate that all 3 members of the MAPK family can alter nuclear protein import in opposite directions depending upon the concentration of ERK-2 used. RanGAP represents the MAP kinase target whereby nuclear transport can be stimulated or inhibited.

Chlamydia pneumoniae stimulates proliferation of vascular smooth muscle cells through induction of endogenous heat shock protein 60.

Chlamydia pneumoniae infection has been linked with atherosclerosis. However, the mechanism responsible for the atherogenic effects of C pneumoniae remains unclear. Heat shock proteins (HSPs) have been found in atherosclerotic lesions. HSPs of HSP70 and HSP90 families are involved in the regulation of cell cycle progression and cell proliferation. We assessed the hypothesis that HSP60 is induced in vascular cells infected with C pneumoniae and stimulates cell proliferation. Rabbit vascular smooth muscle cells (VSMCs) and human umbilical vein endothelial cells (HUVECs) were infected with C pneumoniae. Western blot analysis demonstrated the induction of endogenous HSP60 expression in C pneumoniae-infected VSMCs. C pneumoniae infection significantly increased the number of VSMCs, and the mitogenic effect correlated with the expression level of endogenous HSP60. In contrast to VSMCs, C pneumoniae infection had no effect on the expression level of HSP60 and did not stimulate cell proliferation in HUVECs. Exogenous addition of recombinant chlamydial HSP60 had no mitogenic effect on VSMCs and HUVECs. However, overexpression of HSP60 within VSMCs by infection with adenovirus encoding human HSP60 resulted in a significant increase in cell numbers compared with uninfected VSMCs. These results suggest that overexpression of endogenous HSP60 may be a central intracellular event responsible for the mitogenic effects induced by C pneumoniae infection. In addition to C pneumoniae, other infectious agents and atherogenic risk factors may also stimulate VSMC proliferation and contribute to the lesion formation through the induction of HSP60.