Cardiovascular Events and Parathyroid Hormone—Suggestion of a Further Link

Abstract

Parathyroid Hormone Stimulates Endothelial Expression of Atherosclerotic Parameters through Protein Kinase Pathways.

Eberhard Ritz: Feature Editor

For a long time it has been suspected that parathyroid hormone (PTH) plays a causal role in the genesis of the high cardiovascular risk in uremia. The recent discovery, using vitamin D receptor (VDR) knockout mice, that 1,25(OH)₂ D₃ inhibits renin secretion from the juxtaglomerular apparatus (¹) and that uninhibited renin secretion in VDR knockout mice causes cardiac hypertrophy and other sequelae, as well as the observation of reduced mortality in dialysis patients on active vitamin D (²), have somewhat stolen the limelight from PTH as a potential cardiovascular culprit. The above communication comes as a timely and welcome reminder that PTH can certainly not entirely be discounted.

In their study, Rashid and colleagues studied confluent cultures (passage 2-4) of human umbilical vein endothelial cells (HUVEC), a work horse for studies on endothelial cell function, which they exposed to clinically relevant concentrations (10⁻¹² to 10⁻¹⁰ mol/L) of 1,34-PTH for 24 to 72 h. The main read-out was the receptor for advanced glycation end products (RAGE) as well as IL-6. The main finding was that 1,34-PTH at these concentrations increased the mRNA as well as the protein expression (Western blot) of RAGE. Furthermore, it increased the mRNA but not the secretion (by ELISA) of IL-6.

The authors then proceeded to determine the signaling pathways responsible for the effect of 1,34-PTH. To this end they first pretreated the culture for 30 min with the protein kinase C (PKC) inhibitor Calphostin and the protein kinase A (PKA) antagonist Rp-cAMP. Both inhibitors caused a significant decrease of RAGE mRNA and RAGE protein expression to the level in untreated cells, indicating that both the PKC and PKA pathways are involved. This is in contrast to what is seen in vascular smooth muscle cells where only the pathway via adenylate cylase and PKA is activated (³), but similar to what is seen in most other PTH target cells, including renal epithelia.

Because nitric oxide (NO) is an endothelial cell signaling substance of particular importance, the authors also pretreated the cultures with L-NAME, the inhibitor of NO synthase. Such pretreatment also inhibited RAGE mRNA and protein expression, as well as IL-6 mRNA. This finding argues in favor of a role for endothelial NO synthase (eNOS) and NO in the RAGE and IL-6 expression and is consistent with previous findings of activation of eNOS by PTH (⁴).

As a caveat it must be realized that the HUVEC model has limitations as to how far it can be generalized to apply to endothelial cells of other vascular territories.

Nevertheless, what are the implications of this finding? The relationship between cardiovascular events or mortality and intact PTH concentrations has been postulated for a long time (⁵), but the evidence remains shaky because the relationship is complex: both high and low PTH concentrations predict increased mortality depending on the accompanying changes of calcium and phosphate (⁶).

It is illustrative to have a look at primary hyperparathyroidism, where many of the confounding factors of secondary hyperparathyroidism are not operative, although minor disturbances of renal function may be a confounder in primary hyperparathyroidism as well. In a large national Swedish registry, mortality was increased in patients with primary hyperparathyroidism (⁷), and increased cardiovascular risk or mortality in patients with secondary hyperparathyroidism was found in many other studies as well (^8–10). Two recent retrospective analyses found that long-term mortality was reduced after parathyroidectomy by approximately 15% (^11,12). Long-term data on cardiovascular events and survival in patients on calcimimetics and their relatively selective lowering of PTH are eagerly awaited; cardiovascular benefit would give a new dimension to this compound (¹³). Past experimental studies have already shown lower blood pressure (¹⁴), less morphologic abnormalities of the heart, and lower lipid levels (¹⁵).

Endothelial cells express PTH/PTHrp (PTH-related protein) receptors (¹⁶), as do may other organs unrelated to mineral homeostasis (¹⁷), such as the vasculature (^18,19). In uremia the PTH/PTHrp receptor is downregulated in many organs (e.g., the heart [²⁰], the kidney [²¹] or osteoblasts [²²]). In the context of vascular disease and atherosclerosis, implied by the above authors as a potential corollary of their findings, it is of particular interest that Marti[Combining Acute Accent]n-Ventura et al. (²³), when analyzing inflammatory atherosclerotic plaques of the carotid artery by immunohistochemistry, documented high expression of PTHrp, PTH/PTHrP receptor, and the proinflammatoty chemokine monocyte chemoattractant protein 1 (MCP-1) in the unstable shoulder of the plaques. Such expression was colocalized with inflammatory cells. In vitro studies with vascular smooth muscle cells (not endothelial cells as in the above study) showed that PTHrp increased MCP-1 mRNA, an effect that was specifically inhibited by the inhibitory peptide PTHrp (^7–34), by protein kinase inhibitors, or by parthenolide, a specific the inhibitor of nuclear factor κB. Evidence of increased expression of PTHrp in lesioned vessels has also been provided by other authors (^24,25). The N-terminal fragment of PTHrp is a potent vasodilator and either stimulates or inhibits growth of vascular cells depending on the experimental conditions (^19,26). It is unresolved whether circulating 1,84-PTH (and in the above in vitro studies) acts via the type 1 PTH/PTHrp receptor just as an analog of the autocrine/paracrine local PTHrp, so to speak as a cuckoo's egg.

Of particular interest in the above study is the observation that PTH upregulated the expression of RAGE. RAGE is a multiligand receptor of the Ig superfamily of cell surface molecules and acts as a pattern recognition receptor; in other words, the receptor does not interact with a single specific ligand, but with a broad spectrum of molecules, e.g., advanced glycation products (AGE), but also with high-molecular group box 1- protein, S100/calgranulins, and amyloid-β peptides (^27,28). RAGE also serves as an endothelial adhesion receptor for leukocyte integrins and promotes leukocyte recruitment and extravasation. Activation of RAGE increases the activity of proinflammatory nuclear factor κB, the long-acting inflammatory master switch. It has been argued that this mechanism converts transient cellular stimuli into sustained cellular dysfunction (^27,28). This link between RAGE and inflammation has been summarized in several reviews (²⁹). In our study, expression of RAGE was increased in endothelial cells of nondiabetic uremic patients (³⁰).

This novel potential link between PTH and vascular injury is definitely of interest, but when assessing the impact of PTH on cardiovascular risk, other PTH-induced abnormalities must also be considered, e.g., the effects on cardiomyocytes with increased heart rate, calcium uptake, mitochondrial malfunction, and necrosis of cardiomyocytes (^31,32), as well as activation of interstitial cardiac fibroblasts (³³) and thickening of postcoronary arteries (³⁴), not to forget indirect effects, e.g., on tissue and vascular calcification.

Although the study, like all good studies, raises more questions than it answers, it is certain to stimulate rethinking of entrenched opinions on this subject.