Lipoprotein metabolism in experimental nephrosis.

In experimental nephrosis, a decrease in plasma albumin resulting from proteinuria causes a decreased in the plasma oncotic pressure. The existence of an osmoreceptor, which responds to the low oncotic pressure and produces a factor(s) that signals the liver to increase the secretion of plasma proteins, is postulated. The hyperlipidemia characteristic of the nephrotic syndrome results primarily from increased hepatic secretion of apolipoproteins and lipoproteins representing the entire density spectrum from VLDL, IDL, and LDL to HDL. Not all plasma proteins and apolipoproteins are affected to the same extent. Increased mRNA levels due to increased transcription have been shown for albumin and apolipoprotein A-1 (apoA-1). The increased secretion of VLDL, the major vehicle for triglyceride transport from the liver, appears to be due mainly to posttranscriptional events possibly related to increased lipogenesis. Once proteinuria begins, the demand for amino acids for albumin and apolipoprotein synthesis by the liver is increased. To meet this demand, protein catabolism in the peripheral tissues is increased. One manifestation of this process is a decrease in lipoprotein lipase which reduces VLDL catabolism, contributing to the sustained elevation of plasma VLDL. The spectacular overproduction of apoA-1 in nephrosis in the rat is accompanied by a decreased fractional catabolic rate (FCR), contributing to the maintenance of high levels of HDL. Urinary loss of HDL and its renal catabolism does not account for the decreased FCR. The reason for the decreased FCR is not known. Work with nephrotic rats overexpressing transgenic human apoA-1 has shown that human A-1 forms smaller HDL3-sized particles, rather than the larger HDL2 of the rat. This may contribute to the failure of HDL levels to increase in the human nephrotic syndrome. High plasma VLDL and LDL with normal or low HDL probably account for the increased incidence of coronary artery disease in the nephrotic syndrome.