Bone Marrow Adipocyte-Derived PAI-1 As a Negative Regulator of Hematopoietic Regeneration

Introduction: Bone marrow adipocytes (BMAs) reportedly act as negative regulators of hematopoietic regeneration. This can be observed in obese or elderly patients, both of whom exhibit BMA-rich microenvironment with poor hematopoietic regeneration. Though this relationship is known, the mechanism(s) underlying the suppressive effects of BMAs on hematopoietic regulation are yet to be elucidated. Recently, in our previous study, we demonstrated that blocking plasminogen activator inhibitor type-1 (PAI-1) enhances hematopoietic regeneration through the activation of the fibrinolytic pathways in the bone marrow. As PAI-1 is known as one of the major adipokines, we reasoned that PAI-1 produced from BMAs could have a causative role in the impairment of hematopoietic regeneration in the BMA-rich microenvironment.

Methods: C57B6/J and PAI-1-deficient mice with a C57B6/J background were used. Diet-induced obesity (DIO) C57B6/J mice were fed a 60 kcal% fat diet for 12 weeks prior to performing experiments. After syngeneic hematopoietic stem cell transplantation (HSCT), either TM5614 (a specific inhibitor of PAI-1 molecules) or vehicle (distilled water) were administered daily via oral gavage using a feeding needle for 14 consecutive days.

Results: We compared the hematopoietic regeneration after syngeneic HSCT between femurs (microenvironment with a low BMA content) and tail vertebrae (which consist of abundant BMAs). As expected, the microenvironment in tail vertebrae was significantly associated with higher PAI-1 expression in comparison with that in femurs. Hematopoietic regeneration was less efficient in the tail vertebrae, with a reduced number of total donor hematopoietic and CD34^-Lin^-Sca1⁺c-kit⁺ cells observed than in femurs at 1 and 4 weeks following syngeneic HSCT. This impaired regeneration was significantly alleviated in PAI-1-knockout (Figure a) and PAI-1 inhibitor-treated mice (Figure b). Following this, we examined the effects of PAI-1 inhibitor on hematopoietic regeneration after syngeneic HSCT in DIO mice, which had BMA-rich bone marrow microenvironment associated with higher PAI-1 expression compared with that in normal-weight mice. The comparable homing activity of donor cell was observed in normal-weight and DIO mice by using PKH26 staining. However, DIO mice, in comparison to normal-weight mice, exhibited impaired hematopoietic regeneration, with a reduced number of total donor hematopoietic, CD34^-Lin^-Sca1⁺c-kit⁺, and CD48^-CD150⁺Lin^-Sca1⁺c-kit⁺ SLAM cells at 1 week following HSCT. The impaired hematopoietic regeneration was completely restored by PAI-1 inhibitor administration in these mice (Figure c and d). Furthermore, the number of total donor hematopoietic and CD34^-Lin^-Sca1⁺c-kit⁺ cells at 3 weeks following HSCT was significantly higher in PAI-1 inhibitor treated DIO mice than in normal-weight mice.

Conclusion: Our results indicated that PAI-1 hampers hematopoietic regeneration in the BMA-rich microenvironment and the blockade of PAI-1 activity could be a novel therapeutic approach to facilitate hematopoietic regeneration after HSCT in BMA-rich patients.

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