Abstract
BackgroundFibrosing disorders of the lung, such as idiopathic pulmonary fibrosis, are characterized by progressive extracellular matrix accumulation that is driven by myofibroblasts. The transcription factor megakaryoblastic leukemia-1 (MKL1) mediates myofibroblast differentiation in response to several profibrotic stimuli, but the role it plays in mediating pulmonary fibrosis has not been fully elucidated. In this study, we utilized mice that had a germline deletion of MKL1 (MKL1 (−,-)) to determine the role that MKL1 plays in the development of bleomycin-induced pulmonary fibrosis.MethodsBleomycin or normal saline were intratracheally delivered to 9 to 12 week old female MKL1 (+,+) and MKL1 (−,-) mice. Mice were assessed for weight loss and survival to 28 days. Inflammatory responses were assessed through bronchoalveolar lavage at days 3 and 7 post-treatment. The development of pulmonary fibrosis was characterized using hydroxyproline assay and histological staining. MKL1 (+,+) and MKL1 (−,-) mouse lung fibroblasts were isolated to compare morphologic, gene expression and functional differences.ResultsMKL1 (−,-) mice demonstrated increased survival, attenuated weight loss, and decreased collagen accumulation compared to wild-type animals 28-days after intratracheal instillation of bleomycin. Histological analysis demonstrated decreased trichrome, smooth muscle α-actin, and fibronectin staining in MKL1(−,-) mice compared to MKL1 (+,+) controls. Differential cell counts from bronchoalveolar lavage demonstrated that there was attenuated neutrophilia 3 days after bleomycin administration, but no difference at day 7. Isolated mouse lung fibroblasts from MKL1 (−,-) mice had decreased contractility and deposited less fibronectin matrix compared to wild-type controls, suggesting a defect in key remodeling functions.ConclusionsAltogether, these data demonstrate that MKL1 plays a significant role in mediating the fibrotic response to bleomycin injury. Loss of MKL1 attenuated early neutrophil influx, as well as myofibroblast-mediated remodeling. Targeting MKL1 activity may therefore be a useful strategy in treating pulmonary fibrosis.
Highlights
Fibrosing disorders of the lung, such as idiopathic pulmonary fibrosis, are characterized by progressive extracellular matrix accumulation that is driven by myofibroblasts
megakaryoblastic leukemia-1 (MKL1) is essential for the development of bleomycin-induced pulmonary fibrosis We first explored the effects that MKL1 germline deletion has on the bleomycin-induced model of pulmonary fibrosis
We found that while the average MKL1 (+,+) animal weight decreased to approximately 80% of their initial weight, MKL1 (−,) mice had similar initial weight loss, but began to regain weight from the second week after bleomycin administration, correlating with the typical onset of the fibrotic response (Figure 1A)
Summary
Fibrosing disorders of the lung, such as idiopathic pulmonary fibrosis, are characterized by progressive extracellular matrix accumulation that is driven by myofibroblasts. The transcription factor megakaryoblastic leukemia-1 (MKL1) mediates myofibroblast differentiation in response to several profibrotic stimuli, but the role it plays in mediating pulmonary fibrosis has not been fully elucidated. We have previously reported that myofibroblast differentiation requires signaling via the transcription factors megakaryoblastic leukemia-1 (MKL1) and serum response factor (SRF) [5,6,7]. MKL1 has been observed to be required for myofibroblast differentiation in response to mechanical signals, matrix stiffness, and profibrotic G-protein coupled receptor agonists [11,12,13]. Our group has observed that signaling via protein kinase A (PKA) can protect against bleomycin-induced pulmonary fibrosis in mice, and that this protective effect is likely via inhibition of MKL1/SRF in myofibroblasts [14,15].
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