An in Vitro Study on the Role of Angiogenesis in Iron Deficiency Induced Reactive Thrombocytosis

Introduction: Iron deficiency (ID) is one of the recognized causes of reactive thrombocytosis in children. Factors that are commonly associated with megakaryopoiesis such as thrombopoietin (TPO), interleukin 6 (IL-6) and IL-11 are not altered in patients with iron deficiency and thrombocytosis suggesting the role of alternate mechanisms in controlling this process. We have previously shown using an ID rat model that ID increased the number of megakaryocytes in the bone marrow. We have also shown an increase in VEGFR (FLT1) and CXCR4 staining in bone marrow slides of ID rats. This data suggests that angiogenesis plays a vital role in the development of reactive thrombocytosis in response to ID. In this report, we have expanded our study to identify specific angiogenic signaling molecules associated with ID and used functional assays to validate it.

Methods: For this study, we used the megakaryoblast cell line MEG-01 as an in vitro model of megakaryopoiesis. MEG-01 cells were adapted to grow in chemically defined serum free medium containing iron (iron replete media). For iron deficiency, serum free iron free media was mixed with iron replete media at a 1% v/v concentration (iron deplete media). For our experiments, MEG-01 cells were grown in both iron replete and depleted media for 7 days. Cell viability was measured using the trypan blue exclusion assay. Messenger-RNA expression of iron-related markers (TFR1, TFR2, FLT1, FLT3, FTL, FTH1, TF, HMOX1 and HMOX2) and angiogenic markers (VEGFA, VEGFB, VEGFC, PDGF, ANGPTL1, ANGPTL2, FGF2) was studied using real time PCR. We performed functional validation of angiogenesis with an in vitro tube formation assay using human umbilical vein endothelial (HUVEC) cells. For statistical analysis of the data we performed the t test using graph pad prism software and we considered p<0.05 as statistically significant.

Results: In low iron conditions, MEG-01 cells showed a significant increase in FLT1 (4 fold) and FLT3 (3 fold) expression using real time PCR (p<0.001). Iron deficiency also induced a 2 fold increase in the mRNA expression of angiogenic molecules VEGFB, VEGFC, FGF2 and PDGFA (p<0.001). Using the tube formation assay, we also show that conditioned media collected from iron deficient MEG-01 cells induced increased vessel formation in endothelial cells.

Conclusion: In this study, we were able to validate our earlier in vivo findings on iron deficiency induced reactive thrombocytosis. We show that cells adapt to low iron conditions by upregulating FLT1, FLT3 and FTL. We also show that several markers in the angiogenesis pathway like VEGFB, VEGFC, FGF2 and PDGFA are upregulated in response to iron deficiency. We were also able to show that an increase in these angiogenic molecules induced increased vessel formation in endothelial cells. This report, along with our previous findings, points to the importance of the angiogenic pathway in reactive thrombocytosis induced by iron deficiency.