Cyclic AMP and the reverse transformation reaction

Functional effects of polyamines via activation of human β1- and β2-adrenoceptors stably expressed in CHO cells

Polyamines mediate acute metabolic effects and cardiac hypertrophy associated with β-adrenoceptor stimulation. They may also modulate β-adrenoceptors, causing functional responses in rat atria and tracheal smooth muscle. The aim of this study was to determine whether polyamines interact with human β(1)- and β(2)-adrenoceptors and the functional consequences of such an interaction. Chinese hamster ovary (CHO) cells stably transfected with human β(1)- and β(2)-adrenoceptors were used to evaluate the effect of polyamines binding to β-adrenoceptors, cAMP production and morphological changes, which were pharmacologically validated by investigating the effects of the β-adrenoceptor agonists, isoproterenol and salbutamol. Polyamines interacted with human β(1)- and β(2)-adrenoceptors, as shown by the displacement of [(125)I]iodocyanopindolol in the binding assay. Putrescine showed higher affinity to β(1)- than β(2)-adrenoceptors. Spermidine and spermine produced partial displacement (approximately 50%) and, at the highest concentration, the effect was reversed. Putrescine and spermine acutely increased cAMP and, in a serum-free medium, induced a stellate-like form in cells, which was inhibited by propranolol, a β-blocker. A 10 to 15 h incubation with putrescine produced a spindle-like form and spatial organization via β-adrenoceptor activation, evidenced by the antagonizing effect by propranolol and lack of effect in wild-type CHO cells. Additionally, it decreased cell proliferation independently of β-adrenoceptor activation. Spermine caused cell death via fetal bovine serum-dependent and -independent mechanisms. The results suggest that putrescine may act as a non-selective and low affinity agonist of human β(1)- and β(2)-adrenoceptors, eliciting morphological changes. These findings may be of importance in physiology and in diseases involving β-adrenoceptor functionality.

Protein kinase A effects of an expressed PRKAR1A mutation associated with aggressive tumors

Most PRKAR1A tumorigenic mutations lead to nonsense mRNA that is decayed; tumor formation has been associated with an increase in type II protein kinase A (PKA) subunits. The IVS6+1G>T PRKAR1A mutation leads to a protein lacking exon 6 sequences [R1 alpha Delta 184-236 (R1 alpha Delta 6)]. We compared in vitro R1 alpha Delta 6 with wild-type (wt) R1 alpha. We assessed PKA activity and subunit expression, phosphorylation of target molecules, and properties of wt-R1 alpha and mutant (mt) R1 alpha; we observed by confocal microscopy R1 alpha tagged with green fluorescent protein and its interactions with Cerulean-tagged catalytic subunit (C alpha). Introduction of the R1 alpha Delta 6 led to aberrant cellular morphology and higher PKA activity but no increase in type II PKA subunits. There was diffuse, cytoplasmic localization of R1 alpha protein in wt-R1 alpha- and R1 alpha Delta 6-transfected cells but the former also exhibited discrete aggregates of R1 alpha that bound C alpha; these were absent in R1 alpha Delta 6-transfected cells and did not bind C alpha at baseline or in response to cyclic AMP. Other changes induced by R1 alpha Delta 6 included decreased nuclear C alpha. We conclude that R1 alpha Delta 6 leads to increased PKA activity through the mt-R1 alpha decreased binding to C alpha and does not involve changes in other PKA subunits, suggesting that a switch to type II PKA activity is not necessary for increased kinase activity or tumorigenesis.

cAMP induced alterations of Chinese hamster ovary cells monitored by mass spectrometry

Chinese Hamster Ovary fibroblasts (CHO-K1) have shown different protein contents when undergoing differentiation by 3',5'-cyclic adenosine monophosphate (cAMP), which is known to induce reverse transformation (RT) from malignancy to fibroblast-like characteristics. The mass spectrometry (MS) investigation here reported about the behavior of CHO-K1 cells before and after exposure to cAMP reveals a change in the composition of nuclear proteins associated to an inhibition of the protein expression. Possible implications of this finding on the control of cell reverse transformation are discussed.

Epigenetic reversion of breast carcinoma phenotype is accompanied by changes in DNA sequestration as measured by AluI restriction enzyme

The importance of microenvironment and context in regulation of tissue-specific genes is well established. DNA exposure to or the sequestration from nucleases detects differences in higher order chromatin structure in intact cells without disturbing cellular or tissue architecture. To investigate the relationship between chromatin organization and tumor phenotype, we used an established three-dimensional assay in which normal and malignant human breast cells can be easily distinguished by the morphology of the structures they make (acinus-like versus tumor-like, respectively). We show that these phenotypes can be distinguished also by sensitivity to AluI digestion in which the malignant cells resist digestion relative to nonmalignant cells. Treatment of T4-2 breast cancer cells in three-dimensional culture with cAMP analogs or a phosphatidylinositol 3-kinase inhibitor not only reverted their phenotype from nonpolar to polar acinar-like structures but also enhanced chromatin sensitivity to AluI. By using different cAMP analogs, we show that cAMP-induced phenotypic reversion, polarization, and shift in DNA organization act through a cAMP-dependent protein-kinase A-coupled signaling pathway. Importantly, inhibitory antibody to fibronectin produced the same effect. These experiments underscore the concept that modifying the tumor microenvironment can alter the organization of tumor cells and demonstrate that architecture and global chromatin organization are coupled and highly plastic.

Interplay between exchange protein directly activated by cAMP (Epac) and microtubule cytoskeleton

"Exchange protein directly activated by cAMP" (Epac) is a newly discovered cAMP receptor that mediates the intracellular cAMP actions in addition to the classic cAMP-dependent protein kinase system. In this study, we show that Epac interacts directly with tubulin, co-purifies with cellular microtubules, and co-localizes with the mitotic spindle assembly. Association with microtubules suppresses Epac-mediated Rap1 activation, while the binding of Epac promotes microtubule formation. These results demonstrate that Epac plays an important role in connecting the microtubule cytoskeleton network and intracellular cAMP-signalling.

Useful cell lines derived from the adrenal medulla

Five approaches for the preparation of adrenal chromaffin cell lines have been developed. Initially, continuous chromaffin lines were derived from spontaneous pheochromocytoma tumors of the medulla, either from murine or human sources, such as the rat PC12 cell line and the human KNA and KAT45 cell lines. Over the last few decades, more sophisticated molecular methods have allowed for induced tumorigenesis and targeted oncogenesis in vivo, where isolation of specific populations of mouse cell lines of endocrine origin have resulted in model cells to examine a variety of regulatory pathways in the chromaffin phenotype. As well, conditional immortalization with retroviral infection of chromaffin precursors has provided homogeneous and expandable chromaffin cells for transplant studies in animal models of pain. This same strategy of immortalization with conditionally expressed oncogenes has been expanded recently to create the first disimmortalizable chromaffin cells, with an excisable oncogenic cassette, as might be envisioned for the creation of human chromaffin cell lines. Eventually, as we increase our understanding of regulating the phenotypic fate of chromaffin cells in vitro, stem or progenitor adrenal medullary cell lines will be derived as an alternative source for expansion and clinical use.

Normal cells control the growth of neighboring transformed cells independent of gap junctional communication and SRC activity

The growth of many types of cancer cells can be controlled by surrounding normal cells. However, mechanisms underlying this phenomenon have not been defined. We used a layered culture system to investigate how nontransformed cells suppress the growth of neighboring transformed cells. Direct physical contact between transformed and nontransformed cells was required for growth suppression of transformed cells in this system; communication by diffusible factors was not sufficient. However, significant gap junctional communication was not required, indicating that other intercellular junctions mediated this growth regulatory response. We also report that the Src kinase activity in transformed cells was not directly inhibited by contact with nontransformed cells. Instead, nontransformed cells increased the expression of serum deprivation-response protein and the transcription factor four and a half LIM domain 1 in tumor cells. In addition, these results suggest mechanisms by which normal cells may block Wnt signaling, inhibit insulin-like growth factor activity, and promote host recognition of neighboring tumor cells.

Antioxidant activity of melatonin in Chinese hamster ovarian cells: changes in cellular proliferation and differentiation

Melatonin is an endogenously generated molecule with free radical scavenging and antioxidant properties. Here, we studied the antiproliferative role of melatonin and other antioxidants on transformed Chinese hamster ovarian cells. Melatonin reduces cell proliferation in a dose- and time-dependent manner. Natural antioxidants which appear in edible plants including resveratrol and vitamin E mimicked the effect of melatonin. Flow cytometer analysis revealed that melatonin treatment reduces the number of cells in S-phase and increases cells in both G0/G1 and G2/M gaps. In addition, melatonin, as well as trolox, caused a clear morphological change by inducing the cells to become spindle shaped and fibroblast-like. Its effect is a reversible phenomenon that disappeared when melatonin was withdrawn from the culture medium. GSH levels are increased after melatonin treatment but pharmacologically blockade of GSH synthesis did not abolish melatonin's antiproliferative effect. Reduction of cell proliferation and the apparent induction of cell differentiation overlapped with melatonin's ability to change the intracellular redox state of CHO cells. We conclude that the cellular redox state may be involved in cellular transformation caused by antioxidants such as melatonin and trolox.