Rapid cellular responses

The lipid A moiety of LPS elicits rapid cellular responses from a number of immune responsive cells such as macrophages, lymphocytes, monocytes and endothelial... 

The multiplicity of G proteins coupled to opiate receptors may explain how different opiates can bind to the same receptor yet induce different cellular responses. For example, morphine binds to the cloned rat fi receptor expressed in HEK 293, CHO and COS-7 cells and inhibits cAMP accumulation [80-82]. Morphine can be continuously applied to the cells for up to 16 h, and the potency and magnitude of morphine inhibition of adenylyl cyclase does not diminish [80, 81]. In contrast, the opiate sufentanil can bind to the same cloned fi receptor in HEK 293 cells to inhibit cAMP accumulation. However, sufentanil s actions rapidly desensitize [83]. Since both compounds bind to the same receptor, and the fi receptor is the only receptor these drugs can interact with in these cells, the ability of these two full agonists to differentially regulate the fi receptor must be due to their abilities to affect separate adaptive processes in these cells. 

Hormonal actions on target neurons are classified in terms of cellular mechanisms of action. Hormones act either via cell-surface or intracellular receptors. Peptide hormones and amino-acid derivatives, such as epinephrine, act on cell-surface receptors that do such things as open ion-channels, cause rapid electrical responses and facilitate exocytosis of hormones or neurotransmitters. Alternatively, they activate second-messenger systems at the cell membrane, such as those involving cAMP, Ca2+/ calmodulin or phosphoinositides (see Chs 20 and 24), which leads to phosphorylation of proteins inside various parts of the target cell (Fig. 52-2A). Steroid hormones and thyroid hormone, on the other hand, act on intracellular receptors in cell nuclei to regulate gene expression and protein synthesis (Fig. 52-2B). Steroid hormones can also affect cell-surface events via receptors at or near the cell surface. 

Several serpentine receptors—including the E> adrenoceptor if it is persistently activated—instead traffic to lysosomes after endocytosis and are degraded. This process effectively attenuates (rather than restores) cellular responsiveness, similar to the process of down-regulation described above for the epidermal growth factor receptor. Thus, depending on the particular receptor and duration of activation, endocytosis can contribute to either rapid recovery or prolonged attenuation of cellular responsiveness (Figure 2-12). 

Some cellular responses occur too rapidly following steroid hormone exposure to involve the multi-step process of nuclear receptor activation. For example, 17/6-estradiol can rapidly stimulate adenylate cyclase and cause a near-instantaneous increase in intracellular cAMP in cultured prostate cells. These effects are mediated by the interaction of steroid hormones with cell surface proteins. 

Phorbol esters are tumor promoters capable of binding to and activating protein kinase C (PKC), one critical component in T cell activation.55,56 PMA is a structural analogue of diacylglycerol (DAG), an allosteric activator of PKC. PKC activation via phosphorylation leads to calcium release, resulting in a cascade of cellular responses including rapid proliferation.56 Experimentally, phorbol esters have been used as chemoattractants and to differentiate nonadherent monocytes to adherent macrophage cultures.55,57-60... 

Fig. 6.6 Signaling pathways involved in AlVmediated apoptosis. The stimulation of the AT2 receptor leads to a slow elevation in cellular ceramide, a proapoptotic messenger. The more rapid AT2 response results in the activation of a protein phosphatase, which has been identified as either a protein phosphatase 2A or MAP kinase phosphatase 1. The activated phosphatase catalyzes the dephosphorylation and inactivation of ERK1/2. Because ERK1/2 maintains the viability of Bcl-2, the inactivation of ERK1/2 leads to the dephosphorylation and degradation of the antiapoptotic factor.

Fluorescence microscopy has long been a powerful tool for cell biology as well as drug discovery research. It can measure both the contents and locations of multiple biomolecules or probes in cells simultaneously. Thus it provides unparalleled levels of detailed information about cellular responses to drug treatment or other perturbations. It has been particularly useful for rapidly validating drug leads in multiple cellular systems relevant to their efficacy and toxicity profiles. 

Concurrent with these biomethodological approaches to research has been the rapid advance of physicochemical methodology. These sophisticated techniques of detection have provided means for identifying and measuring infinitely small amounts of pesticides and other environmental toxicants and their metabolites, which may be involved in micro insults to cellular response in various species, including man. 

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