Plasma Ca2+ regulates the synthesis and secretion of PTH
Humans have four parathyroid glands, two located on the posterior surface of the left lobe of the thyroid and two more on the right. Combined, these four glands weigh <500 mg. They are composed largely of chief cells, which are responsible for the synthesis and secretion of PTH. These cells, like cells that secrete other peptide hormones, are highly specialized to synthesize, process, and secrete their product. The major regulator of PTH secretion is ionized plasma Ca2+, although vitamin D also plays a role. Both inhibit the synthesis or release of PTH. In contrast, an increase in plasma phosphorus concentration stimulates PTH release.
PTH Synthesis and Vitamin D
The PTH gene possesses upstream regulatory elements in the 5′ region, including response elements for both vitamins D and A (see p. 90). The vitamin D response element binds a vitamin D receptor (VDR) when the receptor is occupied by a vitamin D metabolite, usually 1,25-dihydroxyvitamin D. The VDR is a member of the family of nuclear receptors, like the steroid hormone and thyroid hormone receptors. Like the thyroid hormone receptor, VDR forms a heterodimer with the retinoid X receptor (RXR) and acts as a transcription factor (see Table 3-6). The receptor has a very high affinity for the 1,25-dihydroxylated form of vitamin D (KD ≅ 10−10 M), less affinity for the 25-hydroxy form (KD ≅ 10−7 M), and little affinity for the parent vitamin (either D2 or D3, see below). Binding of the vitamin D–VDR complex to the VDR response element decreases the rate of PTH transcription.
Processing of PTH
After transport of the mature PTH messenger RNA (mRNA) to the cytosol, PTH is synthesized on ribosomes of the rough endoplasmic reticulum (RER) and begins its journey through the secretory pathway (see pp. 34–35). PTH is transcribed as a prepro-PTH of 115 amino acids (Fig. 52-6). The 25–amino-acid “pre” fragment targets PTH for transport into the lumen of the RER. This signal sequence (see p. 28) appears to be cleaved as PTH enters the RER. During transit through the secretory pathway, the 90–amino-acid pro-PTH is further processed to the mature, active, 84–amino-acid PTH. This cleavage appears quite efficient, and no pro-PTH appears in the storage granules. Conversely, the breakdown of 1-84 PTH or “intact” PTH into its N- and C-terminal fragments—as discussed below—already starts in the secretory granules.
FIGURE 52-6 PTH synthesis. The synthesis of PTH begins with the production of prepro-PTH (115 amino acids) in the RER. Cleavage of the signal sequence in the ER lumen yields pro-PTH (90 amino acids). During transit through the secretory pathway, enzymes in the Golgi cleave the “pro” sequence, yielding the mature or “intact” PTH (84 amino acids), which is stored in secretory granules. Beginning in the secretory granule, enzymes cleave PTH into two fragments. The N-terminal fragment is either 33 or 36 amino acids in length and contains all of the biological activity.
Metabolism of PTH
Once secreted, PTH circulates free in plasma and is rapidly metabolized; the half-life of 1-84 PTH is ~4 minutes. Beginning in the secretory granules inside the parathyroid chief cells and continuing in the circulation—predominantly in the liver—PTH is cleaved into two principal fragments, a 33– or 36–amino-acid N-terminal peptide and a larger C-terminal peptide (see Fig. 52-6). Virtually all the known biological activity of PTH resides in the N-terminal fragment, which is rapidly hydrolyzed, especially in the kidney. However, the half-life of the C-terminal fragment is much longer than that of either the N-terminal peptide or the intact 84–amino-acid PTH molecule. An estimated 70% to 80% of the PTH-derived peptide in the circulation is represented by the biologically inactive C-terminal fragment. The presence of a significant amount of antigenically recognized but biologically inactive C-terminal fragments in the circulation has complicated the use of the usual radioimmunoassay methods for measuring PTH in both clinical and experimental settings. This problem has been solved by the development of sensitive enzyme-linked immunosorbent assays (ELISAs) that use two antibodies that react with two distinct sites on the PTH molecule and measure only the intact PTH hormone (i.e., 84 amino acids). These assays are invaluable in the diagnosis of disorders of PTH secretion, particularly when kidney function is impaired (a circumstance that further prolongs the half-life of the inactive C-terminal metabolites). N52-1
Historical Assays for PTH
Contributed by Eugene Barrett
On pages 1059–1060, we discuss the modern methods for assaying intact PTH. In the past, clinicians exploited the effects of PTH on renal phosphate transport and renal cAMP production to assay biologically active PTH levels. Both the renal threshold (Tm) for phosphate (see p. 786 and Fig. 36-15) and the rate of urinary excretion of cAMP are readily determined in humans and are indirect measures of plasma PTH levels. In the case of cAMP, one actually measures the excretion of “nephrogenous” cAMP; that is, the total urinary cAMP minus the amount of cAMP filtered in the glomeruli.
With the advent of the newer PTH assays, these tests have largely been discarded in clinical settings.
High plasma [Ca2+] inhibits the synthesis and release of PTH
To a first approximation, and ignoring the contributions of vitamin D that are discussed below, regulation of PTH secretion by plasma Ca2+ appears to be a simple negative-feedback loop. The major stimulus for PTH secretion is a decline in the concentration of Ca2+ in the blood (hypocalcemia) and ECF. Hypocalcemia also stimulates synthesis of new PTH, which is necessary because the parathyroid gland contains only enough PTH to maintain a stimulated secretory response for several hours.
Using cultured parathyroid chief cells (which like the parathyroids in vivo respond to very small decreases in the concentration of ionized Ca2+), investigators identified a Ca2+-sensing receptor (CaSR) that resides in the plasma membrane of the parathyroid cell (Fig. 52-7A). This receptor binds Ca2+ in a saturable manner, with an affinity profile that is similar to the concentration dependence for PTH secretion. CaSR is a member of the G protein–coupled receptor (GPCR) family (see pp. 51–66). Coupling of this Ca2+ receptor to Gαq activates phospholipase C, which generates inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) and results in the release of Ca2+ from internal stores and the activation of protein kinase C (PKC; see pp. 60–61). Unlike most endocrine tissues, in which activation of these signaling systems promotes a secretory response, in the parathyroid the rise in [Ca2+]i and activation of PKC inhibit hormone secretion. (Another example is the granular cell of the juxtaglomerular apparatus, in which an increase in [Ca2+]i inhibits secretion of renin; see p. 842). Thus, increasing levels of plasma [Ca2+] decrease PTH secretion (see Fig. 52-7B).
FIGURE 52-7 PTH secretion and its dependence on ionized Ca2+ in the plasma. A, Four parathyroid glands lie on the posterior side of the thyroid. The chief cells synthesize, store, and secrete PTH. Increases in extracellular [Ca2+] inhibit PTH secretion in the following manner: Ca2+ binds to a receptor that is coupled to a heterotrimeric G protein, Gαq, which activates phospholipase C (PLC). This enzyme converts phosphoinositides (phosphatidylinositol 4,5-bisphosphate, or PIP2) to IP3 and DAGs. The IP3 causes the release of Ca2+ from internal stores, whereas the DAG stimulates PKC. Paradoxically, both the elevated [Ca2+]i and the stimulated PKC inhibit release of granules containing PTH. Increased [Ca2+]o also inhibits PTH synthesis. Thus, increased levels of plasma Ca2+lower PTH release and therefore tend to lower plasma [Ca2+]. B, Small decreases in free plasma [Ca2+] greatly increase the rate of PTH release. About half of the total plasma Ca2+ is free. In patients with familial hypocalciuric hypercalcemia (FHH), the curve is shifted to the right; that is, plasma [Ca2+] must rise to higher levels before inhibiting PTH secretion. As a result, these patients have normal PTH levels, but elevated plasma [Ca2+]. ER, endoplasmic reticulum.
The PTH receptor couples via G proteins to either adenylyl cyclase or phospholipase C
The action of PTH to regulate plasma [Ca2+] is secondary to its binding to the PTH 1R receptor (PTH1R). A second PTH receptor, PTH2R, has been identified. However, its role, if any, in the regulation of plasma [Ca2+] is uncertain. Kidney and bone have the greatest abundance of PTH1R. Within the kidney, PTH1R is most abundant in the proximal and distal convoluted tubules. In bone, the preosteoblast and osteoblast appear to be the major target cells. PTH1R is a GPCR (see pp. 51–66) that binds some N-terminal fragments of PTH, the biologically active 1-34 peptide, as well as the 1-84 intact PTH molecule (see Fig. 52-6). PTH1R also binds PTH-related protein (PTHrP), which is discussed below. In contrast, PTH2R is selectively activated by PTH.
PTH1R appears to be coupled to two heterotrimeric G proteins and thus to two signal-transduction systems. Binding of PTH to the receptor stimulates Gαs, which in turn activates adenylyl cyclase and thus releases cAMP and stimulates protein kinase A (see pp. 56–57). The activated PTH receptor also stimulates Gαq, which in turn stimulates phospholipase C (see p. 58) to generate IP3 and DAG. The IP3 releases Ca2+ from internal stores, thus increasing [Ca2+]i and activating Ca2+-dependent kinases. In humans, the bone PTH receptor on osteoblastic cells is identical to that present in renal cortical cells.
As discussed in the next two sections, the net effects of PTH on the kidney and bone are to increase plasma [Ca2+] and to lower plasma [phosphate].
In the kidney, PTH promotes Ca2+ reabsorption, phosphate loss, and 1-hydroxylation of 25-hydroxyvitamin D
PTH exerts a spectrum of actions on target cells in the kidney and bone, as illustrated for Ca2+ in Figure 52-8A and for phosphate in Figure 52-8B. In renal tubules, PTH receptors are located on the basolateral membrane. Binding of PTH to its receptors activates dual intracellular signaling systems and in this manner modifies transepithelial transport.
FIGURE 52-8 Feedback loops in the control of plasma calcium and phosphate levels. The three major regulators are PTH released by parathyroid chief cells; 1,25-dihydroxyvitamin D released by renal proximal-tubule cells; and, in the case of phosphate, FGF23 released by osteocytes. Plasma Ca2+ concentration feeds back on the parathyroid glands, whereas plasma inorganic phosphate (Pi) concentration feeds back on osteocytes.
Stimulation of Ca2+ Reabsorption
A key action of PTH is to promote the reabsorption of Ca2+ in the thick ascending limb (TAL) and distal convoluted tubule (DCT) of the kidney (see p. 789). Most of the ~25 mmol of Ca2+ filtered each day is reabsorbed in the proximal tubule (~65%) and TAL (~25%). The distal nephron is responsible for reabsorbing an additional 5% to 10% of the filtered load of Ca2+, with ~0.5% of the filtered load left in the urine. Thus, when PTH stimulates distal Ca2+ reabsorption, it greatly decreases the amount of Ca2+ excreted in the urine (usually 4 to 5 mmol/day) and tends to raise plasma [Ca2+] (see Fig. 52-8A).
Greater fractional Ca2+ excretion occurs when Ca2+ reabsorption in the proximal tubule, TAL, or DCT is impaired, as occurs with osmotic or loop diuretics or hypoparathyroidism. As discussed below, vitamin D has a synergistic action to promote Ca2+ reabsorption in the DCT.
Inhibition of Phosphate Reabsorption
PTH reduces phosphate reabsorption in the proximal tubule, producing a characteristic phosphaturia and decreasing plasma phosphate levels (see Fig. 52-8B). This phosphaturia results from a PTH-induced redistribution of Na/phosphate cotransporters (NaPi-IIa and NaPi-IIc; see pp. 785–786) away from the apical membrane of the renal proximal tubule and into a pool of subapical vesicles (see pp. 786–787). Note that fibroblast growth factor 23 (FGF23; see p. 787), released by osteocytes in response to high plasma [Pi], also inhibits NaPi-IIa and NaPi-IIc and thus phosphate reabsorption (see Fig. 52-8B). Conversely, a PTH-induced decrease in plasma [Pi] inhibits the release of FGF23, which in turn promotes Pi reabsorption, countering the action of PTH.
As discussed below, PTH stimulates bone to resorb calcium and phosphate from the hydroxyapatite crystals of bone mineral. Thus, PTH stimulates both steps in the movement of phosphate from bone to blood to urine. Elimination of phosphate via the urine plays an important role when elevated levels of PTH liberate Ca2+ and phosphate from bone. At normal plasma Ca2+ and phosphorus concentrations, the Ca × PO4 ion product approaches the solubility product. Clinically, this product is measured as total plasma calcium (in mg/dL) × plasma elemental phosphorus (in mg/dL). Further, PTH-induced increases in [Ca2+]—if accompanied by a rise in phosphate levels—would cause calcium phosphate salts to precipitate out of the soluble phase, at least partly negating the action of PTH to raise plasma [Ca2+]. Thus, PTH-induced phosphaturia diminishes the Ca × PO4 ion product and prevents precipitation when Ca2+ mobilization is needed. The body tightly regulates plasma [Ca2+] but allows plasma levels of phosphate to vary more widely.
In addition to affecting phosphate reabsorption, PTH decreases the proximal-tubule reabsorption of (see p. 829) and of several amino acids. These actions appear to play a relatively minor role in whole-body acid-base and nitrogen metabolism, respectively.
Stimulation of the Last Step of Synthesis of 1,25-Dihydroxyvitamin D
A third important renal action of PTH is to stimulate the 1-hydroxylation of 25-hydroxyvitamin D in the mitochondria of the proximal tubule. The resulting 1,25-dihydroxyvitamin D is the most biologically active metabolite of dietary or endogenously produced vitamin D. Its synthesis by the kidney is highly regulated, and PTH is the primary stimulus to increase 1-hydroxylation. Note that hyperphosphatemia elevates plasma FGF23, which inhibit this hydroxylation (see Fig. 52-8B). Conversely, hypophosphatemia, either spontaneous or induced by the phosphaturic action of PTH, also promotes the production of 1,25-dihydroxyvitamin D. As discussed on pages 1065–1067, the 1,25-dihydroxyvitamin D formed in the proximal tubule has three major actions: (1) enhancement of renal Ca2+ reabsorption, (2) enhancement of Ca2+absorption by the small intestine, and (3) modulation of the movement of Ca2+ and phosphate in and out of bone (Box 52-1).
Familial Hypocalciuric Hypercalcemia
The Ca2+ receptor on the parathyroid cell is mutated in patients with the disorder familial hypocalciuric hypercalcemia (FHH). Plasma Ca2+ level in these patients is typically elevated 10% to 30% above the normal range. These patients generally tolerate this elevated plasma [Ca2+] very well. The finding of a total plasma Ca2+ of 3.0 to 3.3 mM (12 to 13 mg/dL)—the normal range is 2.2 to 2.6 mM (8.8 to 10.6 mg/dL)—is frequently more alarming to the physician than to the patient. Despite having a plasma [Ca2+] that would make most people symptomatic with kidney stones, fatigue, or loss of mental concentration, these patients have remarkably few symptoms, presumably because they have adapted to the hypercalcemia since birth. The amount of Ca2+ present in their urine is typically less than normal (hence the designation hypocalciuric) and much lower than that encountered in persons with hypercalcemia due to other causes. Despite the high plasma [Ca2+], the plasma [PTH] in FHH patients is typically normal. The normal [PTH] suggests that regulation of PTH secretion is intact but the set-point at which the Ca2+ turns off PTH secretion is shifted toward a higher plasma [Ca2+] (see Fig. 52-7B).
FHH is an autosomal dominant disorder, and several different point mutations have been identified in different families. Heterozygotes express both normal and mutated Ca2+ receptors. Rarely, infants are born with the homozygous disorder (i.e., both copies of the receptor are defective). These infants have very severe hypercalcemia (>15 mg/dL) and severe hyperparathyroidism. The condition is life threatening and is characterized by markedly elevated plasma [Ca2+], neuronal malfunction, demineralization of bone, and calcification of soft tissues. These infants die unless the inappropriately regulated parathyroid glands are removed.
Like the parathyroid gland, the renal TAL and DCT have abundant plasma-membrane Ca2+ receptors on their basolateral membranes. These receptors respond to changes in plasma Ca2+ and inhibit Ca2+reabsorption (see p. 789). Thus, with a mutated receptor, renal Ca2+ reabsorption may not be inhibited until plasma [Ca2+] rises to abnormally high levels. The result would be the increased Ca2+ reabsorption and the hypocalciuria characteristic of FHH.
The discovery of CaSR led to the development of a CaSR agonist that mimics Ca2+ (a calcimimetic). This drug has now come into clinical use for treating patients with parathyroid cancer or hyperparathyroidism secondary to chronic renal disease. Calcimimetics decrease the secretion of PTH and secondarily decrease plasma [Ca2+].
In bone, PTH can promote net resorption or net deposition
The second major target tissue for PTH is bone, in which PTH promotes both bone resorption and bone synthesis.
Bone Resorption by Indirect Stimulation of Osteoclasts
The net effect of persistent increases of PTH on bone is to stimulate bone resorption, thus increasing plasma [Ca2+]. Osteoblasts express abundant surface receptors for PTH; osteoclasts do not. Because osteoclasts lack PTH receptors, PTH by itself cannot regulate the coupling between osteoblasts and osteoclasts. Rather, PTH acts on osteoblasts and osteoclast precursors to induce the production of several cytokines that increase both the number and the activity of bone-resorbing osteoclasts. PTH causes osteoblasts to release agents such as M-CSF and stimulates the expression of RANKL, actions that promote the development of osteoclasts (see Fig. 52-4). In addition, PTH and vitamin D stimulate osteoblasts to release interleukin-6 (IL-6), which stimulates existing osteoclasts to resorb bone (see Fig. 52-5).
One of the initial clues that cytokines are important mediators of osteoclastic bone resorption came from observations on patients with multiple myeloma—a malignancy of plasma cells, which are of B-lymphocyte lineage. The tumor cells produce several proteins that activate osteoclasts and enhance bone resorption. These proteins were initially called osteoclast-activating factors. We now know that certain lymphocyte-derived proteins strongly activate osteoclastic bone resorption, including RANKL, lymphotoxin, IL-1, and TNF-α.
Bone Resorption by Reduction in Bone Matrix
PTH also changes the behavior of osteoblasts in a manner that can promote net loss of bone matrix. For example, PTH inhibits collagen synthesis by osteoblasts and also promotes the production of proteases that digest bone matrix. Digestion of matrix is important because osteoclasts do not easily reabsorb bone mineral if the bone has an overlying layer of unmineralized osteoid.
Whereas persistent increases in PTH favor net resorption, intermittent increases in plasma [PTH] have predominately bone-synthetic effects, inducing higher rates of bone formation and mineral apposition. PTH promotes bone synthesis by three mechanisms. First, PTH promotes bone synthesis directly by activating Ca2+ channels in osteocytes, a process that leads to a net transfer of Ca2+ from bone fluid to the osteocyte. The osteocyte then transfers this Ca2+ via gap junctions to the osteoblasts at the bone surface. This process is called osteocytic osteolysis. The osteoblasts then pump this Ca2+ into the extracellular matrix, which contributes to mineralization. Second, PTH decreases the production of sclerostin by osteocytes. Lower levels of plasma sclerostin and dickkopf1 promote osteoblastic differentiation and also inhibit osteoblastic apoptosis. Third, PTH stimulates bone synthesis indirectly in that osteoclastic bone resorption leads to the release of growth factors trapped within the matrix; these include insulin-like growth factor 1 (IGF-1), IGF-2, fibroblast growth factor 2 (FGF2), and transforming growth factor-β. Finally, PTH stimulates osteoblasts to produce OPG and thereby interfere with RANKL activation of osteoclasts.
The PTH 1-34 peptide is now available as a pharmacological agent for the treatment of osteoporosis. Clinical data show marked increases in bone density—particularly within the axial skeleton—in response to injections of PTH 1-34 once or twice daily. The effects on trabecular bone are striking, with less positive responses seen in cortical bone—particularly in the limbs.