Bartonella bacilliformis GroEL; Effect on Growth of Human Vascular Endothelial Cells in Infected Co-Cultures

Bartonella are the only bacteria known to induce angioproliferative lesions of the human vasculature and liver during infection. Previous work from our lab suggests that GroEL participates in the mitogenic response observed in HUVEC cultures supplemented with the soluble fraction of Bartonella bacilliformis . Work in this study shows that exposure to high concentrations of the fraction is actually cytotoxic for HUVECs. To analyze this phenomenon, live B. bacillformis - HUVEC co-cultures were employed to study the effect of excess bacterial GroEL on the host cell during active infection. Four B. bacilliformis strains were generated to produce varying levels of GroEL. HUVEC co-cultures with LSS100, a strain that synthesizes markedly greater quantities of GroEL relative to others, significantly accelerates apoptosis of the co-cultured HUVECs relative to other strains. Acceleration of apoptosis can be inhibited by Z-VAD-FMK, a pan-caspase inhibitor. Time course data show that at 18 h of infection, both LSS100 and control strains significantly inhibit spontaneous apoptosis of co-cultured HUVECs, as previously reported for other Bartonella species. However, by 48 h LSS100 significantly increases apoptosis of the host cell. We hypothesize that intracellular Bartonella GroEL functions as an HSP60 analog, a eukaryotic orthologue known to accelerate procaspase 3 activation by enhancing its vulnerability to upstream activator caspases. These data suggest another strategy whereby Bartonella may regulate host cell growth.