Background: Chronic lymphocytic leukaemia (CLL) is one of the most prevalent leukaemias in Australia and is characterised by the proliferation of non-functional B-cells, in the microenvironments of lymph nodes (LN) and bone marrow. Circulating CLL cells are quiescent and their proteome may well be different from that of an active proliferating cell. In order to understand the drivers of CLL proliferation, it is critical to understand what is occurring in these proliferative niches. LN biopsies are rarely taken in CLL as they are not essential for diagnosis or disease staging, subsequently, this study used archived formalin fixed paraffin embedded (FFPE) LN biopsies for protein extraction.
Methods: We compared FFPE LN from CLL patients (n=14) to healthy LN sections (n=6). Proteins were isolated using heat and xylene to remove the paraffin, rehydration of the tissue followed by mechanical and chemical lysis. Protein identification and quantification was carried out using a SWATH MS/MS approach.
Results: Approximately 1000 proteins were identified, and pathway analysis identified lipid metabolism and degradation as the key differences between CLL and healthy LNs. Metabolic pathways were further explored using a variety of in vitro techniques including; FA uptake assays, microscopy, as well as proteomic/transcriptomic techniques to identify FA uptake receptors. We found that CLL is a lipolytic tumour, preferring the uptake of long-chain FAs over short-chain FAs or glucose and that the process is driven by endocytosis rather than receptor-mediated uptake. Once FAs are internalised, the CLL cells either store the excess fatty acids inside lipid droplets, which can be observed under electron microscopy, or increase beta-oxidation pathways to utilise the lipid energy store.
Significance: These findings have clinical implications in the development of diagnostic imaging techniques that improve understanding of disease activity (Lipid-based PET Scans) and in potential therapeutic approaches, whereby the metabolomics and lipid dependency of this leukaemia are exploited.