**Hormonal Actions and Interactions in Proximal Tubule Cells Associated with the Development of Chronic Kidney Disease**

Akihiko Saito1, Michihiro Hosojima2 and Hiroyoshi Sato1 *1Department of Applied Molecular Medicine 2Division of Clinical Nephrology and Rheumatology Niigata University Graduate School of Medical and Dental Sciences Japan* 

#### **1. Introduction**

As the first tubular segment of the kidney between the Bowman's capsule and the loop of Henle, the proximal tubule is exposed to initial glomerular filtrates. Proximal tubule cells (PTCs) have a variety of functions including 1) reabsorption and intracellular processing of glomerular-filtered substances such as proteins, peptides, glucose, amino acids, uric acid, sodium, potassium, phosphate, bicarbonate and water via apical membrane receptors, transporters, and channels; 2) uptake of substances such as protein-bound compounds via basolateral membrane transporters, followed by metabolism or secretion to the urinary space; and 3) synthesis of bioactive substances that are released to peritubular capillaries (Fig. 1).

Normal functions of PTCs (indicated by arrows) include: 1) reabsorption and intracellular processing of glomerular-filtered substances via apical membrane receptors, transporters, and channels; 2) uptake of substances via basolateral membrane transporters followed by metabolism or secretion into the urinary space; and 3) synthesis of bioactive substances that are released to peritubular capillaries.

Fig. 1. Normal functions of proximal tubule cells (PTCs)

Impairment of these diverse functions is likely to affect systemic hemodynamic and metabolic homeostasis and can lead to the development of chronic kidney disease (CKD) and cardiovascular disease (CVD) (Saito, Kaseda et al. 2010). In particular, PTC injury occurs during the early stages of diabetic nephropathy, a leading cause of CKD.

Hormonal Actions and Interactions in Proximal Tubule Cells

development of proteinuria/albuminuria in diabetic patients.

**2.1.1 Megalin** 

sections.

**2.1.2 Cubilin** 

(Boger et al. 2011).

**2.1.3 Amnionless** 

1999; Leheste et al. 2003) (Fig. 3).

Associated with the Development of Chronic Kidney Disease 307

Megalin is a large (~600 kDa) glycoprotein member of the low-density lipoprotein receptor family that is primarily expressed in clathrin-coated pits (Saito, Sato et al. 2010). Megalinligand complexes are internalized by invagination of clathrin-coated pits mediated by multiple adaptor proteins and motor molecules, forming endosomal vesicles. Acidification of the intravesicular lumen dissociates the ligands from megalin and they are transported to lysosomes for degradation or storage or are secreted into the cytosol for further processing or transport. Megalin is then recycled to the apical membranes through a recycling compartment. Megalin thus plays a critical role in reabsorption and metabolism of glomerular-filtered substances including albumin and low molecular weight proteins. Vitamin D-binding protein (DBP) is an endocytic ligand of megalin, and megalin knockout mice display decreased utilization of vitamin D for its activation in PTCs (Nykjaer et al.

Decreased megalin expression in PTCs has been observed in the early diabetic stages of experimental animals (Tojo et al. 2001; Russo et al. 2007), and it has been suggested that the functions of megalin are also impaired in patients during the early stages of diabetic nephropathy, since low molecular weight proteinuria is frequently observed in patients at these stages (Pontuch et al. 1992; Hong et al. 2003). Therefore, the altered regulation of megalin expression and its functions must be primarily responsible for the early

Megalin also interacts with various membrane receptors and transporters such as the cubilin/amnionless complex, sodium-hydrogen exchanger isoform 3 (NHE3) and sodiumdependent phosphate cotransporter type IIa (NaPi-IIa) as discussed in the following

Cubilin is a 460-kDa peripheral glycoprotein that lacks transmembrane and intracellular segments but is anchored to apical PTC membranes. It was originally identified as the receptor for intrinsic factor-vitamin B12 complex (Seetharam et al. 1988; Seetharam et al. 1997). Cubilin requires interaction with megalin to regulate its endocytic functions (Yammani, Seetharam, and Seetharam 2001; Kozyraki et al. 2001); however, it is bound more firmly by a protein called amnionless forming the complex CUBAM (Fyfe et al. 2004; Coudroy et al. 2005). Cubilin gene defects are the cause of hereditary megaloblastic anaemia 1 and Imerslund-Gräsbeck syndrome, also known as selective vitamin B12 malabsorption with proteinuria (Aminoff et al. 1999). Cubilin is also involved in the absorption of various protein ligands present in glomerular filtrates, including albumin, transferrin and DBP (Christensen, Verroust, and Nielsen 2009). A recent meta-analysis of genome-wide association studies identified a missense cubilin gene variant associated with levels of albuminuria in both the general population and in diabetic individuals

Amnionless is a 38–50 kDa membrane protein with a single transmembrane domain that was initially identified as a component required for normal development of the trunk mesoderm derived from the middle streak (Kalantry et al. 2001). In addition, defects of the

amnionless gene cause hereditary megaloblastic anaemia (Tanner et al. 2003).

In this review, we focus on actions and interactions of various hormones and hormone-like substances on the regulation of apical membrane receptors and transporters that mediate three major functions of PTCs: 1) receptor-mediated protein endocytosis, 2) sodium reabsorption, and 3) phosphate reabsorption, which are particularly associated with the pathophysiology of CKD and the increased risk of CVD (Fig. 2).
