EGFR Activation Increases Parathyroid Hyperplasia and Calcitriol Resistance in Kidney Disease

Abstract

Calcitriol, acting through vitamin D receptors (VDR) in the parathyroid, suppresses parathyroid hormone synthesis and cell proliferation. In secondary hyperparathyroidism (SH), VDR content is reduced as hyperplasia becomes more severe, limiting the efficacy of calcitriol. In a rat model of SH, activation of the EGF receptor (EGFR) by TGF-␣ is required for the development of parathyroid hyperplasia, but the relationship between EGFR activation and reduced VDR content is unknown. With the use of the same rat model, it was found that pharmacologic inhibition of EGFR activation with erlotinib prevented the upregulation of parathyroid TGF-␣, the progression of growth, and the reduction of VDR. Increased TGF-␣/EGFR activation induced the synthesis of liver-enriched inhibitory protein, a potent mitogen and the dominant negative isoform of the transcription factor CCAAT enhancer binding protein-␤, in human hyperplastic parathyroid glands and in the human epidermoid carcinoma cell line A431, which mimics hyperplastic parathyroid cells. Increases in liver-enriched inhibitory protein directly correlated with proliferating activity and, in A431 cells, reduced VDR expression by antagonizing CCAAT enhancer binding protein-␤ transactivation of the VDR gene. Similarly, in nodular hyperplasia, which is the most severe form of SH and the most resistant to calcitriol therapy, higher TGF-␣ activation of the EGFR was associated with an 80% reduction in VDR mRNA levels. Thus, in SH, EGFR activation is the cause of both hyperplastic growth and VDR reduction and therefore influences the efficacy of therapy with calcitriol.

Activator Protein 2α Mediates Parathyroid TGF-α Self-Induction in Secondary Hyperparathyroidism

“…In the course of CKD, however, progressive reductions in parathyroid vitamin D receptor, induced by increasing TGF-␣/EGFR-driven signaling, antagonize calcitriol actions in the parathyroid glands, generating a feed-forward loop for enhanced AP2/ TGF-␣ levels and calcitriol resistance. 18 The identification of a similar association between AP2 and TGF-␣ levels in human hyperplastic glands from patients with CKD suggests that AP2-mediated TGF-␣ self-induction also contributes to the progression of human SH. Indeed, parathyroid AP2 levels strongly correlated with the severity of hyperplastic growth.…”

“…To overcome this limitation, we used A431 cells, a validated model of TGF-␣ self-induction. 18 Similar to hyperplastic parathyroid cells, TGF-␣ overexpression drives A431 cell growth. 22 In these cells, a 10-bp mutation at the core AP2 binding site of the human TGF-␣ promoter was sufficient to reduced markedly TGF-␣ induction of its own gene despite TGF-␣-induced increases in AP2.…”

“…11,12 More significant, suppression of TGF-␣ activation of the EGFR by erlotinib, a potent and highly specific EGFR-tyrosine kinase inhibitor, arrested not only parathyroid growth but also TGF-␣ selfupregulation. 17,18 These findings underscore the importance of controlling parathyroid TGF-␣ expression to ameliorate the progression of SH. The goal of these studies was the identification of the pathogenesis of the elevated parathyroid TGF-␣ of experimental and human SH.…”

“…As expected for a mediator of TGF-␣ self-upregulation, parathyroid AP2 levels, in whole-cell extracts from human parathyroid glands, strongly correlated with PCNA, a marker of the severity of the hyperplasia ( Figure 4A). Because parathyroid TGF-␣ expression and growth rates differ markedly between diffuse and nodular hyperplasia, 18 we simultaneously assessed AP2 and TGF-␣ expression using immunofluorescence staining in areas of diffuse and nodular growth within the same parathyroid gland ( Figure 4A). Histologically, diffuse hyperplasia was defined by areas with increased number of parenchymal cells with normal lobular structures.…”

Cyclooxygenase 2 Promotes Parathyroid Hyperplasia in ESRD

“…15,16 Recently, EGF receptor (EGFR) activation by TGF-␣ has also been suggested to play an important role in the growth of the PTG and the downregulation of VDR, which may be associated with vitamin D resistance in advanced SHPT in ESRD. 17 EGFR activation may lead to activation of cyclin D1, an important contributor to parathyroid hyperplasia in humans. 18 Cyclooxygenase (COX) catalyzes the rate-limiting step in the synthesis of prostaglandins (PGs) from arachidonic acid (AA).…”