C-peptide stimulates ERK1/2 and JNK MAP kinases via activation of protein kinase C in human renal tubular cells

Accumulating evidence indicates that replacement of C-peptide in type 1 diabetes ameliorates nerve and kidney dysfunction, but the molecular mechanisms involved are incompletely understood. C-peptide shows specific binding to a G-protein-coupled membrane binding site, resulting in Ca²⁺ influx, activation of mitogen-activated protein kinase signalling pathways, and stimulation of Na⁺, K⁺-ATPase and endothelial nitric oxide synthase. This study examines the intracellular signalling pathways activated by C-peptide in human renal tubular cells.

Human renal tubular cells were cultured from the outer cortex of renal tissue obtained from patients undergoing elective nephrectomy. Extracellular-signal-regulated kinase 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK) and Akt/protein kinase B (PKB) activation was determined using phospho-specific antibodies. Protein kinase C (PKC) and RhoA activation was determined by measuring their translocation to the cell membrane fraction using isoform-specific antibodies.

Human C-peptide increases phosphorylation of ERK1/2 and Akt/PKB in a concentration- and time-dependent manner in renal tubular cells. The C-terminal pentapeptide of C-peptide is equipotent with the full-length C-peptide, whereas scrambled C-peptide has no effect. C-peptide stimulation also results in phosphorylation of JNK, but not of p38 mitogen-activated protein kinase. MEK1/2 inhibitor PD98059 blocks the C-peptide effect on ERK1/2 phosphorylation. C-peptide causes specific translocation of PKC isoforms δ and ɛ to the membrane fraction in tubular cells. All stimulatory effects of C-peptide were abolished by pertussis toxin. The isoform-specific PKC-δ inhibitor rottlerin and the broad-spectrum PKC inhibitor GF109203X both abolish the C-peptide effect on ERK1/2 phosphorylation. C-peptide stimulation also causes translocation of the small GTPase RhoA from the cytosol to the cell membrane. Inhibition of phospholipase C abolished the stimulatory effect of C-peptide on phosphorylation of ERK1/2, JNK and PKC-δ.

C-peptide signal transduction in human renal tubular cells involves the activation of phospholipase C and PKC-δ and PKC-ɛ, as well as RhoA, followed by phosphorylation of ERK1/2 and JNK, and a parallel activation of Akt.