Membrane-hormone interaction

Very little is known about the motions of lipid bilayers at elevated pressures. Of particular interest would be the knowledge of the eifect of pressure on lateral diifusion, which is related to biological functions such as electron transport and some hormone-receptor interactions. However, pressure eifects on lateral diifusion of pure lipid molecules and of other membrane components have yet to be studied carefully. 

Fig. 12. A pictorial representation of adenylate cyclase and hormone receptors interacting with a membrane. The GTP control component is also shown. The different hormone receptors may not interact directly with the adenylate cyclase and may be diffusing freely in the membrane until a hormone is bound.

A number of other, more specialized systems are available to the reproductive toxicologist to answer specific mechanistic questions. In particular, the hormonal control of reproductive function and its perturbation by toxicants have received much attention. Such investigations can use intact cells to investigate the downstream consequences of toxicants on hormone-receptor interactions or use cells that respond to specific hormones (e.g., the MCF-7 breast cancer cell line and estrogen Soto et al., 1995). Receptor biology/ligand binding can be examined in membrane preparations of specific cell types or in recombinant cell systems (e.g., human and yeast cells Klein et al.,... 

The locus of the encounter between hormone and receptor may be extracellular, cytosolic, or nuclear, depending on the hormone type. The intracellular consequences of hormone-receptor interaction are of at least six general types (1) a change in membrane potential results from the opening or closing of a hormone-gated... 

Clearly, major differences seem suggested in the mechanisms of hormone-membrane receptor interactions leading to various post-binding events [11], For FSH, as an example, there have been reports of hormone effects on membrane potentials of cultured Sertoli cells [12] and on conductance of artificial lipid membranes [13], as well as on amino acid transport systems [14]. Indeed, there has been one report of receptor-mediated gonadotropin action without receptor occupancy... 

As indicated earlier, the intent of this section was not to be global with respect to the scope of its coverage, but rather to discuss in general terms some considerations common to the study of ligands which interact with membrane receptors and, thereby, elicit post-binding events. Many of the examples chosen have been drawn from my experience with the follitropin-gonadal receptor system, but they provide instances of problems, concerns and caveats in use of techniques and interpretation of results that are common to this particular field of study. The reader is referred to the specific examples of hormone receptor interactions to follow, wherein aspects of the problems not germaine to this section, such as, for example, techniques for purification of solubilized receptors, are considered in detail. 

There are many ways in which hormone-receptor interactions may be studied, the classic method being the Scatchard technique, named in honor of George Scatchard. This technique is applicable to any protein-small molecule interaction, and it provides a means of determining the heterogeneity of binding sites, dissociation constants, and the number of binding sites per receptor unit. The last may be a protein molecule, a cell, a cell membrane fragment, or a unit volume of cytosol with a known protein content. 

Figure 16.18 Overview of signal transmission from hormone-receptor interaction on the outside cell surface to cAMP production on the cytosol side of the membrane through stimulation of adenylate cyclase. Hs is the stimulatory hormone, and R is its receptor. Gs is the stimulatory G protein. is an inhibitory hormone, and Rj is its cell surface receptor. Gj is the inhibitory G protein. The last inhibits adenylate cyclase when combined with GTP.

Less well-defined but particularly important in terms of the function of non-neuronal cells are so-called receptor-operated channels [6,7]. By definition these are channels in the plasma membrane which open in response to hormone-receptor interaction without a change in membrane potential. The mechanism of their opening may either be by a direct coupling of receptor (possibly via a G protein) with the channel, or by an indirect coupling via the generation of an intracellular messenger such as cAMP or the putative messenger, inositol 1,3,4,5-tetrakisphosphate. 

The following polypeptide hormones each interact with receptors in the plasma membrane of their target cells. Which one triggers a signaling pathway that is directly stimulated by treatment of the cell with an inhibitor of cyclic AMP phosphodiesterase ... 

On the postsynaptic side, there are specific receptors located in the membrane on to which the transmitter binds, in a similar way to the type of hormone-receptor interaction proposed in the previous chapter. (It is also possible that cAMP is involved in the postsynaptic response to some transmitters.) The transmitter-receptor results in a change in the postsynaptic membrane structure. If the receptor is an excitatory one, this may result in an influx of Ca++ ions large enough for the postsynaptic membrane to become depolarized. If a sufficient number of synapses transmit excitatory messages to the postsynaptic nerve at around the same time, the result will be a general depolarization, and the second nerve wil 14 fire or the muscle contract. 

Very little is known about the motions of lipid bilayers at elevated pressures. Of particular interest would be the effect of pressure on lateral diffusion, which is related to biological functions such as electron transport and some hormone-receptor interactions. Pressure effects on lateral diffusion of pme lipid molecules and of other membrane components have yet to be carefully studied, however. Figure 9 shows the pressure effects on the lateral self diffusion coefficient of sonicated DPPC and POPC vesicles [86]. The lateral diffusion coefficient of DPPC in the liquid-crystalline (LC) phase decreases, almost exponentially, with increasing pressure from 1 to 300 bar at 50 °C. A sharp decrease in the D-value occurs at the LC to GI phase transition pressure. From 500 bar to 800 bar in the GI phase, the values of the lateral diffusion coefficient ( IT0 cm s ) are approximately constant. There is another sharp decrease in the value of the lateral diffusion coefficient at the GI-Gi phase transition pressure. In the Gi phase, the values of the lateral diffusion coefficient ( 1-10"" cm s ) are again approximately constant. 

Cells appear to he able to recognize cells of like kind, and thus to imite into coherent organs, principally because of specialized glycoproteins (Chap. 2) on the ceU membranes and through local hormone-receptor interactions (Chap. 6). 

It has been widely accepted that the mode of action of certain hormones is a direct interaction with enzymes. In the case of thyroid hormones, however, inconsistencies in the data are so fundamental that some investigators now seriously doubt whether a hormone-enzyme interaction does, indeed, represent their mechanism of action. J or hormonal activity to be exerted on enzymes, it has been assumed that the effect must be mediated through acceleration or inhibition of a pacemaker. The variety of enzymes affected by the in vim administration of thyroxine makes it most unlikely that altered activity of an appropriate pacemaker will, in fact, be demonstrated. Further, it has already been shown that many of the observed changes in enzyme activity are secondary to effects on vitamin metabolism, metal ions, or membranes. In addition, all the in vitro effects on enzymes have required for their demonstration concentrations of thyroxine... 

The most obvious conclusions that can be drawn from this discussion is that the nature of the hormone-oocyte interaction which initiates the several events of maturation remains unknown. Several lines of evidence, including that discussed above, implicates the oocyte surface as the initial site of interaction. Whether steroids act only at this level, however, or subsequently function within the cytoplasm is not clear and remains to be tested. Likewise, while the current evidence all points to steroid uptake by oocytes as a nonspecific process, the possible involvement of specific steroid receptors cannot be ruled out. In this regard, continued uptake studies on intact oocytes would appear to be fruitless, but the possible advantages of similar studies on isolated membranes or specific subcellular (molecular) fractions could not be more revealing. ... 

In a series of ingenious experiments on ACTH and dynorphin Schwyzer has also shown that their pharmacological potency and hydrophobic membrane interactions are critically dependent on the covalent linkage of message and address to form amphiphilic molecules. Amphiphilicity is caused in these cases by the amphiphilic primary structures, in contrast to the amphiphilicity resulting from the secondary and tertiary structure of some other peptide hormones. Such an amphiphilicity was postulated recently by Kaiser and Kezdy [32] for hormones such as insulin and -endorphin [33]. 

The first demonstration of intracellular steroid-hormone receptors was by Jensen and Jacobson (1962) for estradiol. Levinson et al. (1972) showed that hormone binding to the glucocorticoid receptor in cultured rat hepatoma cells takes place inside the cell membrane rather than at the cell surface, but did not exclude the existence of other biological actions of steroids mediated by surface membrane receptors. Indeed, Ozegovic et al. (1977) have shown that aldosterone binds to isolated kidney plasma membranes and have proposed that this steroid-membrane interaction may reflect an early event in the transmembrance movement of aldosterone. 

---