Insulin, glucagon and the receptor-mediated control of cyclic AMP concentrations in liver

Abstract

Over the past eight years my research interests have been primarily connected with various aspects concerning the molecular mechanisms of action of the receptors for glucagon and insulin. At the time my work started it had only recently been appreciated that integral membrane proteins were mobile entities able to diffuse, freely within the lateral plane of the bilayer. This exciting discovery led us (Houslay et al., 1976a,b, 1977, 1980a, 1983~; Houslay, 1980) and others (Schramm et al., 1977; Tolkovsky & Levitzki, 1978; Martin et al., 1979), using independent techniques, to demonstrate that hormone receptors were able to migrate independently in the membrane lipid bilayer. Thus, activation of adenylate cyclase, mediated by the stimulatory guanine nucleotide regulatory protein N,/G, (Houslay, 1983, 1984~; Gilman, 1984), was achieved through productive collisions between these components in the plane of the bilayer. The precise nature of the complexes formed between receptors, N, and adenylate cyclase, and their ‘lifetimes’, appears to vary somewhat depending upon the guanine nucleotide concentration, temperature and system under study. Thus these aspects still require further definition. That hormone-stimulated adenylate cyclases are multicomponent systems consisting of a distinct (stimulatory) receptor(s), a (stimulatory) guanine nucleotide regulatory protein (N,) and the catalytic unit of the enzyme inserted asymmetrically in the bilayer (see Houslay et al., 1980a), prompted us to define the influence that the lipid bilayer might exert on the functioning of this information transfer system in liver plasma membranes (see Houslay & Gordon, 1983; Houslay, 1985~). We were able to demonstrate that the glucagon-stimulated system, in liver plasma membranes, was strikingly sensitive to changes in the physical properties (Dipple & Houslay, 1978) and chemical composition (Houslay et al., 19766; Houslay & Palmer, 1978; Houslay et al., 1986) of the bilayer. Thus increases in bilayer fluidity were shown to activate adenylate cyclase (Dipple & Houslay, 1978; Gordon et al., 1980; Needham & Houslay, 1982; Needham et al., 1982; Whetton et al., 1982, 1983a,b,c), whereas decreases in bilayer fluidity led to inhibition of activity (Gordon et al., 1983; Whetton et al., 1984). Indeed, due to both the asymmetric disposition of the protein components and the asymmetry of the lipid bilayer (Whetton et al., 1982), selective effects can be exerted on the ‘resting’ (basal) and ‘hormone-stimulated’ activity of adenylate cyclase, hence affecting the net (fold) stimulation elicited by hormone action. This is most readily appreciated by using charged, anionic (Houslay et al., 19806) and cationic (Houslay et af. , 198 l), lipophilic drugs, which perturb the two halves of the lipid bilayer selectively with appropriate selective effects on adenylate cyclase activity. We were also able to show that cholesterol optimized the functioning of adenylate cyclase in liver plasma membranes (Whetton et al., I983a,b; Houslay, 1985~). Too high concentrations Twenty-Second Colworth Medal Lecture