Glomerular Proteoglycans in Diabetes: Partial Structural Characterization and Metabolism of De Novo Synthesized Heparan-35SO4 and Dermatan-35SO4 Proteoglycans in Streptozocin-Induced Diabetic Rats

Abstract

The metabolism of glomerular proteoglycans was studied in an effort to understand the mechanisms leading to reduction of glomerular basement membrane (GBM) heparan sulfate (heparan-SO₄) proteoglycan in diabetes. Glomeruli were isolated from control and streptozocin-induced diabetic rats after exposure to [35S]sulfate. A pool of rapidly metabolized ³⁵S-glycosaminoglycans (GAG), predominantly heparan-³⁵SO₄, was present in GBMs from controls but not diabetics, whereas intact isolated glomeruli from the two groups contained similar quantities of ³⁵S-macromolecules after 4 and 16 h in vitro. Glomeruli from diabetics contained less ³⁵S-proteoglycan than controls after 16 h in vivo. A more rapid disappearance of [³⁵s[sulfate from serum and an increased inorganic sulfate concentration in diabetes may account for this difference. Glomeruli from diabetics contained more heparan-³⁵SO₄ and less dermatan-³⁵SO₄ proteoglycan than control glomeruli in vitro. Diabetic glomerular heparan-³⁵SO₄ proteoglycan and its GAG chains had hydrodynamic sizes similar to controls (M_r, 13 and 1.25 × 10⁴, respectively). A heparin-releasable heparan-³⁵SO₄ proteoglycan detected in isolated control glomeruli by gel electrophoresis was present in chase medium of glomeruli from diabetics in the absence of heparin. Two dermatan-³⁵SO₄ proteoglycans were synthesized in vitro. One had size and charge properties similar to glomerular heparan-₃₅SO₄ proteoglycan. A second, larger dermatan-³⁵SO₄ proteoglycan accumulated in tissue over 16 h. It was partially excluded from Sepharose CL-6B columns and eluted from Sepharose CL-4B columns at K_av = 0.32. The hydrodynamic sizes of both tissue forms of dermatan-³⁵SO₄ proteoglycan were similar in diabetics and controls. Differences in the biochemical characteristics of the major de novo synthesized glomerular proteoglycan pools could not be invoked to explain altered metabolism of GBM heparan sulfate in diabetic animals. These changes may result from diminished affinity of heparan sulfate proteoglycan for extracellular matrix or cell surfaces and may account for altered glomerular ultrafiltration properties in diabetes mellitus.