Umbilical cord blood-derived stromal cells regulate megakaryocytic proliferation and migration through SDF-1/PECAM-1 pathway

Megakaryocytes participate in the occurrence of bleomycin-induced pulmonary fibrosis

Pulmonary fibrosis is characterized by the remodeling of fibrotic tissue and collagen deposition, which mainly results from aberrant fibroblasts proliferation and trans-differentiation to myofibroblasts. Patients with chronic myelogenous leukemia, myeloproliferative disorder, and scleroderma with pulmonary fibrosis complications show megakaryocyte infiltration in the lung. In this study, we demonstrated that the number of CD41⁺ megakaryocytes increased in bleomycin (BLM)-induced lung fibrosis tissues through the Chemokine (CXCmotif) ligand 12/Chemokine receptor 4 (CXCL12/CXCR4) axis. Pharmacological inhibition of the CXCL12/CXCR4 axis with WZ811 prevented migration of CD41⁺ megakaryocytes induced by BLM-injured lung tissue ex vivo and in vivo. In addition, WZ811 significantly attenuated lung fibrosis after BLM challenge. Moreover, megakaryocytes directly promoted fibroblast proliferation and trans-differentiation to myofibroblasts. We conclude that thrombopoietin (TPO) activated megakaryocytes through transforming growth factor β (TGF-β) pathway to promote fibroblast proliferation and trans-differentiation to myofibroblasts, which is abolished by treatment with selective TGF-βR-1/ALK5 inhibitors. Therefore, CD41⁺ megakaryocytes migrate to injured lung tissue partially through the CXCL12/CXCR4 axis to promote the proliferation and trans-differentiation of fibroblasts through direct contact and the TGF-β1 pathway.

CD44 regulation of endothelial cell proliferation and apoptosis via modulation of CD31 and VE-cadherin expression

CD44 has been implicated in a diverse array of cell behaviors and in a diverse range of signaling pathway activations under physiological and pathophysiological conditions. We have documented a role for CD44 in mediating vascular barrier integrity via regulation of PECAM-1 (CD31) expression. We now report our findings on the roles of CD44 in modulating proliferation and apoptosis of microvascular endothelial cells via its modulation of CD31 and VE-cadherin expression and the Hippo pathway. In this report, we demonstrate persistent increased proliferation and reduced activations of both effector and initiator caspases in high cell density, postconfluent CD44 knock-out (CD44KO), and CD31KO cultures. We found that reconstitution with murine CD44 or CD31 restored the proliferative and caspase activation rates to WT levels. Moreover, we have confirmed that the CD31 ecto-domain plays a key role in specific caspase cascades as well as cell adhesion-mediated cell growth and found that CD31 deficiency results in a reduction in VE-cadherin expression. Last, we have shown that both CD44KO and CD31KO endothelial cells exhibit a reduced VE-cadherin expression correlating with increased survivin expression and YAP nuclear localization, consistent with inactivation of the Hippo pathway, resulting in increased proliferation and decreased apoptosis. These findings support the concept that CD44 mediates several of its effects on endothelia through modulation of adhesion protein expression, which, in addition to its known modulation of junctional integrity, matrix metalloproteinase levels and activation, interactions with cortical membrane proteins, and selected signaling pathways, plays a key role as a critical regulator of vascular function.