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Thiazolidinediones (TZDs) improve insulin resistance by activating a nuclear hormone receptor, peroxisome proliferator-activated receptor γ (PPARγ). However, the use of TZDs is associated with plasma volume expansion through a mechanism that remains to be clarified. Here we showed that TZDs rapidly stimulate sodium-coupled bicarbonate absorption from the renal proximal tubule in vitro and in vivo. TZD-induced transport stimulation is dependent on PPARγ-Src-EGFR-ERK and observed in rat, rabbit and human, but not in mouse proximal tubules where Src-EGFR is constitutively activated. The existence of PPARγ-Src-dependent nongenomic signaling, which requires the ligand-binding ability, but not the transcriptional activity of PPARγ, is confirmed in mouse embryonic fibroblast cells. The enhancement of the association between PPARγ and Src by TZDs supports an indispensable role of Src in this signaling. These results suggest that the PPARγ-dependent nongenomic stimulation of renal proximal transport is also involved in TZD-induced volume expansion.

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ϩ -HCO 3 Ϫ cotransporter (NBC-1) cause proximal renal tubular acidosis (pRTA) associated with ocular abnormalities. One pRTA patient had increased serum amylase, suggesting possible evidence of pancreatitis. To further delineate a link between NBC-1 inactivation and pancreatic dysfunction, immunohistochemical analysis was performed on rat and human pancreas using antibodies against kidney-type (kNBC-1) and pancreatic-type (pNBC-1) transporters. In rat pancreas, the anti-pNBC-1 antibody labeled acinar cells and both apical and basolateral membranes of medium and large duct cells. In human pancreas, on the other hand, the anti-pNBC-1 antibody did not label acinar cells, although it did label the basolateral membranes of the entire duct system. The labeling by anti-kNBC-1 antibody was detected in only a limited number of rat pancreatic duct cells. To examine the effects of pRTA-related mutations, R342S and R554H, on pNBC-1 function, we performed functional analysis and found that both mutants had reduced transport activities compared with the wild-type pNBC-1. These results indicate that pNBC-1 is the predominant variant that mediates basolateral HCO 3 Ϫ uptake into duct cells in both rat and human pancreas. The loss of pNBC-1 function is predicted to have significant impact on overall ductal HCO 3 Ϫ secretion, which could potentially lead to pancreatic dysfunction. Na ϩ -HCO 3 Ϫ cotransporter; pRTA; pancreatic duct cells; pancreatitis THE NA ϩ -HCO 3 Ϫ COTRANSPORTER (NBC-1), originally cloned from amphibian kidney (32), has multiple functions. In the renal proximal tubules, it mediates HCO 3 Ϫ efflux from cells (7, 36, 42), whereas in other epithelial cells, such as the pancreatic duct cells and the corneal endothelium, it mediates HCO 3 Ϫ influx into cells (22,23, 41). The expression of NBC-1 in nonepithelial cells such as trabecular meshwork cells or neurons has been also demonstrated (35, 40). Up to now, three NBC-1 variants have been identified: kNBC-1 from kidney, pNBC-1 from pancreas, and hNBC-1 from heart. Although kNBC-1 differs from pNBC-1 only at the NH 2 terminus, pNBC-1 is identical to hNBC-1 at the amino acid level and differs only in its 5Ј-untranslated region (1,8,10). These NBC-1 variants originate from the same SLC4A4 gene by alternative splicing (2). It is therefore predictable that mutational inactivation of NBC-1 variants may induce a variety of clinical manifestations, including renal and extrarenal phenotypes. Indeed, we have recently shown that mutations in the common coding region of NBC-1, corresponding to R298S and R510H in kNBC-1 or R342S and R554H in pNBC-1, cause renal proximal tubular acidosis (pRTA) associated with ocular abnormalities such as band keratopathy, glaucoma, and cataracts (19). The expression study in ECV304 cells showed that the kidneytype mutants, R298S and R510H, have reduced transport activities compared with the wild-type kNBC-1 (19). The expression of both kNBC-1 and pNBC-1 in several ocular tissues has been confirmed (6, 40). It remains to be clarified, however, w...

Scavenger receptor class B type I (SR-BI) is a high-density lipoprotein (HDL) receptor that mediates the selective uptake of HDL cholesterol and cholesterol secretion into bile in the liver. Previously, we identified an SR-BI-associated protein, termed PDZK1, from rat liver membrane extracts. PDZK1 contains four PSD-95͞ Dlg͞ZO-1 (PDZ) domains, the first of which in the N-terminal region is responsible for the association with SR-BI. PDZK1 controls hepatic SR-BI expression in a posttranscriptional fashion both in cell culture and in vivo. In this study, we demonstrated that the C-terminal region of PDZK1 is crucial for up-regulating SR-BI protein expression. Metabolic labeling experiments and phosphoamino acid analysis revealed that PDZK1 is phosphorylated at Ser residues within this region. Point-mutation analysis demonstrated that PDZK1 is phosphorylated at Ser-509. Interestingly, a mutant PDZK1, in which Ser-509 was replaced with Ala, lost the ability to up-regulate SR-BI protein. We identified Ser-509 of PDZK1 as the residue that is phosphorylated by the cAMP-dependent PKA in vitro as well as in cell culture. Ser-509 of PDZK1 in rat liver was also phosphorylated, as shown by an Ab that specifically detects phosphorylated Ser-509. Administration of glucagon to Wistar rats increased PDZK1 phosphorylation as well as hepatic SR-BI and PDZK1 expression while it decreased plasma HDL levels, indicating that PDZK1 phosphorylation is hormonally regulated. These findings suggest that phosphorylation of PDZK1 has an important role in the regulation of hepatic SR-BI expression and, thus, influences plasma HDL levels.glucagon ͉ high-density lipoprotein ͉ PKA