Studies of the mitogen-activated protein kinases and phosphatidylinositol-3 kinase in the lens. 1. The mitogenic and stress responses.

The current understanding of the cellular signal transduction system is that cells initially respond to outside stimuli, such as growth factors or neurotransmitters, through ligand binding to the respective growth factor receptors or the G-protein-coupled receptors, to initiate transduction of the stimulus. This is followed by a series of association-dissociation and phosphorylation-dephosphorylation processes among the components of a well-defined and intricate infrastructure between the cell membrane and the cytosolic protein kinases to activate and initiate nuclear target genes for cell proliferation, differentiation and other cellular functions. Although some past reports have indicated this signaling machinery is present in the lens, certain pathways, namely the mitogen-response pathway (Raf-MEK-ERK cascade), the stress-response pathways (p38 and SAPK/JNK cascades) and the survival pathway (PI-3K-Akt), have not been thoroughly explored in an intact lens. These pathways were studied using porcine lenses cultured under mitogenic (10 ngml(-1) growth factor) or osmotic stress (30 mM galactose) conditions to examine the cellular response in the epithelial layer, using unstimulated lenses as controls. It was found that all the key members in the Raf-MEK-ERK cascade and PI-3K-Akt cascade were present and that growth factors had a differential stimulatory effect on them. Basic-FGF was the most potent stimulator for ERK followed by EGF and IGF-1, while PDGFab and VEGF were less active. The opposite was true for their stimulatory effect on PI-3K. Hyperglycemic-induced osmotic stress stimulated p38 but not SAPK/JNK, while bFGF could stimulate SAPK/JNK but not p38. Both stimuli activated the Raf-MEK-ERK and PI-3K-Akt pathways. Osmotic-induced activation could be normalized using an aldose reductase inhibitor.