Vitamin K3 as a Substrate for the Human Organic Cation Transporter OCT1

Abstract

The human organic cation transporter (hOCT1 or OCT1) encoded by the gene SLC22A1 is expressed in many tissues, including the whole blood, brain, lung, mammary gland, intestine, kidney, liver, and bone marrow. Of all these tissues, hOCT1 is highly expressed in the liver, suggesting that it has a predominant role in this organ. It is generally expressed in organs which are critical for the removal of small organic cations, drugs, and environmental toxins. It also functions for the uptake of some vitamins, neurotransmitters, and drugs. Its isoform, hOCT2 from gene SLC22A2, serves a similar function. Preliminary data suggested that Menadione could be a substrate for OCT1 transporter. Menadione, or vitamin K3, is a synthetic form that is administered to individuals with blot clotting problems, and as vitamin K supplement in low-income countries. Menadione is known for being toxic at high concentrations, as it generates reactive oxygen species (ROS) through redox cycling that can damage the DNA. No particular transporter of vitamin K has been identified yet, and some believe it to passively diffuse through the cell membrane. In this work, we show that OCT1 is involved in menadione transport. OCT1 deleted HEK293 cells are more resistant to menadione treatment by clonogenic survival assay. We assessed the effects of menadione by evaluating the levels of peroxiredoxin 1 (PRDX1), where it got more upregulated in wild type HEK293 cells in comparison with the OCT1 deleted mutants. Cells upregulate the expression of PRDX1 to counteract the effects of ROS through elimination of the generated ROS and to safeguard telomeres from DNA damage. We also measured ROS levels using the dye CMXRos MitoTracker, and that dye showed higher fluorescence in control HEK293 cells than in OCT1 Knock-outs. Moreover, our results show that OCT1 deletion causes the upregulation of OCT2 as a compensatory mechanism and suggests that OCT2 also plays a role in menadione transport since double depletion causes much higher resistance to menadione. The thesis concludes with the major findings and the potential direction for future studies.

Date of Award	2025
Original language	American English
Awarding Institution	HBKU College of Health & Life Sciences