Leukemia Cells Remodel Adipocyte Niches and Their Progenitor Functions to Generate Leukemia Favoring Niche
Authors: Kumar Bijender , Orellana Marvin , Brooks Jamison , Sargur Madabushi Srideshikan , E Parra Liliana , Zuro Darren , Wang Qiong , Chen Ching-Cheng , Hui Susanta 
Published: October 2019

Abstract
Increasing evidence suggests that the cancer cells take shelter in different osteoblastic and adipocytic niches, where they hide from chemotherapy and continue to survive. As yet, how leukemia cells alter the bone marrow (BM) adipocytic niches to facilitate their expansion and assist them in evading chemotherapy is unclear. We have previously shown that the acute myeloid leukemia (AML) cells directly or through their exosomes, reprogram BM osteoblastic niche which facilitates their expansion and suppress normal hematopoiesis(Kumar B et al, Leukemia 2018,32(3):575-587). In this study , we provide further evidences that AML and Acute lymphoid leukemia (ALL) transformed the BM adipocytic niche to facilitate their expansion and suppress normal hematopoiesis.
Using MLL-AF9 (AML) knock-in mouse, MLL-AF9 or BCR-ABL(p190, ALL) HSC transduction transplantation leukemia mice models, we performed flow cytometry analysis to show that the leukemia cells stimulate expansion of BM derived CD45-Ter119-CD31-CD166-Sca1+CD140a+(PaS)MSCs population compared to normal mice (p=0.04, p=0.001 and p=0.002 respectively).Further, the BM osteoblasts specific Osteocalcin mRNA expression in sorted stroma cells and flow cytometry based osteoblasts population (CD45-Ter119-CD31- CD166+Sca1-) numbers were also significantly reduced in AML (p=0.04) and ALL (p=0.02) mice models suggesting bone loss with the leukemia development. Similar to osteoblasts loss, mature adipocytes (Perilipin, PPARg mRNA) were also significantly reduced in the ALL/AML mice compared to control. The triglyceride content and white adipose tissue(WAT) mass was diminished in leukemic mice , suggesting leukemia may have utilized adipocyte for survival. Adipocyte loss in the leukemia mice was accompanied by long term hematopoietic stem cells(LT-HSC ) and erythroid megakaryocyte progenitor (MEP) populations reduction in the leukemic mice (p=0.01 and p=0.02 respectively).
To dissect the mechanism of adipocytes reduction is either due to adipocyte loss or adipocytes maturation defect in leukemic mice, we analyzed different stromal progenitors in normal and leukemic mice and identified that the leukemia cells stimulate the growth of BM derived adipocytic committed progenitors (CD45-Ter119-CD31-CD166-Sca1+CD140a+CD29+CD24-) and blocked the chondrocyte/osteoblastic/adipocytic multipotent progenitors (CD45-Ter119-CD31- CD166-Sca1+CD140a+CD29+CD24+) population (p=0.02,p=0.01 respectively).Despite the increase in number of MSCs and adipocytic progenitors, the in-vitro adipocytic differentiation potential of sorted adipocyte committed progenitors was severely compromised in ALL and AML compared to control. The WAT western blot analysis showed significantly increased expression of ATGL and pHSL(Ser-563) expression involved in triglyceride lipolysis in the leukemic mice .The ALL leukemic adipocytic stroma had increased expression of IL-1β and IL-6 cytokine levels compared to normal stroma and provided more survival advantage to leukemia cells in in-vitro co-culture experiments in nutrient deprived conditions and during chemo-radiotherapy treatment. Further, the ATGL and HSL pharmacological inhibitors rescued leukemia induced lipolysis, reduced leukemia proliferation and increased chemotherapy induced apoptosis in leukemia cells.
Overall, this data strongly suggests the notion of progressive decline in functional LT-HSCs & normal hematopoiesis, adipocytes and osteoblasts numbers with leukemia progression due to activation of lipolytic enzymes resulting in increased availability of fatty acids for leukemia expansion and is a common feature in both lymphoid and myeloid leukemias.

Disclosures
No relevant conflicts of interest to declare.