# The Effect of Radixin on the Function and Expression of Organic Anion Transporting Polypeptide 1B1

## TL;DR

This study shows that radixin, a scaffold protein, modulates the function and cell surface expression of OATP1B1, a key drug transporter in the liver.

## Contribution

The study identifies radixin as a novel regulatory protein that influences OATP1B1 activity through phosphorylation.

## Key findings

- Radixin knockdown increases OATP1B1-mediated substrate uptake.
- Phospho-mimic radixin (radixin-D) reduces OATP1B1 activity and surface expression.
- PKC-mediated phosphorylation of radixin is essential for suppressing OATP1B1 function.

## Abstract

Organic anion transporting polypeptide 1B1 (OATP1B1) is a key hepatic uptake transporter for drug uptake. Change of OATP1B1 function will affect the pharmacokinetics of various clinically important therapeutic agents. The present study reveals that the scaffold protein radixin regulates the function OATP1B1. Knockdown of radixin significantly increased OATP1B1-mediated substrate uptake. Conversely, overexpression of a phospho-mimic radixin mutant (radixin-D) suppressed both transport activity and cell surface expression of OATP1B1, while wild-type or phospho-dormant (radixin-A) forms had no effect. Importantly, radixin directly interacted with OATP1B1, and protein kinase C (PKC) activation enhanced the phosphorylation of radixin bound to the transporter. Radixin knockdown abolished PKC-induced suppression of OATP1B1 function and surface levels, demonstrating that PKC-mediated radixin phosphorylation is essential for this regulatory mechanism. These findings establish radixin as a dynamic modulator of OATP1B1, the phosphorylation of which influences the location and activity of the transporter.

Organic anion transporting polypeptide 1B1 (OATP1B1) is selectively expressed at the basolateral membrane of human hepatocytes and plays a crucial role in the absorption of various xenobiotic compounds, including many important clinical drugs. Oligomerization with regulatory proteins is a common mechanism for regulating membrane protein functions. In the present study, we found that knocking down the scaffold protein radixin, which is the major member of the ERM family expressed in the liver, significantly enhanced the uptake function of OATP1B1. On the other hand, the overexpression of the phospho-mimic form of radixin (radixin-D) reduced the uptake function and cell surface level of OATP1B1, while the wild-type and phospho-dormant form of radixin (radixin-A) did not exhibit the same effect. Further investigation revealed that radixin interacts with OATP1B1. Activation of protein kinase C (PKC), which our previous study showed accelerates the internalization of OATP1B1, was found to increase the phosphorylation level of radixin associated with OATP1B1. The knockdown of radixin significantly diminished the suppressive effect of PKC on the function and cell surface levels of OATP1B1. These results suggested that OATP1B1 forms complexes with radixin, which may be phosphorylated by PKC, leading to reduced cell surface expression and activity of the transporter.