Evidence of an intestinal phosphate transporter alternative to type IIb sodium-dependent phosphate transporter in rats with chronic kidney disease

Abstract

Background Phosphate is absorbed in the small intestine via passive flow and active transport.NaPi-IIb, a type II sodium-dependent phosphate transporter, is considered to mediate active phosphate transport in rodents. To study the regulation of intestinal phosphate transport in chronic kidney disease (CKD), we analyzed the expression levels of NaPi-IIb, pituitary-specific transcription factor 1 (PiT-1) and PiT-2 and the kinetics of intestinal phosphate transport using two CKD models. Methods CKD was induced in rats via adenine orThy1 antibody injection. Phosphate uptake by intestinal brush border membrane vesicles (BBMV) and the messenger RNA (mRNA) expression of NaPi-IIb, PiT-1 and PiT-2 were analyzed. The protein expression level of NaPi-IIb was measured by mass spectrometry (e.g. liquid chromatography tandem mass spectrometry). Results In normal rats, phosphate uptake into BBMV consisted of a single saturable component and its Michaelis constant (Km) was comparable to that of NaPi-IIb. The maximum velocity (Vmax) correlated with mRNA and protein levels of NaPi-IIb. In the CKD models, intestinal phosphate uptake consisted of two saturable components. The Vmax of the higher-affinity transport, which is thought to be responsible for NaPi-IIb, significantly decreased and the decrease correlated with reduced NaPi-IIb expression. The Km of the lower-affinity transport was comparable to that of PiT-1 and -2. PiT-1 mRNA expression was much higher than that of PiT-2, suggesting that PiT-1 was mostly responsible for phosphate transport. Conclusions This study suggests that the contribution of NaPi-IIb to intestinal phosphate absorption dramatically decreases in rats with CKD and that a low-affinity alternative to NaPi-IIb, in particular PiT-1, is upregulated in a compensatory manner in CKD.

Targeting NaPi-IIb for Hyperphosphatemia in Chronic Kidney Disease Patients - The Dead End?

“…9 Taken together, the results of these studies suggest a limited effect of NaPi-IIb inhibitors for the treatment of hyperphosphatemia in CKD patients. This lack of effect may be due to the decreased NaPi-IIb intestinal expression in advanced CKD as suggested by a recent experimental study; this suggests that other alternative phosphate transporter may predominate in the setting of advance kidney disease 10 Thus, low-affinity transporters, such as PiT-1 or PiT-2 gain importance for intestinal phosphate absorption; inhibition of these transporters might represent a novel therapeutic approach to ameliorate hyperphosphatemia in kidney diseases.…”

Phosphate Balance and CKD–Mineral Bone Disease

“…133 An association between reduced calcitriol and reduced intestinal phosphate absorption has been shown in animal models. 134 Increased PTH leads to higher bone turnover, releasing calcium and phosphate, 9,135 and increased FGF23…”

The Complexities of Organ Crosstalk in Phosphate Homeostasis: Time to Put Phosphate Sensing Back in the Limelight

“…However, the relative contribution of PiT1 and PiT2 to intestinal phosphate absorption is still a matter for deliberation. The expression of PiT1 is segment-specific and comparable to NPT2B, with strong expression in the duodenum and jejunum [ 62 , 63 ], whereas, PiT2 is expressed all along the intestine at low levels [ 64 ]. Historically, a role for the SLC20 transporters in transcellular sodium-dependent phosphate absorption by the intestine has generally been dismissed, as SLC34A2 deletion reduces transcellular phosphate absorption by ~90% [ 27 ].…”

“…A recent study however, suggests that SLC20 sodium phosphate co-transporters may well participate in intestinal phosphate absorption, as the use of the pan inhibitor, EOS789, in a rat model of CKD, had a stronger effect on reducing hyperphosphatemia than the use of a NPT2B-specific inhibitor alone [ 66 ]. In addition, the same group has also reported that whereas the contribution of NPT2B to transcellular absorption is dominant under normal conditions, SLC20-associated transcellular phosphate transport is likely to play a key role in intestinal phosphate absorption in rats with CKD [ 64 ]. Based on the relative expression levels of SLC20 proteins, the authors suggest that PiT1 rather than PiT2 is responsible for the low affinity phosphate absorption observed in rats with CKD [ 64 ].…”

“…In addition, the same group has also reported that whereas the contribution of NPT2B to transcellular absorption is dominant under normal conditions, SLC20-associated transcellular phosphate transport is likely to play a key role in intestinal phosphate absorption in rats with CKD [ 64 ]. Based on the relative expression levels of SLC20 proteins, the authors suggest that PiT1 rather than PiT2 is responsible for the low affinity phosphate absorption observed in rats with CKD [ 64 ]. In contrast, the widespread and low level expression of PiT2 in epithelial and non-epithelial cells throughout the intestine [ 64 ], together with its questionable role in transcellular phosphate absorption, indicates that this protein may potentially function as an intestinal phosphate sensor [ 59 ].…”