Aberrant cell proliferation is one of the hallmarks of carcinogenesis, and cholesterol is thought to play an important role during cell proliferation and cancer progression. In the present study we examined the pathways that could contribute to enhanced proliferation rates of HBL-100 cells in the presence of apolipoprotein E-depleted high-density lipoprotein subclass 3 (HDL3). When HBL-100 cells were cultivated in the presence of HDL3 (up to 200 μg/ml HDL3 protein), the growth rates and cellular cholesterol content were directly related to the concentrations of HDL3 in the culture medium. In principle, two pathways can contribute to cholesterol/cholesteryl ester (CE) uptake from HDL3, (i) holoparticle- and (ii) scavenger-receptor BI (SR-BI)-mediated selective uptake of HDL3-associated CEs. Northern- and Western-blot analyses revealed the expression of CLA-1 (CD-36 and LIMPII analogous 1), the human homologue of the rodent HDL receptor SR-BI. In line with CLA-1 expression, selective uptake of HDL3-CEs exceeded HDL3-holoparticle uptake between 12- and 58-fold. Competition experiments demonstrated that CLA-1 ligands (oxidized HDL, oxidized and acetylated low-density lipoprotein and phosphatidylserine) inhibited selective HDL3-CE uptake. In line with the ligand-binding specificity of CLA-1, phosphatidylcholine did not compete for selective HDL3-CE uptake. Selective uptake was regulated by the availability of exogenous cholesterol and PMA, but not by adrenocorticotropic hormone. HPLC analysis revealed that a substantial part of HDL3-CE, which was taken up selectively, was subjected to intracellular hydrolysis. A potential candidate facilitating extralysosomal hydrolysis of HDL3-CE is hormone-sensitive lipase, an enzyme which was identified in HBL-100 cells by Western blots. Our findings demonstrate that HBL-100 cells are able to acquire HDL-CEs via selective uptake. Subsequent partial hydrolysis by hormone-sensitive lipase could provide ‘free’ cholesterol that is available for the synthesis of cellular membranes during proliferation of cancer cells.
Macrophage cholesteryl ester hydrolases and hormone-sensitive lipase prefer specifically oxidized cholesteryl esters as substrates over their non-oxidized counterparts
