Prostaglandin E2 Is Rapidly Produced In Response to Bone Marrow Injury and Improves Survival of Primitive Hematopoietic Cells

Abstract 407

Since the hematopoietic system is exquisitely sensitive to environmental and iatrogenic injury, the bone marrow microenvironment likely provides protective mechanisms during times of injury or stress. We have previously demonstrated that prostaglandin E2 (PGE2), which can be produced by many cell types in the bone marrow, targets both the bone marrow microarchitecture and primitive hematopoietic cells when administered systemically to mice (Porter, Frisch et. al., Blood, 2009). Since PGE2 is a local mediator of injury and is known to play a protective role in other cell types, we hypothesized that it could be an important microenvironmental regulator of HSPCs during times of injury. To test this hypothesis, we injured mice with a sub-lethal dose of gamma radiation, 6.5 Gy TBI, and sacrificed mice at varying time points from 1 hour to 6 days post-radiation. Bone marrow supernatant was collected and used for quantification of local PGE2 levels by ELISA. We found that, compared to non-irradiated mice, the PGE2 levels were increased greater than two-fold by 4 hours after irradiation (p=0.0030; n=3–6 mice/group), and these levels remain elevated until at least 6 days after injury (p<0.0001 by ANOVA). These data clearly demonstrate that PGE2 production is rapidly upregulated following bone marrow injury. To determine if HSPCs could be responding to this increase in local PGE2, we sorted Lin⁻ c-Kit⁺ Sca1⁺ (LSK) cells from murine bone marrow and assayed the expression of the four PGE2 receptors, EP1-EP4. RT-PCR analysis demonstrated that all four receptors are expressed on LSK cells, suggesting that PGE2 could be acting on these primitive hematopoietic cells during times of injury. We next tested whether supplying additional PGE2 to mice could protect hematopoietic cells after injury. Mice were subjected to 6.5 Gy TBI and were treated with 0.5 mg/kg 16,16-dimethyl-PGE2 (dmPGE2) immediately after radiation and once daily thereafter until time of sacrifice. At 24 hours after radiation injury, mice that were treated with dmPGE2 had greater than 8-fold more surviving LSK cells, a population which still retains HSC repopulating activity in competitive transplantation studies, in their bone marrow compared with vehicle treated mice (n=4/group, p=0.046). Similarly, at 72 hr post-radiation, the dmPGE2 treated mice continued to have almost 2-fold greater numbers of LSK cells remaining viable in their bone marrow compared with vehicle treated mice (n=2–3/group). These data suggest that dmPGE2 treatment after bone marrow injury may provide protection, at least in the days immediately following injury, to primitive hematopoietic cells that remain capable of regenerating the hematopoietic system. To further support this idea, we also pretreated uninjured bone marrow cells in vitro with PGE2 (1 μ M) for 90 minutes and then exposed them to the chemotherapeutic agent cytarabine (Ara-C, 10 μ M for 4 hours). Pretreatment with PGE2 results in lower levels of apoptotic LSK cells compared with vehicle pre-treated LSK cells (30.26% vs. 39.02%; n=9/group; 3 independent experiments; p=0.0012). This result correlates with our in vivo radiation injury data and suggests that PGE2 may target primitive hematopoietic cells and render them more resistant to cell death from injury.

Taken together, these results suggest that PGE2, which is released in the bone marrow after radiation exposure, may be an important microenvironmental regulator of HSPC response to injury, by preventing cell death, and/or increasing their recovery. Amplification of this physiological signal by treatment with exogenous PGE2 could provide a beneficial means of protecting hematopoietic cells in clinical situations of hematopoietic system injury and bone marrow transplantation, allowing patients to tolerate bone marrow suppressive treatments or to recover more easily. Further, these results also bring forth a potential concern about the safety of blocking prostaglandin synthesis by using anti-inflammatory medications during times of bone marrow injury.