Long-Term Excessive Dietary Phosphate Intake Increases Arterial Blood Pressure, Activates the Renin-Angiotensin-Aldosterone System, and Stimulates Sympathetic Tone in Mice

Controlled dietary phosphate loading in healthy young men elevates plasma phosphate and FGF23 levels

Increased dietary inorganic phosphate (Pi) intake stimulates renal Pi excretion, in part, by parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23) or dopamine. High dietary Pi may also stimulate sympathetic outflow. Rodent studies provided evidence for these regulatory loops, while controlled experiments in healthy humans examined periods of either a few hours or several weeks, and often varied dietary calcium intake. The effects of controlled, isolated changes in dietary Pi intake over shorter periods are unknown. We studied the effects of a low or high Pi diet on parameters of mineral metabolism in 10 healthy young men. Participants received a standardized diet (1000 mg phosphorus equivalent/day) supplemented with either a phosphate binder (low Pi diet) or phosphate capsules (750 mg phosphorus, high Pi diet) in a randomized cross-over trial for 5 days with a 7-day washout between diets. High Pi intake increased plasma Pi levels and 24-h excretion and decreased urinary calcium excretion. High Pi intake increased intact FGF23 (iFGF23) and suppressed plasma Klotho without affecting cFGF23, PTH, calcidiol, calcitriol, Fetuin-A, dopamine, epinephrine, norepinephrine, metanephrine, or aldosterone. Higher iFGF23 correlated with lower calcitriol and higher PTH. These data support a role for iFGF23 in increasing renal Pi excretion and reducing calcitriol in healthy young men during steady-state high dietary Pi intake. High dietary Pi intake elevated blood Pi levels in healthy young subjects with normal renal function and may therefore be a health risk, as higher serum Pi levels are associated with cardiovascular risk in the general population.

Leveraging quantitative systems pharmacology and artificial intelligence to advance treatment of chronic kidney disease mineral bone disorder

Chronic kidney disease mineral bone disorder (CKD-MBD) is a complex clinical syndrome responsible for the accelerated cardiovascular mortality seen in individuals afflicted with CKD. Current approaches to therapy have failed to improve clinical outcomes adequately, likely due to targeting surrogate biochemical parameters as articulated by the guideline developer, Kidney Disease: Improving Global Outcomes (KDIGO). We hypothesized that using a Systems Biology Approach combining machine learning with mathematical modeling, we could test a novel approach to therapy targeting the abnormal movement of mineral out of bone and into soft tissue that is characteristic of CKD-MBD. The mathematical model describes the movement of calcium and phosphate between body compartments in response to standard therapeutic agents. The machine-learning technique we applied is reinforcement learning (RL). We compared calcium, phosphate, parathyroid hormone (PTH), and mineral movement out of bone and into soft tissue under four scenarios: standard approach (KDIGO), achievement of KDIGO guidelines using RL (RLKDIGO), targeting abnormal mineral flux (RLFLUX), and combining achievement of KDIGO guidelines with minimization of abnormal mineral flux (RLKDIGOFLUX). We demonstrate through simulations that explicitly targeting abnormal mineral flux significantly decreases abnormal mineral movement compared with standard approach while achieving acceptable biochemical outcomes. These investigations highlight the limitations of current therapeutic targets, primarily secondary hyperparathyroidism, and emphasize the central role of deranged phosphate homeostasis in the genesis of the CKD-MBD syndrome.NEW & NOTEWORTHY Artificial intelligence is a powerful tool for exploration of complex processes but application to clinical syndromes is challenging. Using a mathematical model describing the movement of calcium and phosphate between body compartments combined with machine learning, we show the feasibility of testing alternative goals of therapy for Chronic Kidney Disease Mineral Bone Disorder while maintaining acceptable biochemical outcomes. These simulations demonstrate the potential for using this platform to generate and test hypotheses in silico rapidly, inexpensively, and safely.

Ablation of Vitamin D Signaling in Cardiomyocytes Leads to Functional Impairment and Stimulation of Pro-Inflammatory and Pro-Fibrotic Gene Regulatory Networks in a Left Ventricular Hypertrophy Model in Mice

The association between vitamin D deficiency and cardiovascular disease remains a controversial issue. This study aimed to further elucidate the role of vitamin D signaling in the development of left ventricular (LV) hypertrophy and dysfunction. To ablate the vitamin D receptor (VDR) specifically in cardiomyocytes, VDRfl/fl mice were crossed with Mlcv2-Cre mice. To induce LV hypertrophy experimentally by increasing cardiac afterload, transverse aortic constriction (TAC) was employed. Sham or TAC surgery was performed in 4-month-old, male, wild-type, VDRfl/fl, Mlcv2-Cre, and cardiomyocyte-specific VDR knockout (VDRCM-KO) mice. As expected, TAC induced profound LV hypertrophy and dysfunction, evidenced by echocardiography, aortic and cardiac catheterization, cardiac histology, and LV expression profiling 4 weeks post-surgery. Sham-operated mice showed no differences between genotypes. However, TAC VDRCM-KO mice, while having comparable cardiomyocyte size and LV fibrosis to TAC VDRfl/fl controls, exhibited reduced fractional shortening and ejection fraction as measured by echocardiography. Spatial transcriptomics of heart cryosections revealed more pronounced pro-inflammatory and pro-fibrotic gene regulatory networks in the stressed cardiac tissue niches of TAC VDRCM-KO compared to VDRfl/fl mice. Hence, our study supports the notion that vitamin D signaling in cardiomyocytes plays a protective role in the stressed heart.

The basics of phosphate metabolism

Phosphorus is an essential mineral that is, in the form of inorganic phosphate (Pi), required for building cell membranes, DNA and RNA molecules, energy metabolism, signal transduction and pH buffering. In bone, Pi is essential for bone stability in the form of apatite. Intestinal absorption of dietary Pi depends on its bioavailability and has two distinct modes of active transcellular and passive paracellular absorption. Active transport is transporter mediated and partly regulated, while passive absorption depends mostly on bioavailability. Renal excretion controls systemic Pi levels, depends on transporters in the proximal tubule and is highly regulated. Deposition and release of Pi into and from soft tissues and bone has to be tightly controlled. The endocrine network coordinating intestinal absorption, renal excretion and bone turnover integrates dietary intake and metabolic requirements with renal excretion and is critical for bone stability and cardiovascular health during states of hypophosphataemia or hyperphosphataemia as evident from inborn or acquired diseases. This review provides an integrated overview of the biology of phosphate and Pi in mammals.