Mitogen-activated protein (MAP) kinases comprise a family of ubiquitous proline-directed, protein-serine/threonine kinases, which participate in signal transduction pathways that control intracellular events including acute responses to hormones and major developmental changes in organisms. MAP kinases lie in protein kinase cascades. This review discusses the regulation and functions of mammalian MAP kinases. Nonenzymatic mechanisms that impact MAP kinase functions and findings from gene disruption studies are highlighted. Particular emphasis is on ERK1/2.

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We showed previously that ERK1/2 were activated by glucose and amino acids in pancreatic ␤ cells. Here we examine and compare signaling events that are necessary for ERK1/2 activation by glucose and other stimuli in ␤ cells. We find that agents that interrupt Ca 2؉ signaling by a variety of mechanisms interfere with glucose-and glucagon-like peptide (GLP-1)-stimulated ERK1/2 activity. In particular, calmodulin antagonists, FK506, and cyclosporin, immunosuppressants that inhibit the calcium-dependent phosphatase calcineurin, suppress ERK1/2 activation by both glucose and GLP-1. Ca 2؉ signaling from intracellular stores is also essential for ERK1/2 activation, because thapsigargin blocks ERK1/2 activation by glucose or GLP-1. The glucosesensitive mechanism is distinct from that used by phorbol ester or insulin to stimulate ERK1/2 but shares common features with that used by GLP-1.

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Glucose sensing is essential for the ability of pancreatic ␤-cells to produce insulin in sufficient quantities to maintain blood glucose within the normal range. Stress causes the release of adrenergic hormones that increase circulating glucose by promoting glucose production and inhibiting insulin release. We have shown that extracellular signalregulated kinases 1 and 2 (ERK1/2) are responsive to glucose in pancreatic ␤-cells and that glucose activates ERK1/2 by mechanisms independent of insulin. Here we show that glucose-induced activation of ERK1/2 is inhibited by epinephrine through the ␣ 2 -adrenergic receptor. Epinephrine and the selective ␣ 2 -adrenergic agonist UK14304 reduced insulin secretion and glucose-stimulated ERK1/2 activation in a pertussis toxin-sensitive manner, implicating the ␣ subunit of a Gi family member. ␣ 2 -adrenergic agonists also reduced stimulation of ERK1/2 by glucagon-like peptide 1 and KCl, but not by phorbol ester or nerve growth factor. Our findings suggest that ␣ 2 -adrenergic agonists act via a Gi family member on early steps in ERK1/2 activation, supporting the idea that ERK1/2 are regulated in a manner that reflects insulin demand.

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Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) possesses both growth-inhibitory and -potentiating effects on cells that are independent of IGF action and are mediated through specific IGFBP-3 binding proteins/receptors located at the cell membrane, cytosol, or nuclear compartments and in the extracellular matrix. We have here characterized transferrin (Tf) as one of these IGFBP-3 binding proteins. Human serum was fractionated over an IGFBP-3 affinity column, and a 70-kDa protein was eluted, sequenced, and identified (through database searching and Western immunoblot) as human Tf. Tf bound IGFBP-3 but had negligible affinity to the other five IGFBPs, and iron-saturated holo-Tf bound IGFBP-3 more avidly than unsaturated Tf. Biosensor interaction analysis confirmed that this interaction is specific and sensitive, with a high association rate similar to IGF-I, and suggested that binding occurs in the vicinity of the IGFBP-3 nuclear localization site. As an independent confirmation of this interaction, using a yeast two-hybrid system, we cloned Tf from a human liver complementary DNA library as an IGFBP-3 protein partner. Tf treatment blocked IGFBP-3-induced cell proliferation in bladder smooth muscle cells, and IGFBP-3-induced apoptosis in prostate cancer cells. In summary, we have employed a combination of techniques to demonstrate that Tf specifically binds IGFBP-3, and we showed that this interaction has important physiological effects on cellular events.

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