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The proliferative response of IMR-90 fibroblasts at low and high population doubling level (PDL) to protein kinase C activation has been investigated to clarify whether the reduced mitogenic responsiveness of senescent cells can be ascribed to an alteration in protein kinase C signal transduction pathway. The results show that the signaling pathway leading to DNA synthesis through protein kinase C activation, appears to be modified in senescent IMR-90 human fibroblasts. High PDL fibroblasts exhibit a different sensitivity to phorbol 12-myristate 13-acetate (PMA) and dioctanoylglycerol (diC8); high glucose reduced responsiveness to PMA only in these cells. In addition, high PDL fibroblasts are characterized by an increase in diacylglycerol (DAG) cellular mass that could contribute to the different regulatory properties of the signaling pathway. On the other hand, the ability of the cyclooxygenase inhibitor indomethacin to strikingly improve the proliferative response of high PDL cells to PMA indicates that an altered overall metabolism of arachidonate may represent a crucial step in the reduced mitogenic response involving protein kinase C activation.

The involvement of the glycosylphosphatidylinositol/inositol phosphoglycan (gly-PtdIns/IPG) system in the stimulation of macromolecular syntheses in human fibroblasts has been investigated. The study demonstrates that an insulin sensitive gly-PtdIns/IPG system is present in human fibroblasts, that IPG can significantly stimulate DNA, RNA, and protein synthesis, and that the action of insulin on DNA synthesis as well as that of IPG can be significantly reduced by a specific anti-IPG antibody. These results strongly support the hypothesis that the gly-PtdIns/IPG system is involved in the signal transduction pathway leading to the stimulation of cell proliferation.

Bradykinin (BK), a peptide released during inflammatory response, has been investigated for its ability to regulate glucose metabolism in human fibroblasts. The peptide is able to significantly increase glycolytic flux in these cells. The strict relationship between the glycolytic rate and the levels of fructose 2,6-bisphosphate (Fru-2,6-P2) strongly suggests that the metabolite plays a key role in the regulation of glucose metabolism by bradykinin. The mechanism by which bradykinin increases Fru-2,6-P2 content involves the activation of 6-phosphofructo-2-kinase (PFK-2), the enzyme responsible for the synthesis of the metabolite. The study of the multiple signalling systems triggered by bradykinin demonstrates the involvement of the rise in intracellular Ca2+ concentration and of protein kinase C mediated pathway in the mechanism by which bradykinin increases Fru-2,6-P2 content and PFK-2 activity.

1. Insulin is able to stimulate lactate production and to enhance fructose 2,6-bisphosphate (Fru-2,6-P2) content in 3T3-L1 adipocytes. 2. Phorbol 12-myristate 13-acetate is more efficacious than insulin in rising Fru-2,6-P2 content and less effective in the stimulation of glycolysis. 3. 3T3-L1 adipocyte 6-phosphofructo-l-kinase appears to be very sensitive to exogenous Fru-2,6-P2. 4. Insulin treatment does not affect the maximum activity of 6-phosphofructo-1-kinase whereas it markedly increases the affinity of pyruvate kinase for phosphoenolpyruvate. 5. The role of Fru-2,6-P2 in the insulin induced enhancement of glycolytic flux is discussed.