Dyslipoproteinemia in an inbred rat strain with spontaneous chronic progressive nephrotic syndrome

Rats of the Milan Normotensive Strain (MNS) develop a dyslipoproteinemia that is associated with a spontaneous, age-dependent and slowly progressive nephropathy characterized by proteinuria and hypoalbuminemia (nephrotic syndrome). We assumed that the MNS strain might be a suitable model for studying the features of nephrotic dyslipoproteinemia and its relationship with proteinuria, hypoalbuminemia, and hepatic apolipoprotein production. Plasma lipoproteins were investigated in MNS rats at various ages (4-48 weeks) and in another rat strain (Milan Hypertensive Strain, MHS), genetically related to MNS but free of nephropathy, that was used as control. In MNS rats, abnormal proteinuria was detectable at 20 weeks and increased 2-fold up to 34 weeks with no reduction of plasma albumin (compensated stage). During this stage we found increased levels of plasma cholesterol (+ 34%), high density lipoprotein-1 (HDL1) (+ 73%), and HDL2 (+ 31%) that were positively correlated with proteinuria but not with plasma albumin. The later stage (34-48 weeks) (nephrotic stage) was characterized by a further increase of proteinuria, moderate hypoalbuminemia (- 25%), a 2-fold increase of plasma cholesterol, triacylglycerols, low density lipoprotein (LDL), and HDL1, and a 1.2-fold increase of HDL2. In this stage the levels of LDL, HDL1, and HDL2 were positively correlated with proteinuria, and negatively correlated with plasma albumin. The most striking change in apolipoproteins was a progressive increase of the relative content of apoA-I in HDL (in 48-week-old MNS rats the A-I/E ratio was 3-fold that found in MHS rats) that was associated with a similar increase of plasma apoA-I. None of these lipoprotein changes were observed in age-matched MHS rats. At the end of the compensated stage, the hepatic levels of A-I, B, A-II, and albumin mRNA were 5.3-, 3.5-, 1.3-, and 2.0-fold, respectively, those found in age-matched MHS rats. During the nephrotic stage, albumin mRNA continued to increase, whereas A-I, B, and A-II mRNAs decreased toward the levels found in age-matched MHS rats. Thus, nephrotic dyslipoproteinemia in MNS rats starts to develop in the compensated stage before the onset of hypoalbuminemia, is characterized by an early elevation of HDL1 + HDL2, and is associated with an increased content of hepatic mRNAs of some apolipoproteins, especially apoA-I. The slow progression of nephrotic syndrome with the long-standing proteinuria and no reduction in plasma albumin renders the MNS strain the most suitable animal model for the study of the effect of proteinuria on plasma lipoprotein metabolism.