Transcriptional regulation of insulin receptor substrate 1 by protein kinase C

Insulin receptor substrate-1 (IRS-1) is involved in insulin signal transduction distal to receptor occupation. Targeted disruption of IRS-1 leads to insulin resistance and hyperglycemia in mice, which suggests that altered IRS-1 expression could contribute to the insulin resistance seen in non-insulin-dependent diabetes mellitus. In vitro studies using phorbol esters have implicated the protein kinase C (PKC) pathway as being involved in the pathogenesis of insulin resistance. Using the MCF-7 breast cancer cell, a role for PKC in regulating IRS-1 expression was examined. In an MCF- 7 cell line (MCF-7-PKC-α) that exhibits multiple alterations in PKC isoform expression, IRS-1 content was reduced to negligible levels relative to parental MCF-7 cells. This decrease in IRS-1 content was associated with a 30-fold reduction in IRS-1 transcription. In parental MCF-7 cells, PKC inhibitors (GF109203X (bisindolylmaleimide I) and staurosporine) reduced IRS- 1 content. Chronic exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA; >8 h) reduced IRS-1 content and down-regulated the novel PKC-δ isoform. Bryostatin 1 inhibited TPA-induced depletion of both IRS-1 and PKC-δ expression in MCF-7 cells. Associated with TPA-induced reduction in IRS-1 content was a reduction in IRS-1 transcription. These data demonstrate that PKC can modulate IRS-1 content and suggest a potential role for PKC-δ in positively regulating IRS-1 expression.