Genomic pharmacology: more intracellular sites for drug action

Regulation of ErbB Receptors by the Ca2+ Sensor Protein Calmodulin in Cancer

Overexpression and mutations of the epidermal growth factor receptor (EGFR/ErbB1/HER1) and other tyrosine kinase receptors of the ErbB family (ErbB2/HER2, ErbB3/HER3 and ErbB4/HER4) play an essential role in enhancing the proliferation, the migratory capacity and invasiveness of many tumor cells, leading to cancer progression and increased malignancy. To understand these cellular processes in detail is essential to understand at a molecular level the signaling pathways and regulatory mechanisms controlling these receptors. In this regard, calmodulin (CaM) is a Ca2+-sensor protein that directly interacts with and regulates ErbB receptors, as well as some CaM-dependent kinases that also regulate these receptors, particularly EGFR and ErbB2, adding an additional layer of CaM-dependent regulation to this system. In this short review, an update of recent advances in this area is presented, covering the direct action of Ca2+/CaM on the four ErbB family members mostly in tumor cells and the indirect action of Ca2+/CaM on the receptors via CaM-regulated kinases. It is expected that further understanding of the CaM-dependent mechanisms regulating the ErbB receptors in future studies could identify new therapeutic targets in these systems that could help to control or delay cancer progression.

Neurotensin in reward processes

Neurotensin (NTS) is a neuropeptide neurotransmitter expressed in the central and peripheral nervous systems. Many studies over the years have revealed a number of roles for this neuropeptide in body temperature regulation, feeding, analgesia, ethanol sensitivity, psychosis, substance use, and pain. This review provides a general survey of the role of neurotensin with a focus on modalities that we believe to be particularly relevant to the study of reward. We focus on NTS signaling in the ventral tegmental area, nucleus accumbens, lateral hypothalamus, bed nucleus of the stria terminalis, and central amygdala. Studies on the role of NTS outside of the ventral tegmental area are still in their relative infancy, yet they reveal a complex role for neurotensinergic signaling in reward-related behaviors that merits further study. This article is part of the special issue on 'Neuropeptides'.

Immunohistochemical localization of activated EGF receptor in human eccrine and apocrine sweat glands

Epidermal growth factor (EGF) is secreted into sweat from secretory cells of human sweat glands. The function of EGF in sweat is poorly understood. The biological function of EGF is exerted by the binding of EGF to the receptor (EGFR) and its activation. Therefore, we immunohistochemically localized the activated form of EGFR in human eccrine and apocrine sweat glands to assess the functional importance of the EGF-EGFR system in human sweat glands. Frozen sections of human skin were stained with a monoclonal antibody (MAb) specific for tyrosine-phosphorylated (activated) EGFR and with an MAb that stains both activated and non-activated EGFR. In the secretory portion of eccrine sweat glands, nuclei of the secretory cells were stained with the anti-activated EGFR MAb. In coiled and straight portions of eccrine sweat ducts, nuclei of luminal and peripheral cells were stained with the antibody specific for activated EGFR. Luminal cell membranes and luminal cytoplasm of inner ductal cells possessed non-activated EGFR. In the secretory portion of apocrine sweat glands, activated EGFRs were present in cytoplasm and nuclei of secretory cells. These data suggest that EGF, already known to be present in the cytoplasm of secretory cells in eccrine and apocrine sweat glands, activates EGFR in the nuclei of secretory cells themselves in an intracrine manner. Because ductal cells do not express EGF, EGF in the sweat secreted from the secretory cells should activate EGFR in the ductal cells in a paracrine manner. (J Histochem Cytochem 49:597-601, 2001)

Correlative ultrastructural distribution of neurotensin receptor proteins and binding sites in the rat substantia nigra

Neurotensin (NT) produces various stimulatory effects on dopaminergic neurons of the rat substantia nigra. To gain insight into the subcellular substrate for these effects, we compared by electron microscopy the distribution of immunoreactive high-affinity NT receptor proteins (NTRH) with that of high-affinity 125I-NT binding sites in this region of rat brain. Quantitative analysis showed a predominant association of immunogold and radioautographic labels with somata and dendrites of presumptive dopaminergic neurons, and a more modest localization in myelinated and unmyelinated axons and astrocytic leaflets. The distributions of immunoreactive NTRH and 125I-NT binding sites along somatodendritic plasma membranes were highly correlated and homogeneous, suggesting that membrane-targeted NTRH proteins were functional and predominantly extrasynaptic. Abundant immunocytochemically and radioautographically labeled receptors were also detected inside perikarya and dendrites. Within perikarya, these were found in comparable proportions over membranes of smooth endoplasmic reticulum and Golgi apparatus, suggesting that newly synthesized receptor proteins already possess the molecular and conformational properties required for effective ligand binding. By contrast, dendrites showed a proportionally higher concentration of immunolabeled than radiolabeled intracellular receptors. A fraction of these immunoreactive receptors were found in endosomes, suggesting that they had undergone ligand-induced internalization and were under a molecular conformation and/or in a physical location that precluded their recognition by and/or access to exogenous ligand. Our results provide the first evidence that electron microscopic immunocytochemistry of the NT receptor identifies sites for both the binding and trafficking of NT in the substantia nigra.

Axonal and dendritic transport of internalized neurotensin in rat mesostriatal dopaminergic neurons

Previous studies have demonstrated that neurotensin is internalized and retrogradely transported in neurons of the substantia nigra following its intracerebral injection in the neostriatum. The aim of the present study was to compare the intracellular distribution of retrogradely transported material with that observed following internalization of the peptide at the somatodendritic level and to confirm that the internalization was confined to dopamine neurons. To document somatodendritic internalization, slices (350 microns) from the rat ventral midbrain were incubated in vitro with 20 mM fluoresceinylated neurotensin, a fluorescent derivative of neurotensin, and immunostained 5-60 min later for tyrosine hydroxylase. To document retrograde transport, rats were injected with the same compound into the neostriatum and the brains processed for tyrosine hydroxylase immunohistochemistry 4.5 and 8 h later. Confocal laser microscopic examination of superfused slices revealed that fluoresceinylated neurotensin was internalized at the level of the perikarya and processes of neurons in the substantia nigra, ventral tegmental area and interfascicular nucleus. At short time intervals, the label was detected in the form of small, intensely fluorescent particles distributed within the cytoplasm of both perikarya and dendrites. At longer time intervals, these fluorescent particles were larger, less numerous and confined to the perikarya where they eventually clustered against the nucleus. Following intrastriatal injection of fluoresceinylated neurotensin, retrogradely labeled cells were apparent throughout the substantia nigra, pars compacta, as well as in the lateral part of the ventral tegmental area. Here again, the label took the form of small fluorescent particles, comparable in size, shape and distribution to those detected following superfusion of midbrain slices. In both labeling conditions, fluoresceinylated neurotensin was almost exclusively confined to tyrosine hydroxylase-immunoreactive cells. These results indicate that neurotensin is internalized throughout the terminal and dendritic arborization of mesostriatal dopamine cells and that the internalized peptide is transported centripetally from both locations to the soma of the cells. The clustering of fluorescent particles in the perinuclear region of the cells further suggests that the internalized process may play a role in the long term transcellular signalling.

Retrograde axonal transport of neurotensin in rat nigrostriatal dopaminergic neurons. Modulation during ageing and possible physiological role

Biochemical and anatomical data are reported which demonstrate for the first time the existence of a retrograde axonal transport process for a neuropeptide, neurotensin, in rat brain. Neurotensin receptors are mainly located in the striatum on nerve terminals of the nigrostriatal dopaminergic pathway. Thus, the association of specific neurotensin receptors on a well defined pathway provides an excellent model to investigate the existence of such a process. Two hours after the intrastriatal injection of iodinated neurotensin, radioactivity started to accumulate in the ipsilateral substantia nigra. The levels were maximal during the fourth hour. The appearance of this labelling was prevented by injection of a large excess of unlabelled neurotensin or of neurotensin 8-13, an active neurotensin fragment, but not by neurotensin 1-8 which had no affinity for neurotensin receptors. These results suggest that the appearance of radioactivity in the ipsilateral substantia nigra was dependent on the initial binding of this peptide to its receptors in the striatum. HPLC studies demonstrated that the radioactivity found in the substantia nigra corresponded to intact neurotensin and to degradation products of this peptide. Moreover, it has been shown that this retrograde transport was microtubule-dependent and occurred in dopaminergic nigrostriatal neurons. Light and electron microscopic data confirmed and extended the present results. Four and a half hours after intrastriatal injection of iodinated neurotensin, silver grains were mainly detected in dopaminergic perikarya of the substantia nigra pars compacta. The vast majority were associated with neuronal elements and their localization within cell bodies suggests that retrogradely transported neurotensin may be processed along a variety of intracellular pathways including those mediating recycling in the rough endoplasmic reticulum and degradation in lysosomes. However, the presence of silver grains over the nucleus, as well as the increase in tyrosine-hydroxylase mRNA expression in the ipsilateral substantia nigra 4 hr after intrastriatal injection of neurotensin support the concept that neurotensin alone, or associated with its receptor, might be involved in the regulation of gene expression. Finally, we have demonstrated that in old rats the quantity of retrogradely transported neurotensin was significantly decreased as compared to that observed in young adult rats. This retrograde axonal transport of a neuropeptide may represent, as already suggested for growth factors, an important dynamic process conveying information from nerve terminals to the cell body.

From receptor internalization to nuclear translocation. New targets for long-term pharmacology

Receptors involved in intercellular communication at the cell surface share the capacity to desensitize through molecular and cellular mechanisms. Cellular desensitization is a rapid and dynamic process whereby membrane receptors internalize in response to an excess of agonists. The internalized receptors may recycle rapidly or undergo down-regulation when following a degradative pathway. However, receptor internalization does not necessarily mean degradation; it also represents the initial step of a retrograde signalling system whereby an "interiorized" message, the ligand-receptor complex, can be transported in contrast to second messengers, along axons or in the cytoplasm leading to long-term effects in the nucleus. Such "third messengers" have to undergo nuclear translocation to serve as transcriptional regulators in the control of gene expression. The "third messengers" are thus cytoplasmic proteins, including the receptor itself, which may be associated with internalized vesicles and released by mechanisms which have not yet been elucidated. They represent already good targets for the development of new drugs, and multi-targeting and synergistic approaches are likely to increase their usefulness.

Drug-metabolizing enzymes in ligand-modulated transcription

Genes encoding many of the so-called drug-metabolizing enzymes (DMEs) are present in both prokaryotes and eukaryotes, suggesting that these genes arose on this planet more than 3.5 billion years ago--long before animal-plant divergence (estimated to be about 1.2 billion years ago) and long before the use and commercial development of drugs. What, therefore, are the real functions of DMEs? Several years ago I proposed that DMEs are upstream in the regulatory cascade of numerous signal transduction pathways, i.e. necessary for maintaining physiologically "safe", or "acceptable", steady-state levels of all small non-protein endogenous ligands (M(r) = 250 +/- 200) in each cell. Innumerable foreign chemicals and drugs mimic these small endogenous ligands, thus binding to a particular receptor and acting either as an agonist or antagonist in activating or inhibiting genes effecting growth, differentiation, apoptosis, homeostasis and neuroendocrine functions. Discussed in this review are additional examples consistent with this theory and not described in previous reviews, including: (i) insect-plant symbiosis; (ii) "cross-talk" amongst genes in the aromatic hydrocarbon-responsive [Ah] battery; (iii) signal transduction pathways involving the arachidonic acid cascade; and (iv) the explanation in carcinogen-screening studies as to why a maximum, or half maximum, tolerated dose (MTD, MTD50) of many test compounds might cause cell division and tumorigenesis in experimental animals.

Epidermal growth factor induced tyrosine phosphorylation of nuclear proteins associated with translocation of epidermal growth factor receptor into the nucleus

Treatment of human squamous carcinoma cells (HN5 cells) with epidermal growth factor (EGF) caused a time-dependent increase in tyrosine phosphorylation of six nuclear proteins of molecular mass 166, 140, 117, 95, 86 and 79 kDa. The major tyrosine phosphorylated protein was indistinguishable from the plasma membrane form of the epidermal growth factor receptor and was shown by enzyme linked immunosorbent assay (ELISA) to be translocated into the nucleus from extra-nuclear sites upon ligand stimulation. Using immunoelectron microscopy of both isolated nuclei and whole cells, epidermal growth factor receptor (EGF-R) was found to be associated with the chromatin and, to a lesser extent, with the inner surface of the nuclear membrane. Tyrosine phosphorylation of proteins other than EGF-R was particularly notable in the nucleoli. These observations suggest that EGF-R may exert some of its physiological functions by directly inducing tyrosine phosphorylation of specific nuclear proteins. Translocation of EGF-R to the nucleus may provide a vital link between plasma membrane signalling and gene activation.