Abstract
The CD36/FAT scavenger receptor/fatty acids transporter regulates cellular lipid accumulation important for inflammation, atherosclerosis, lipotoxicity and initiation of cellular senescence. During stress‐induced premature senescence (SIPS), a number of stimuli (e.g., oxLDL, β‐amyloid and 4‐hydroxynonenal) may lead to up‐regulation of CD36‐mediated lipid uptake triggering signaling and the expression of lipid‐regulating and inflammatory genes involved in the senescence‐associated secretory phenotype (SASP) characterized by secretion of cytokines, chemokines, lipids and proteases. Both, SIPS and SASP may be modulated by vitamin E (alpha‐tocopherol, αT), since it inhibits CD36 expression, cell surface exposition and CD36‐associated signaling. Here we compared the regulatory effects of αT, αTP (alpha‐tocopheryl phosphate, αTP) and αTP/βCD (a nanocarrier complex between αTP and β‐cyclodextrin (βCD)) and investigated their regulatory effects on lipid accumulation, phagocytosis and senescence in THP‐1 monocytes and macrophages. Both, αTP and αTP/βCD inhibited CD36 surface exposition stronger that αT leading to more pronounced CD36‐mediated events such as inhibition of DiI‐labelled oxLDL uptake, phagocytosis of fluorescent Staphylococcus aureus bioparticles, and cell proliferation. When compared to βCD, the complex of αTP/βCD extracted cholesterol from cellular membranes with higher efficiency and was associated with delivery of αTP to the cells, thus forming a Dual‐Action Lipid‐lowering complex (DALC). Interestingly, both, αTP and more so αTP/βCD inhibited lysosomal senescent‐associated beta‐galactosidase (SA‐β‐gal) activity and increased lysosomal pH, suggesting CD36‐mediated uptake into the endo‐lysosomal phagocytic compartment. Accordingly, the observed pH increase was more pronounced with αTP/βCD in macrophages when compared to monocytes, possibly due to their increased ability to phagocytose the αTP/βCD complex, whereas no significant change of pH occurred with αT, alpha‐tocopherol acetate (αTA) or βCD. In contrast to αT and αTA, the αTP molecule is di‐anionic at neutral pH (calculated pKa1 1.64 and pKa2 6.07), but upon moving into the acidic endo‐lysosomal compartment becomes protonated and thus is acting as a base. Moreover, it is expected to be retained in lysosomes since it still carries one negative charge, as it occurs also with lysosomotropic drugs such as chloroquine and hydroxychloroquine. Thus, treatment with αTP or αTP/βCD and/or inhibition of conversion of αTP to αT as it occurs in aged cells may counteract CD36‐mediated overlapping inflammatory, senescent, and atherosclerotic events.
