Protein Kinase C-γ Activation in the Early Streptozotocin Diabetic Rat Lens

Abstract

Purpose: The purpose of this study is to demonstrate the early activation of the protein kinase C-γ (PKC-γ) pathway in the streptozotocin (STZ)-induced diabetic rat lens. Methods: Twelve-week-old male and female Sprague-Dawley rats were injected with 80 mg/kg (body weight) of STZ (N-[methylnitrosocarbamoyl]-D-glucosamine) intraperitoneally. Very high glucose (VHG) diabetes was defined as a nonfasting blood glucose level of at least 450 mg/dl, confirmed by daily monitoring with Accu-Check Advantage test strips, and occurred about 2 weeks after STZ administration. All assayed lenses were from VHG or age-matched control rats, harvested within 24 hr of VHG detection. PKC-γ activation was measured by enzyme activity assay and by Western blotting to show autophosphorylation on Thr514. Cellular insulin-like growth factor-1 (IGF-1), PKC-γ phosphorylation of Cx43 on Ser368, and activation of phospholipase C-γ 1 (PLC-γ 1), extracellular signal-regulated kinase (ERK1/2), and caspase-3 were determined by Western blotting. Endogenous diacylglycerol (DAG) levels were measured with a DAG assay kit. Lens gap junction activity was determined by the microinjection/Lucifer yellow dye transfer assay. Electron microscopy was applied to affirm fiber cell damage in the VHG diabetic lenses. Results: In the lenses of VHG diabetic rats, PKC-γ enzyme was activated. PKC-γ could be further activated by 400 nM phorbol-12-myristate-13-acetate (PMA), but the PKC-γ protein levels remained constant. No elevation of IGF-1 level was observed. Western blots showed that activation of PKC-γ may be due to activation of PLC-γ 1, which synthesized endogenous DAG, a native PKC activator. The level of PKC-γ –catalyzed phosphorylation of Cx43 on Ser368 and resulting inhibition of lens gap junction dye transfer activity was increased in the VHG diabetic lenses. At this early time period, the diabetic lens showed no activation of either caspase-3 or ERK1/2. Only a single fiber cell layer deep within the cortex (∼ 90 cell layers from capsule surface) showed vacuoles and damaged cell connections. Conclusions: Early activation of PLC-γ 1 and elevated DAG were observed within VHG diabetic lenses. These were correlated with activation of PKC-γ, phosphorylation of Cx43 on Ser368, and inhibition of dye transfer. Abnormal signaling from PKC-γ to Cx43 in the epithelial cells/early fiber cells, observed within VHG diabetic lenses, may be responsible for fiber cell damage deeper in the lens cortex.

KEYWORDS:

• diacylglycerol
• gap junctions
• insulin-like growth factor-1
• protein kinase C-γ
• streptozotocin diabetic rat