Homocysteine transport by human aortic endothelial cells: identification and properties of import systems

Area of Science:

• Cardiovascular disease research
• Endothelial cell biology
• Amino acid transport mechanisms

Background:

• Hyperhomocysteinemia is a known risk factor for cardiovascular disease.
• The mechanisms of L-homocysteine transport in human vascular endothelium are not well understood.
• Understanding these transport pathways is crucial for elucidating the role of homocysteine in endothelial dysfunction.

Purpose of the Study:

• To investigate the specific L-cysteine transport systems responsible for L-homocysteine uptake in human aortic endothelial cells (HAEC).
• To characterize the kinetics and substrate specificity of these transporters for L-homocysteine, L-homocystine, and L-cysteine.
• To determine the relative contribution of different transport systems to L-homocysteine import.

Main Methods:

• Primary human aortic endothelial cells (HAEC) were cultured.
• Uptake of radiolabeled [35S]L-homocysteine, [35S]L-homocystine, and [35S]L-cysteine was measured.
• Inhibitors specific to various L-cysteine transport systems (e.g., X(AG), ASC, A, L, x(c)) were used to block transporter activity.
• Kinetic analysis using Michaelis-Menten parameters (Km) was performed.

Main Results:

• L-homocysteine uptake in HAEC is mediated by four L-cysteine transport systems: sodium-dependent X(AG), ASC, and A, and sodium-independent system L.
• The relative contribution of these systems to L-homocysteine uptake was X(AG) ≥ L > ASC > A.
• Michaelis-Menten constants (Km) indicated varying affinities for L-homocysteine across these systems (19.0 µM for L to 1000 µM for A).
• L-homocystine and L-cysteine uptake showed overlapping but distinct patterns, with system x(c) active for L-homocystine and L-cysteine but not L-homocysteine.

Conclusions:

• Human aortic endothelial cells utilize multiple L-cysteine transporters to import L-homocysteine.
• These transporters provide entry pathways for L-homocysteine and its dimer, L-homocystine, into the vascular endothelium.
• The findings contribute to understanding the cellular mechanisms underlying hyperhomocysteinemia's impact on cardiovascular health.