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Regulation of Megalin Expression in Cultured Proximal Tubule Cells by Angiotensin II Type 1A Receptor- and Insulin-Mediated Signaling Cross Talk

Division of Clinical Nephrology and Rheumatology (M.H., K.Y., R.K., A.Su., H.K., A.T., K.I., T.T., F.G.) and Department of Applied Molecular Medicine (H.S., T.S., A.K., N.I., A.Sa.), Niigata University Graduate School of Medical and Dental Sciences, Niigata 951-8510, Japan; Center for Transdisciplinary Research (H.S.) and Niigata University Health Administration Center (Y.S.), Niigata University, Niigata 950-2181, Japan; Department of Pharmacology (A.N.), Faculty of Medicine, Kagawa University, Kagawa 761-0793, Japan; and Department of Pharmaceutical Sciences (T.J.T.), Texas Tech University Health Sciences Center, Amarillo, Texas 79106

Address all correspondence and requests for reprints to: Akihiko Saito, M.D., Ph.D., Department of Applied Molecular Medicine, Niigata University Graduate School of Medical and Dental Sciences, 1-757, Asahimachi-dori, Chuo-ku, Niigata, 951-8510 Japan. E-mail: akisaito{at}med.niigata-u.ac.jp.

Impairment of proximal tubular endocytosis of glomerular-filtered proteins including albumin results in the development of proteinuria/albuminuria in patients with chronic kidney disease. However, the mechanisms regulating the proximal tubular function are largely unknown. This study aimed to investigate the role of angiotensin II type 1A receptor (AT_1AR)- and insulin-mediated signaling pathways in regulating the expression of megalin, a multiligand endocytic receptor in proximal tubule cells (PTCs). Opossum kidney PTC-derived OK cells that stably express rat AT_1AR but are deficient in endogenous angiotensin II receptors (AT_1AR-OK cells) were used for this study. Treatment of the cells with angiotensin II suppressed mRNA and protein expression of megalin at 3- and 24-h incubation time points, respectively. Cellular uptake and degradation of albumin and receptor-associated protein, megalin’s endocytic ligands were suppressed 24 h after angiotensin II treatment. The AT_1AR-mediated decrease in megalin expression was partially prevented by ERK inhibitors. Insulin competed with the AT_1AR-mediated ERK activation and decrease in megalin expression. Inhibitors of phosphatidylinositol 3-kinase (PI3K), a major component of insulin signaling, also suppressed megalin expression, and activation of the insulin receptor substrate (IRS)/PI3K system was prevented by angiotensin II. Collectively the AT_1AR-mediated ERK signaling is involved in suppressing megalin expression in the OK cell line, and insulin competes with this pathway. Conversely, the insulin-IRS/PI3K signaling, with which angiotensin II competes, tends to stimulate megalin expression. In conclusion, there is AT_1AR- and insulin-mediated competitive signaling cross talk to regulate megalin expression in cultured PTCs.