Abstract
Erythropoiesis is a tightly regulated multistage process in which hematopoietic stem cells (HSCs) proliferate progressively and differentiate to form mature red blood cells (RBCs). This process is metabolically demanding, requiring the coordinated activity of multiple organs to utilize iron and establish homeostasis. Central to erythropoiesis is the production of erythropoietin (Epo), driven by tissue oxygen status. Liver kinase B1 (LKB1) acts as a central regulator to control cellular metabolism, cell polarity and proliferation during development and stress response. As all of these fundamental cellular processes regulated by LKB1 are present in erythropoiesis, we hypothesize that LKB1 may be involved in this coordination.
LKB1 exerts its control of metabolism, polarity, and proliferation through AMP-activated protein kinase (AMPK) and 12 other AMPK-related effector kinases. LKB1 regulates quiescence and metabolic homeostasis of HSCs through AMPK-dependent and AMPK-independent mechanisms. Whole-body knockout studies in mice have revealed that loss of AMPK results in shortened erythrocyte life span, increased osmotic fragility, hemolytic destruction of RBCs, splenomegaly, and anemia.
Several questions still remain: Does LKB1 affect steady-state erythropoiesis? Does LKB1 play a role in the erythroid iron deprivation response? How does LKB1 affect the maturation of erythroblasts to become mature RBCs?
To determine the role of LKB1 in erythropoiesis, we crossed mice carrying loxP-flanked STK11/LKB1 alleles with EpoR-GFP-Cre mice, which will lead to gene excision just prior to erythroid commitment. These animals were then studied in steady-state and stress conditions. Mice with erythroid loss of LKB1 were found to have elevated serum Epo levels but unexpectedly had normal RBC counts with no evidence of anemia. An increase in Epo is normally accompanied by an increase in RBCs. These findings suggest a poor response to Epo – which could be due to shortened lifespan of the RBCs, ineffective erythropoiesis, or resistance to Epo. Mice with erythroid loss of LKB1 had RBC lifespans similar to those of control animals. Erythroid LKB1 was found to be dispensable for an appropriate response to anemic challenge in mice. Also, LKB1 was found to have a very limited role in a mouse model of iron deprivation. Ex vivo analysis revealed that erythroid loss of LKB1 results in accelerated maturation of erythroid progenitors as evidenced by an accumulation of CD71/Ter119 double positive cells when compared to control. Inhibition of the LKB1 downstream kinase AMPK recapitulates this accelerated maturation. Conversely, agonist stimulation of AMPK delays maturation. These findings reveal a previously unreported role for LKB1. Therefore, LKB1 is identified as a regulator of Epo homeostasis and the Epo-EpoR axis.
