Dissertation committee:
Κ. Δημόπουλος Καθηγητής ΕΚΠΑ (επιβλέπων), Δημόκριτος Δ. Κλέτσας Ερευνητής Α', Ινστιτούτο Βιολογίας, ΕΚΕΦΕ , Σμ. Αντωνοπούλου Καθηγήτρια Χαροκόπειου Πανεπιστημίου
Summary:
Vitamin D3 and its analog paricalcitol, are beneficial in human and
experimental chronic kidney disease (CKD), the leading cause of which is
diabetic nephropathy (DN). Apart from their general efficacy to the treatment
of secondary hyperparathyroidism in haemodialysis patients these two molecules
also improve the inflammatory status and proteinuria in these patients. In one
approach to study the beneficial role of paricalcitol we studied its effects on
PAF biosynthesis as well as other inflammatory mediators implicated in CKD. In
a separate approach, we used immortalized human podocytes (HGEC) to assess the
expression profile of the podocytic-associated proteins podocalyxin and nephrin
after treatment with 1,25-dihydroxyvitamin D3 (calcitriol) and its analogue
paricalcitol. In vitro paricalcitol inhibited PAF/thrombin-induced platelet
aggregation and the inhibitory effect was comparable with that of PAF/thrombin
antagonists. In addition, paricalcitol inhibited in vitro the PAF-CPT activity
in platelets and leukocytes and increased PAF-AH activity in leukocytes.
However, much higher concentrations of paricalcitol were needed to inhibit λύσο-
PAF-AT activity. Similarly, in haemodialysis patients, paricalcitol treatment
reduced PAF-CPT activity in platelets and leukocytes and increased PAF-AH
activity in leukocytes, while λύσο-PAF-AT activity remained unaltered. On the
other hand paricalcitol therapy reduced the expression levels of IL-8, IL-1b,
and TNF-a and increased VEGF levels. These results further support the
beneficial effects of vitamin D treatment in haemodialysis patients, since it
strongly affects PAF/thrombin activities, PAF-metabolism and IL-8, IL-1b, TNF-a
and VEGF circulating levels. HGEC exhibit high-glucose-mediated downregulation
of podocalyxin and nephrin, loss of which has been linked with loss of the
permselective renal barrier and proteinuria. Calcitriol and paricalcitol
reversed high glucose-induced decrease of nephrin and significantly enhanced
podocalyxin expression in podocytes cultured in high glucose. HGEC express VDR
and retinoid X receptor (RXR). In the presence of calcitriol and paricalcitol
VDR expression was up-regulated and VDR co-localized with RXR in the nucleus.
VDR knock-down abolished the protective action of calcitriol and paricalcitol
on podocalyxin expression indicating that podocalyxin activation of expression
is partly mediated by VDR. Furthermore VDR specifically regulates podocalyxin
expression by bounding to a site upstream of the podocalyxin promoter. Vitamin
D analogues maintain and furthermore, re-activate the expression of specialized
components of podocytes including podocalyxin, hence they provide protection
against loss of the permselective renal barrier, with molecular mechanisms
elucidated herein.
Keywords:
Vitamin D3, Paricalcitol, Heamodialysis patients, Podocytes, Platelet activating factor
