transforming-growth-factor-alpha and Vitamin-D-Deficiency

Chronic kidney disease is characterized by Vitamin D deficiency and activation of the renin-angiotensin-aldosterone system. Increasing data show that vitamin D receptor agonists (VDRAs) exert beneficial effects in renal disease and possess anti-inflammatory properties, but the underlying mechanism remains unknown. Emerging evidence suggests that "a disintegrin and metalloproteinase" (ADAM)/epidermal growth factor receptor (EGFR) signalling axis contributes to renal damage. Aldosterone induces EGFR transactivation regulating several processes including cell proliferation and fibrosis. However, data on tubular epithelial cells is scarce. We have found that, in cultured tubular epithelial cells, aldosterone induced EGFR transactivation via TGF-α/ADAM17. Blockade of the TGF-α/ADAM17/EGFR pathway inhibited aldosterone-induced proinflammatory gene upregulation. Moreover, among the potential downstream mechanisms, we found that TGF-α/ADAM17/EGFR inhibition blocked ERK and STAT-1 activation in response to aldosterone. Next, we investigated the involvement of TGF-α/ADAM17/EGFR axis in VDRA anti-inflammatory effects. Preincubation with the VDRA paricalcitol inhibited aldosterone-induced EGFR transactivation, TGF-α/ADAM-17 gene upregulation, and downstream mechanisms, including proinflammatory factors overexpression. In conclusion, our data suggest that the anti-inflammatory actions of paricalcitol in tubular cells could depend on the inhibition of TGF-α/ADAM17/EGFR pathway in response to aldosterone, showing an important mechanism of VDRAs action.

Topics: ADAM Proteins; ADAM17 Protein; Aldosterone; Cell Line; Cell Proliferation; Epithelial Cells; ErbB Receptors; Ergocalciferols; Gene Expression Regulation; Humans; Inflammation; Kidney Tubules; Receptors, Calcitriol; Renal Insufficiency, Chronic; Renin-Angiotensin System; Signal Transduction; STAT1 Transcription Factor; Transforming Growth Factor alpha; Vitamin D Deficiency

1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] is known to influence cell proliferation/maturation, whereas epidermal growth factor (EGF) is a potent stimulant of proliferation. Recently, hypocalcemia of vitamin D (D) deficiency was shown to significantly perturbe hepatic regeneration, which could be only partly restored by normalizing extracellular calcium, whereas normalization of 1,25-(OH)2D3 fully restored the process. To define the calcium- and/or D3-sensitive mechanisms associated with liver growth, a study of the initial events transduced by EGF was initiated by probing EGF receptor (EGFR) density and affinity, its subsequent autophosphorylation, and the level of its steady state transcript. Studies were carried out in D-depleted rats kept either untreated or supplemented with D3, 1,25-(OH)2D3, or calcium alone. The hepatic EGFR number (picomoles per mg microsomal protein) was significantly affected by hypocalcemic D-depleted (0.82 +/- 0.2), but responded with similar increases to calcium (1.7 +/- 0.09; P < 0.05), D3 (1.6 +/- 0.3; P < 0.05), and 1,25-(OH)2D3 (2.1 +/- 0.3; P < 0.01). The EGFR mRNA level revealed, however, no significant effect of the calcium or D3 status, indicating that posttranscriptional events were playing an important role. Phosphorylation studies showed that EGFR autophosphorylation and tyrosine protein kinase activity paralleled receptor density, with the lowest autophosphorylation values obtained in hypocalcemic D-depleted rats (D-depleted hypocalcemic vs. D3 repleted, P < 0.007). When normalized for receptor number, however, EGFR autophosphorylation increased in D-depleted hypocalcemic rats to a level comparable to that observed in all other groups. To dissociate the effect of the D3 hormone from that of calcium alone on EGFR, D-depleted rats were treated with the nonhypercalcemic 1,25-(OH)2D3 analog 22-OXA-1,25-(OH)2D3 (OCT), with or without calcium supplementation. Hypocalcemic OCT-treated rats did not exhibit any increase in EGFR number (0.6 +/- 0.1) compared to D-depleted hypocalcemic rats, but the addition of dietary calcium to OCT restored extracellular calcium concentrations and EGFR density (1.8 +/- 0.2; P < 0.002) to values comparable to those observed after D3 or 1,25-(OH)2D3 treatment. EGFR autophosphorylation was also decreased in hypocalcemic OCT-treated animals (P < 0.03), but after normalization for receptor density, full restoration of EGFR autophosphorylation was achieved. Our data demonstrate that in normal hepat

Topics: Animals; Calcitriol; Calcium; Cholecalciferol; Epidermal Growth Factor; ErbB Receptors; Female; Hypocalcemia; Liver; Male; Phosphorylation; Rats; Rats, Sprague-Dawley; RNA, Messenger; Transforming Growth Factor alpha; Vitamin D Deficiency