Mild Stretch-Induced Injury Increases Susceptibility to Interleukin-1β-Induced Release of Matrix Metalloproteinase-9 from Astrocytes

Traumatic brain injury (TBI) results in the activation of glia and the release of proinflammatory cytokines, including interleukin (IL)-1 beta. The response of astrocytes to mild TBI has not been well studied. We used an in vitro model of cell stretch to investigate the effects of mild mechanical insult on astrocyte injury (lactate dehydrogenase and propidium iodide), and on mediators of inflammation including IL-1 beta, the chemokine CX3CL1, and nitrite. Here, we tested the hypothesis that a mild mechanical insult would increase susceptibility of astrocytes to delayed exposure to IL-1 beta, including enhanced release of the matrix metalloproteinease-9 (MMP-9). We investigated the role of the mitogen protein-activated kinase (MAPK) pathway in these responses. Cells subjected to a mild stretch show an increase in activation of the ERK1/2 and JNK pathways, and an increase in lactate dehydrogenase (LDH), but no change in the levels of inflammatory mediators. An early increase in LDH was dependent on ERK activation. Exposure to IL-1 beta, or to stretch alone, did not increase MMP-9. In contrast, the combination of mild stretch followed by IL-1 beta resulted in greater activation of the ERK pathway compared to either stimulus alone, and also resulted in an increase in the production of MMP-9 by astrocytes. Inhibition of the ERK pathway suppressed the increase in MMP-9 induced by the combination of stretch and IL-1 beta treatment. These results suggest that a primary mild mechanical injury renders astrocytes more susceptible to a secondary exposure to a proinflammatory cytokine such as IL-1 beta via the activation of the ERK pathway, and suggest a mechanism by which a mild head injury may confer increased susceptibility to neurologic injury caused by a subsequent insult.