Contribution of Phosphate and FGF23 to CKD Progression

Abstract

Purpose of the review:

Progressive forms of chronic kidney disease (CKD) exhibit kidney inflammation and fibrosis that drive continued nephron loss; however, factors responsible for the development of these common pathologic features remain poorly defined. Recent investigations suggest pathways involved in maintaining urinary phosphate excretion in CKD may be contributing to kidney function decline. This review provides an update on recent evidence linking altered phosphate homeostasis to CKD progression.

Recent findings:

High dietary phosphate intake and increased serum concentrations of fibroblast growth factor 23 (FGF23) both increase urinary phosphate excretion and are associated with increased risk of kidney function decline. Recent investigations have discovered high concentrations of tubular phosphate promote phosphate-based nanocrystal formation that drives tubular injury, cyst formation, and fibrosis.

Summary:

Studies presented in this review highlight important scientific discoveries that have molded our current understanding of the contribution of altered phosphate homeostasis to CKD progression. The collective observations from these investigations implicate phosphaturia, and the resulting formation of phosphate-based crystals in tubular fluid, as unique risk factors for kidney function decline. Developing a better understanding of the relationship between tubular phosphate handling and kidney pathology could result in innovative strategies for improving kidney outcomes in patients with CKD.

Introduction

Chronic kidney disease (CKD) is a global health burden that affects roughly 9–10% of the world’s population. Regardless of the initiating cause of CKD, there are common attributes that promote disease progression, including tubular epithelial cell injury, interstitial inflammation, and fibrosis (1). In spite of this, the development of effective therapies for preventing kidney function decline has been greatly hindered by limited knowledge of the mechanisms promoting these shared pathologic features (1, 2). Efforts to improve kidney outcomes would be greatly enhanced by gaining a more nuanced understanding of the pathways driving tubular injury, inflammation, and fibrosis in CKD. Recent observations suggesting a critical relationship between derangements in phosphate balance and kidney pathology may provide key insight into a long-overlooked contribution of phosphate to CKD progression.

Our understanding of phosphate metabolism in CKD has been greatly advanced by the discovery of the phosphaturic hormone fibroblast growth factor 23 (FGF23) and by a more complete grasp of pathways regulating phosphate transport in both the intestine and kidney. It is now clear that the urinary fractional excretion of phosphate progressively increases with kidney function decline to maintain serum phosphate levels in the normal range until late-stage disease (3, 4). However, recent evidence suggests this process of enhanced individual nephron phosphate excretion may be detrimental to tubular health and promote further nephron loss. In the current review, we provide a background on the initial studies describing this relationship between derangements in phosphate balance and kidney damageand highlight more recent observations that indicate a contribution of FGF23 and phosphaturia to CKD progression.

Altered phosphate homeostasis in CKD

The kidneys serve a vital role in regulating total body phosphate balance. As such, as dietary phosphate intake varies, urinary phosphate excretion must match these changes to maintain serum phosphate concentrations in a narrow range. Abnormalities in this balance result in hyperphosphatemia and a multitude of organ toxicities ranging from defects in bone architecture to cardiac and vascular damage. When functional nephron numbers decline, as in CKD, individual nephron phosphate excretion must increase to prevent hyperphosphatemia. This concept is supported by early micropuncture studies conducted on nephrons isolated from CKD rats which demonstrated tubular phosphate concentrations to be dramatically elevated to values far exceeding the normal supersaturation point of this mineral (5). Similarly, cross-sectional studies in patients with CKD have found that urinary fractional excretion of phosphate progressively increases across CKD stages beginning as early as stage II CKD (4). This increased urinary phosphate excretion is induced primarily by a rise in serum FGF23, a phosphaturic hormone derived from osteocytes and osteoblasts in bone, that works in concert with circulating parathyroid hormone (PTH) to inhibit phosphate reabsorption in the proximal tubule (3). Importantly, these changes in phosphate handling are likely to be universally observed in all forms of advanced-stage CKD, regardless of the initial cause of kidney damage.

Dietary phosphate influences CKD progression

The combination of a widespread use of phosphate-based additives in processed foods and a high consumption of animal protein in the Western diet have led to a steady rise in phosphate consumption over the last several decades (6). It is currently estimated that the average daily phosphate ingestion for individuals residing in industrialized countries is roughly double the recommended dietary allowance for this mineral (7). While it remains unclear if this high consumption of phosphate has detrimental effects in individuals with normal kidney function, an impressive number of studies have found that increased dietary phosphate promotes CKD progression.

The relationship between dietary phosphate and CKD progression was first demonstrated in rats undergoing subtotal nephrectomy (8–10). Interestingly, the ingestion of a high phosphate diet in rodents with CKD was associated with increased mineral deposition in the remnant kidney. On the contrary, other investigators demonstrated that feeding subtotal nephrectomy rats a phosphate-restricted diet was protective of kidney function (11–13). While these earlier studies focused solely on the effects of dietary phosphate in rodents with surgically-induced CKD, more recent studies have confirmed this association in alternative CKD models. As such, our group observed that in Col4a3^−/− mice, a murine model of human Alport disease, severe phosphate restriction ameliorated histologic evidence of renal tubular injury, despite having no obvious impact on glomerular pathology (14). Importantly, the renoprotective effect of phosphate restriction occurred despite persistent elevations in serum FGF23, suggesting that FGF23 itself was not directly responsible for the observed relationship between dietary phosphate and kidney pathology in this model. More recently, we and others expanded these investigations to rodent models of cystic kidney disease (15**, 16*). In line with observations in other CKD models, the consumption of high dietary phosphate was found to exacerbate cyst formation in polycystic kidney disease (PKD) rats (15**), while dietary phosphate restriction dramatically reduced kidney cyst burden and early markers of interstitial fibrosis in PKD mice (16*).

While it is difficult to replicate long-term dietary intervention studies in humans, several clinical observations may support the concept of dietary phosphate consumption being a modifiable risk factor for CKD progression in humans. First, multiple studies have demonstrated hyperphosphatemia to be an independent risk factor for CKD progression and the risk of developing end-stage kidney disease (ESKD) (17, 18). Second, results from several small clinical trials suggest that dietary phosphate restriction can slow CKD progression, especially in patients with more advanced stages of CKD (19–21). However, since investigators in these clinical trials restricted dietary protein intake to achieve a lower dietary phosphate consumption, it is unclear if the observed outcomes in these trials were solely due to changes in dietary phosphate, as opposed to altered protein intake. Moreover, not all trials have revealed a benefit of phosphate restriction on CKD progression. A single prospective study conducted by Williams et al. found no apparent effect of dietary phosphate lowering on CKD progression in a cohort of 95 patients with a variety of CKD etiologies followed for an average of 19 months post-randomization (22). Importantly, the phosphate-restricted group in this study did exhibit a reduction in urinary phosphate excretion, suggesting that their intervention was indeed effective at reducing phosphate intake. Additional studies testing the effects of phosphate binders on long-term clinical outcomes in CKD patients have demonstrated no clear benefit for either a change in estimated glomerular filtration rate (eGFR) or risk of ESKD (23); however, none of these investigations were specifically designed to study kidney function endpoints as their primary outcome. Overall, while the animal studies in this field are quite impressive, it remains difficult to draw definitive conclusions on the impact of dietary phosphate restriction based on human data alone, as these investigations suffer from potential confounding from concurrent protein restriction, seem grossly underpowered to detect more subtle effect sizes, or were of insufficient duration to detect a more long-term impact on kidney outcomes.

Serum FGF23 predicts kidney function decline

Serum FGF23 levels are increased in all forms of CKD and have been associated with increased risk for mortality in this population (24, 25). While much attention has been dedicated to the contribution of chronically elevated FGF23 to the development of cardiovascular pathology (24, 26), substantial evidence suggests a strong relationship between serum FGF23 concentrations and adverse kidney outcomes. This concept was first introduced in 2007 by Fliser et al. in a study which performed FGF23 measurements in 277 non-diabetic patients with CKD (27). An important observation from this work was that in a subset of 177 patients that were followed prospectively, individuals with serum FGF23 concentrations above 35 pg/mL were at increased risk of a doubling of serum creatinine or progression to ESKD. Similarly, in a separate investigation from the EMPATHY trial, which included a cohort of patients with diabetic retinopathy, hyperlipidemia, and an eGFR >30mL/min/1.73m² (28), baseline FGF23 levels above 53 pg/mL were associated with an increased risk for the composite outcome of doubling of serum creatinine or ESKD over a 5-year follow up period (29**). Moreover, a subsequent study revealed FGF23 to be a strong predictor of incident ESKD. In an analysis of 13,488 patients with normal baseline kidney function from the Atherosclerosis in Communities Study stratified according to baseline serum FGF23 concentrations, patients in the highest quintile (>54.6 pg/mL) were at an increased risk (HR 2.10; 95%CI 1.31–3.36, p<0.001) for incident ESKD compared to patients in the lowest quintile (<32.0pg/mL) over a median follow up time of 19 years (30). Despite compelling results from these investigations, not all studies support a positive association between serum FGF23 and adverse kidney outcomes. For example, a study by Isakova, et al. (31) showed FGF23 was not independently associated with an increased risk for incident kidney disease in a population of patients with type 2 diabetes over a median follow up of 4.7 years.

An association between FGF23 and adverse kidney outcomes has also been investigated in kidney transplant recipients and patients with autosomal dominant polycystic kidney disease (ADPKD). In a cohort of 984 kidney transplant recipients, baseline serum FGF23 was found to be an independent predictor of all-cause mortality and allograft loss over a median follow up of 37 months (32). Regarding kidney outcomes in patients with ADPKD, Chonchol et al. analyzed the association of baseline serum FGF23 and kidney disease progression in 1,002 patients with ADPKD from the HALT-PKD patient cohort (33). This investigation found higher baseline serum FGF23 was associated with a more rapid decline in eGFR, a greater increase in height-adjusted total kidney volume (htTKV), and all-cause mortality over a median follow up of 5.4 years; however, their multivariate modeling excluded several important traditional predictors of kidney function decline in ADPKD, including htTKV and PKD genotype. By contrast, a more recent study by El Ters et al. (34*) which analyzed data from 192 patients included in the Consortium for Radiologic Imaging Studies of PKD (CRISP) cohort (35) observed high serum FGF23 to similarly predict a more rapid decline in eGFR over the 13-year study period. The predictive value of FGF23 on kidney function decline in this cohort was indeed observed to be independent of both htTKV and PKD genotype.

Evidence for phosphaturia and phosphate-based crystal formation as promoters of tubular injury and kidney cyst formation

While the mechanisms whereby high dietary phosphate and high serum FGF23 may contribute to CKD progression remain largely undefined, it is noteworthy that both these clinical scenarios result in increased concentrations of tubular phosphate, suggesting that phosphaturia itself may be the direct culprit driving these adverse kidney outcomes (8–10, 27, 30). Prior studies in both animals and humans have unequivocally demonstrated that a reduction in nephron numbers confers an increased risk for kidney injury related to acute phosphate loading (9, 10, 36, 37); however, the impact of lesser degrees of chronic phosphaturia on CKD progression remains unclear. As such, there is currently no published evidence to suggest that patients with hereditary forms of chronic phosphaturia, such as X-linked hypophosphatemic rickets (XLH), are at increased risk for developing CKD. However, a recent investigation in patients with hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a condition resulting in chronic urinary phosphate wasting, did find a high prevalence of kidney cyst formation in this patient population (38).

As previously noted, increased phosphaturia in patients exhibiting a progressive loss of functional nephrons would be predicted to favor tubular precipitation of phosphate-based crystals. It has been well-established in the nephrolithiasis literature that nanocrystal formation is a potent toxin to tubular epithelial cells and is capable of triggering local inflammatory pathways that are widely implicated in the pathophysiology of kidney fibrosis (39). This concept of phosphate-based nanocrystal formation in CKD is further supported by studies describing the presence of microscopic crystals by histologic analyses performed on kidney tissue obtained from either autopsy or kidney biopsies from patients with CKD (39, 40), with calcium-phosphate crystals being the primary mineral composition observed in these studies.

Further support for a direct role of nanocrystal formation as a driver of tubular damage in CKD comes from a study by Torres et al. which provided significant evidence that tubular crystal formation could exacerbate cystic kidney disease progression in rodent models of PKD (15**). In these studies, exogenous treatments to stimulate tubular deposition of calcium-phosphate and calcium-oxalate crystals hastened cyst growth by activating mTOR signaling, an established pathway promoting cystogenesis in ADPKD. Furthermore, mTOR inhibition blunted this tubular dilation in response to crystal deposition. Additionally, in an analysis of human samples from patients with ADPKD, the authors observed an inverse correlation between urinary citrate, an endogenous inhibitor of crystal formation in tubular fluid, and cyst burden, as quantified by radiographic measurements of total kidney volume. It is also noteworthy that one of the few effective strategies proven to slow cystic kidney disease progression in humans, induction of a water diuresis by blocking vasopressin signaling in the kidney (41, 42), would also be expected to dramatically reduce tubular crystal formation by lowering the supersaturation of mineral compounds in tubular fluid.

In a recent high-impact paper, Shiizaki et al. provided important mechanistic insight into potential pathways responsible for crystal-induced tubular damage in CKD (29**). They observed that mice exposed to high phosphate diets had luminal deposition of calcium-phosphate crystals in the proximal tubule that was associated with increased expression of markers of tubular injury, inflammation, and fibrosis. Moreover, these effects were dependent on expression of toll-like receptor 4 (TLR4), as mice deficient in this receptor demonstrated no evidence of tubular crystal deposition. Additional in vitro studies conducted by this group demonstrated that prolonged exposure of tubular epithelial cells to phosphate-based crystals led to endocytosis of these crystals and the downstream activation of p38/NF-κB pathways. Interestingly, while in vivo uptake of calcium phosphate crystals was dependent on TLR4 expression, in vitro stimulation of p38/NF-κB by these crystals was independent of TLR4 activation. Taken together, these data imply that TLR4 serves a role in crystal attachment to tubular epithelial cell membranes, but separate pathways are required for crystal endocytosis and the activation of downstream inflammatory pathways.

Conclusion

The studies presented in this review highlight important scientific discoveries that have built the foundation for our current understanding of the contribution of dietary phosphate consumption and urinary phosphate excretion to tubular injury, kidney inflammation, and interstitial fibrosis. Taken together, the collective observations from these investigations implicate phosphaturia, and the resulting formation of phosphate-based crystals in tubular fluid, as unique risk factors for kidney function decline. Importantly, since increments in serum FGF23 and enhanced urinary phosphate excretion are physiologic changes that occur in all forms of advanced kidney disease, tubular phosphate handling may represent a novel universal target for preventing CKD progression. On the contrary, recent strategies to treat hyperphosphatemia in CKD rodents by enhancing urinary phosphate excretion should be received with an abundance of caution (43–45), as the evidence presented in this review suggests that such an approach may actually hasten kidney function decline in the long-term. Ongoing studies should further clarify the independent contribution of dietary phosphate, FGF23, and phosphaturia to CKD progression and better define if phosphate-based nanocrystals mediate these relationships. Developing a better understanding of these pathophysiologic mechanisms may uncover completely novel strategies for improving kidney outcomes in patients with CKD.

Key Points.

• Altered phosphate homeostasis is an important, modifiable risk factor that influences CKD progression.

• Increased urinary phosphate concentrations promote deposition of phosphate-based crystals in the kidney leading to increased inflammation, fibrosis, and kidney function decline.

• FGF23 levels predict progression of CKD and increases in renal cyst growth and could be a useful biomarker to inform clinical decision making.

• A better understanding of the mechanisms involved in phosphate-induced tubular injury may reveal novel therapeutic strategies to slow the progression of CKD.