Bacterial heat shock protein-60 increases epithelial cell proliferation through the ERK1/2 MAP kinases

Heat shock proteins (hsp) have important roles in the regulation and protection of both prokaryotic and eukaryotic cells, especially during environmental stress. Hsps are also important bacterial virulence factors. We investigated whether bacterial hsp60 can alter epithelial cell mitogen-activated protein kinase (MAPK) signaling and cell proliferation. Human skin keratinocytes (HaCaT cell line) were cultured in the presence of hsp60 purified from Actinobacillus actinomycetemcomitans, an important oral pathogen. Protein kinases in the ERK1/2 and p38 MAPK signaling pathways were probed with kinase-specific and phosphorylation-site-specific antibodies on Western blots. In quiescent cultures, hsp60 increased ERK1/2 phosphorylation in a sustained manner and p38 phosphorylation transiently. Hsp60 also increased epithelial cell proliferation by about 30%. Inhibition of the ERK1/2 pathway by PD 98059 (a MEK1 inhibitor) reversed partially ERK1/2 phosphorylation and totally cell proliferation indicating that the ERK1/2 MAPK pathway is involved in the hsp60-induced cell growth. This was supported by findings that hsp60 stimulated phosphorylation of RSK1/2 and cyclic AMP response element-binding protein and increased expression of transcription factors c-Jun and c-Fos. Recombinant human hsp60 did not alter ERK1/2 or p38 phosphorylation and had no effect on epithelial cell proliferation. Inhibition of p38 MAPK pathway by SB 203580 increased both ERK1/2 phosphorylation and cell proliferation demonstrating that the inhibitor can either directly or indirectly activate the ERK1/2 MAPK pathway. The results show that exogenous bacterial hsp60 is able to activate ERK1/2 phosphorylation and thereby cause increased epithelial proliferation. In case of mucosal infection this effect may either lead to increased wound repair or participate in the pathological mechanism of some bacterial diseases that involve increased epithelial proliferation.