Cyclin and cyclin-dependent kinase expression in the remnant glomerulus.

Abstract

Recent studies suggest that subtotal renal ablation is associated with an early phase of mesangial cell proliferation. Because the mechanism(s) responsible for this response have not been elucidated, the study presented here sought to determine if changes in expression of positive cell cycle regulators, including pRb, cyclin E, and cdk2, occurred in the glomerulus during the early period of compensatory renal hypertrophy that follows 5/6 renal ablation. A first group of rats underwent sham operation and served as the control group. A second group of rats underwent 5/6 renal ablation. Ninety-six hours after subtotal ablation, protein was extracted from sieved glomeruli for Western blot analysis of proliferating cell nuclear antigen (PCNA) and retinoblastoma protein expression (pRb). RNA was extracted from sieved glomeruli for Northern blot analysis of cyclin E and cdk2 mRNA levels, and renal cortical tissue was subjected to immunohistochemical analysis of PCNA. On average, one PCNA-positive nucleus was present every 22 glomerular profiles in normal rats. The number of PCNA-positive nuclei increased fivefold in glomeruli, 96 h after renal ablation (P < 0.05). pRb was present only in the unphosphorylated state in normal glomeruli, but the increase in PCNA was accompanied by the appearance of phosphorylated pRb in remnant glomeruli. mRNA levels for cyclin E increased twofold in remnant glomeruli, whereas mRNA levels for cdk2 were unchanged. It was concluded that renal ablation leads to cell cycle progression in the glomerulus and an increase in the G1 cyclin, cyclin E, is associated with the appearance of phosphorylated retinoblastoma protein.