Summary: | 博士 === 高雄醫學院 === 醫學研究所 === 86 === Diabetes mellitus and renal disease is the 5th and 7th leading
cause of death in Taiwan. Hence, they consume a major portion of
medical resources in our country. Furthermore, diabetic
nephropathy is a major cause of diabetic morbidity and
mortality. Unfortunately, diabetic patients will almost always
enter end-stage renal disease (ESRD) once they develop
proteinuria. Therefore, the study for the pathogenesis of
diabetic nephropathy has become a major topic in biomedical
research.Hyperglycemia and advanced glycation end-product (AGE)
are two of the essential factors in diabetic nephropathy. The
pathology of diabetic nephropathy is characterized by cellular
hyperplasia, hypertrophy and the expansion of extracellular
matrix which result in renal fibrosis and ESRD. These processes
are intimately associated with cytokines/growth factors, esp.
transforming growth factor-b (TGF-b). The distal nephron is also
important in diabetic nephropathy, although most studies
regarding diabetic nephropathy were focused on glomerulopathy
and occasionally proximal tubule. Therefore, we performed a
series of studies in high glucose and AGE-cultured distal
tubule-like MDCK cells. We found that, unlike other renal cells,
high glucose did not increase TGF-b production, but it did
increase the responsiveness of MDCK cells to TGF-b, which
includes: inhibition of cellular mitogenesis, induction of
cellular hypertrophy, increase of cell cycle-regulatory
retinoblastoma protein (pRb) dephosphorylation and inhibition of
cdc2 kinase activity. Affinity-labeling experiments showed that
high glucose may increase TGF-b responsiveness by increasing
type I and II TGF-b receptor protein expressions. This is the
first demonstration that distal tubule is unique in that it
responds to high glucose by increasing TGF-b (which may be
derived from paracrine sources) responsiveness but not the
production of endogenous TGF-b.Regarding the roles of
intracellular signal transduction pathways in diabetic
nephropathy, protein kinase C (PKC) had been shown to be
important. However, the roles of various PKC isoenzymes in
diabetic tubulopathy is still not known. Thus, we showed that
high glucose induced PKC activation, PKCi and PKCe activation,
cytosolic translocation of PKCi(l) and membrane translocation of
PKCe. As for the roles of various transcription factors, only
AP-1 had been suggested to be involved in diabetic
glomerulopathy. Thus, we first showed that high glucose induced
activation of transcription factors AP-1 and NF-kB in MDCK cells
concomitantly with the induction of type II TGF-b receptor mRNA.
We speculate that the above changes in the signal transduction
pathways may be involved in the induction of type II TGF-b
receptor mRNA, although this speculation awaits further
confirmation.Regarding the effects of AGE, we found that, unlike
high glucose, AGE inhibited cellular mitogenesis while inducing
cellular hypertrophy in the MDCK cells. Moreover, AGE induced
the production of bioactive TGF-b in these cells. Importantly,
neutralizing anti-TGF-b1 antibody reversed the above AGE-induced
effects. Therefore, endogenous TGF-b1 may mediate the above AGE-
induced effects in the MDCK cells.Our experiments showed that
distal tubular cells behave differently from glomerular and
proximal tubular cells in high glucose and AGE cultures. Whereas
AGE induced bioactive TGF-b, high glucose only induced the
expression of type I and II TGF-b receptors. We conclude that
the complex interaction between high glucose, AGE, TGF-b, TGF-b
receptors and cell cycle-regulatory proteins (pRb and cdc2) may
play important roles in diabetic nephropathy. In addition, PKCi(
l), PKCe, AP-1 and NF-kB may mediate some of the above high
glucose and AGE-induced effects.
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