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In several models of progressive glomerular disease, mesangial cell proliferation,
phenotypic change and increased growth factor expression precede up-regulation of
genes for extracellular matrix components (ECM) and mesangial expansion. To examine
these events in diabetic nephropathy (DN) we conducted sequential studies of glomeruli
in rats with streptozotocin induced DN. We found prominent mesangial cell proliferation
at three days (4.34 +/- 2.24 PCNA + cells/glom vs. 1.6 +/- 0.74 in controls, P < 0.001)
associated with increased alpha-actin expression. PDGF B-chain mRNA was slightly increased
at day one, and PDGF B-chain immunostaining was slightly increased at days one and
six. Staining for bFGF was significantly increased at three days (2.2 +/- 0.6 vs.
1.2 +/- 0.1 in controls, P < 0.01). There was also an early increase in platelets
in glomeruli of diabetic animals, and platelet depletion significantly inhibited the
early phase of proliferation. In addition to mesangial cell proliferation, a prominent
glomerular macrophage infiltration began at day three and peaked at day 30 (3.94 +/-
1.47 vs. 2.08 +/- 1.13 in controls, P < 0.01). TGF-beta mRNA increased at days 14
and 30. Insulin treatment prevented mesangial cell proliferation, actin expression,
and macrophage infiltration, and normalized TGF-beta expression at 14 and 30 days.
These multiple cellular events preceded any detectable increases in glomerular gene
expression or deposition of collagen I, IV or laminin.