Signal transduction mechanisms involved in hormonal Ca2+ fluxes.
This article reviews literature up to mid-1988 covering recent developments pertaining to agonist-induced Ca2+ signaling in various cell types. A large amount of experimental evidence supports a mechanism involving specific guanine nucleotide-binding proteins (G-proteins) as transducing factors between occupancy of a wide variety of receptors by many different agonists and activation of polyphosphoinositide specific phospholipase C enzymes. Although many different G-proteins and phospholipase C enzymes have been purified and cloned, successful reconstitution of the components has not been achieved. Hence, many questions concerning the specificity of coupling between particular receptors to a particular G-protein and phospholipase C subtype remain unresolved. Phospholipase C subtypes isolated from the membrane and soluble fractions of the cell are directly activated by Ca2+ and, preferentially, hydrolyse phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 4-phosphate (PIP). The role of the G-protein is to stimulate inositol lipid breakdown at free Ca2+ concentrations (0.1-0.2 microM) typical of unstimulated cells. Overwhelming evidence supports the concept that Ins 1,4,5-P3, the product of PIP2 hydrolysis, is responsible for the initial agonist-induced Ca2+ transient by mobilization of Ca2+ from a specialized intracellular store. An Ins 1,4,5-P3 receptor has been purified that may correspond to the postulated Ins 1,4,5-P3 gated Ca2+ channel. Despite a growing understanding of the complexities of the metabolism of Ins 1,4,5-P3 and a successful purification of many enzymes involved, including the ATP-dependent 3-kinase that converts Ins 1,4,5-P3 to Ins 1,3,4,5-P4, the role of Ins 1,3,4,5-P4 as a putative second messenger remains enigmatic. Multiple forms of protein kinase C have been described and the role is well established for a 1,2-diacylglycerol, the second product of PIP2 hydrolysis, as its physiological activator. Although protein kinase C has been shown to phosphorylate and modulate the activity of several proteins involved in the Ca2+ signaling pathway and Ca2+ transport, the physiological significance of the protein kinase C in agonist-stimulated cell function requires further elucidation. The extension of measurements of hormone-induced Ca2+ changes to single cells has shown that the occurrence of Ca2+ oscillations is a common phenomena. Elucidation of the biochemical mechanisms causing this oscillatory response and its physiological significance represents an important challenge for future studies.
Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia 19104.
J R
JR
Williamson
J R
JR
Monck
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D006728:Hormones / Q000502:physiology*; D006801:Humans; D015398:Signal Transduction / Q000502:physiology*
D006728:Hormones; D002118:Calcium