Expression of heat shock proteins in periapical granulomas

Cells from virtually all organisms respond to a variety of stresses by the rapid synthesis of a highly conserved set of polypeptides termed heat shock proteins (HSPs). HSPs protect cells under adverse conditions such as infection, inflammation, and disease. We hypothesize that endodontic infection might result in an imbalance in the expression of heat shock genes, accounting for different clinical outcomes in periapical lesions.

We analyzed the expression of 44 HSPs genes using a pathway-specific real-time polymerase chain reaction array in 93 human periapical granulomas and 24 healthy periodontal ligament tissues collected postoperatively. Observed variations in the expression of HSP genes were also analyzed based on the classification of periapical granulomas as active or inactive. In addition, U937 cells were differentiated into macrophages, infected with different concentrations of purified Escherichia coli lipopolysaccharide (LPS), and used as templates for the HSP gene array. Protein expression was assessed by immunohistochemistry.

The expression of HSP genes was significantly increased in granulomas compared with healthy periodontal ligament (P < .00001). Among the 44 HSP genes, DNAJC3, HSPA4, HSPA6, and HSPB1 showed the highest expression levels in both granulomas and LPS-treated macrophages. DNAJC3, HSPA6, and HSPB1 were highly expressed in active lesions, whereas HSPA4 expression was higher in inactive lesions (P < .005). Higher concentrations of LPS led to increased HSP expression in macrophages (P < .0001). Immunocytochemistry confirmed the expression and colocalization of HSPB1 and HSPA6 proteins in the cytoplasm of LPS-infected macrophages.

The observed differential expression patterns of HSPs in periapical granulomas and LPS-infected macrophages suggest that HSP genes and proteins are involved in periapical lesion development and may account for different clinical outcomes. Understanding the role of the heat shock response might provide additional insights into the process of periapical lesion development.