Antidepressants stimulate lipoprotein(a) macropinocytosis via serotonin-enhanced cell surface binding

Abstract

The regulation of Lp(a) clearance from circulation by cellular uptake remains enigmatic. While multiple receptors have been implicated in Lp(a) uptake, each receptor only partially accounts for this uptake, and no endocytic mechanism has yet been prescribed for Lp(a) internalisation into the cell. In this study, we define macropinocytosis as the endocytic pathway responsible for internalising Lp(a). In both liver and macrophage cells, Lp(a) uptake is dependent on extracellular calcium and is inhibited by amiloride or its derivative EIPA. The uptake of apo(a) alone is mediated by macropinocytosis, indicating the apo(a) protein component is a primary regulator of Lp(a) uptake. Importantly, we found that common antidepressants modulate Lp(a) macropinocytosis in cell-type dependent manners. In macrophages, the tricyclic antidepressant, imipramine and selective serotonin reuptake inhibitors (SSRIs) citalopram, fluoxetine and sertraline all inhibit Lp(a) uptake. In liver cells, imipramine and citalopram strongly stimulated Lp(a) uptake, while sertraline inhibited uptake. Imipramine and citalopram enhanced Lp(a) uptake by enhancing cell surface binding of Lp(a) rather than upregulating macropinocytosis per se, indicating that immobilisation of Lp(a) on the plasma membrane is an important regulatory step in Lp(a) macropinocytosis. As the liver is the major catabolic route for Lp(a), we dissected the functional consequences of imipramine and citalopram stimulated uptake in liver cells. Both drugs increased Lp(a) delivery to Rab11-positive recycling endosomes, but not late endosomes or lysosomes, resulting in increased apo(a) recycling into the cellular media. Given that free apo(a) is reported to undergo consistent proteolysis in the extracellular space or in circulation, these results support the potential for the already approved drugs, imipramine and citalopram, as promising therapeutics to lower Lp(a) levels. This approach displays special utility in the large group of people who suffer from depression and anxiety for whom these drugs are commonly prescribed.

Serotonin: an overlooked regulator of endocytosis and endosomal sorting?

“…Finally, serotonin has a high affinity for membranes ( Josey et al, 2020 ), meaning that at high local concentrations it would rapidly insert into the plasma membrane ( Dey et al, 2021 ). Serotonin exposure in both Dey et al (2021) and our study ( Redpath et al, 2021 preprint) was over a timescale of minutes to hours, a longer timeframe of serotonin exposure than would likely be caused by serotonin vesicle release. These studies show that serotonin is clearly capable of enhancing cargo binding and endocytosis, but it remains to be determined how acute versus chronic serotonin treatment modulates endocytosis.…”

“…These experiments were conducted in the presence of serotonin receptor and transporter inhibitors ( Dey et al, 2021 ), strongly suggesting that changes in serotonin levels on/in the plasma membrane enhance endocytic uptake. Corroborating these findings are our recent experiments showing that plasma membrane binding and endocytosis of the circulating lipoprotein, lipoprotein(a) [Lp(a)], is potentially enhanced independent of serotonin receptors and transporter ( Redpath et al, 2021 preprint). Treatment with the antidepressants (SERT inhibitors) imipramine and citalopram, or serotonin itself, significantly increased Lp(a) binding to the plasma membrane and subsequent endocytosis into HepG2 liver cells in a macropinocytosis-dependent manner ( Redpath et al, 2021 preprint).…”

“…Corroborating these findings are our recent experiments showing that plasma membrane binding and endocytosis of the circulating lipoprotein, lipoprotein(a) [Lp(a)], is potentially enhanced independent of serotonin receptors and transporter ( Redpath et al, 2021 preprint). Treatment with the antidepressants (SERT inhibitors) imipramine and citalopram, or serotonin itself, significantly increased Lp(a) binding to the plasma membrane and subsequent endocytosis into HepG2 liver cells in a macropinocytosis-dependent manner ( Redpath et al, 2021 preprint). HepG2 cells express only trace levels of 5-HT 1A and 5-HT 1D receptors [The Human Protein Atlas ( Thul et al, 2017 )], indicating that serotonin receptor and transporter-independent effects are enhancing Lp(a) plasma membrane binding and uptake.…”

“…Although it is not yet clear if the state of plasma membrane order can directly translate from one leaflet of the membrane to the other ( Fujimoto and Parmryd, 2017 ), it is tempting to speculate that serotonin-induced lipid disorder could modulate recruitment of endocytic sorting regulators. In our recent study, we found that imipramine and citalopram treatment enhanced Lp(a) delivery to Rab11 + recycling endosomes ( Redpath et al, 2021 preprint), indicating that serotonin-induced membrane order changes may translate into functional differences of endosomal cargo sorting, although this remains to be directly tested with serotonin treatment.…”