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Insulin resistance as well as pancreatic ␤-cell failure can be induced by elevated free fatty acid (FFA) levels. We studied the mechanisms of FFA-induced apoptosis in rat and human ␤-cells. Chronic treatment with high physiological levels of saturated fatty acids (palmitate and stearate), but not with monounsaturated (palmitoleate and oleate) or polyunsaturated fatty acids (linoleate), triggers apoptosis in ϳ20% of cultured RIN1046-38 cells. Apoptosis restricted to saturated FFAs was also observed in primary cultured human ␤-cells, suggesting that this mechanism is potentially relevant in vivo in humans. To further analyze FFAinduced signaling pathways leading to apoptosis, we used RIN1046-38 cells. Apoptosis was accompanied by a rapid (within 15 min) nuclear translocation of protein kinase C (PKC)-␦ and subsequent lamin B1 disassembly. This translocation was impaired by the phospholipase C inhibitor U-73122, which also substantially reduced apoptosis. Furthermore, lamin B1 disassembly and apoptosis were decreased by cell transfection with a dominant-negative mutant form of PKC-␦. These data suggest that nuclear translocation and kinase activity of PKC-␦ are both necessary for saturated fatty acidinduced apoptosis. Diabetes 52:991-997, 2003

The proteasome is a multiprotein complex involved in the degradation of ubiquitinated proteins. Three proteasome inhibitors, calpain inhibitor I, lactacystin and MG132, induced apoptosis in several human malignant glioma cell lines. Although proteasome inhibitors induced p53 accumulation in a cell line retaining wild-type p53 activity, p53 activity was dispensable for apoptosis since transdominant-negative p53 abrogated p53-dependent p21 induction but did not modulate apoptosis. Further, p21 was induced by higher concentrations of proteasome inhibitors in a p53-independent manner both in p53 wild-type and in p53 mutant cell lines. Although there was a strong G2/M arrest in response to proteasome inhibition in glioma cells, this G2/M arrest was also observed in p21^–/– colon carcinoma cells, suggesting that p21 is dispensable for the G2/M arrest associated with proteasome inhibition. Interestingly, the p21^–/– cells were more resistant to protease inhibitors than parental p21^+/+ cells. In summary, our data indicate that proteasome inhibition induces a p21-independent G2/M arrest and p53-independent apoptosis in human malignant glioma cells.

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