Influence of pentoxifylline on natural cytotoxicity and expression of granzymes and PI-9, a specific granzyme B inhibitor.

Pentoxifylline (PTX) is an unspecific inhibitor of phosphodiesterase activity that increases intracellular concentration of cyclic nucleotides, mainly cAMP. Since PTX improves microcirculatory blood flow, it is commonly and often chronically used in peripheral vascular diseases. On the other hand PTX also displays a variety of immunomodulatory activities. PTX inhibits natural cytotoxicity and it has previously been suggested that it could partially act also through its influence on perforin/granzyme-dependent pathways. However, the underlying mechanisms are obscure and it remains unknown whether PTX inhibits natural cytotoxicity influencing only leukocytes or also acting on target cells. In this study, we show that PTX inhibits expression of granzyme A in human leukocytes probably due to suppression of phosphodiesterase activity. Contrary, PTX does not affect expression of granzyme B and H. On the other hand we hypothesized that PTX could inhibit natural cytotoxicity not only affecting leukocytes but also due to generation of resistance to leukocyte-mediated cytotoxicity in target cells e.g. through overexpression of PI-9, a specific granzyme B inhibitor. We found that at the mRNA level, PTX stimulates expression of PI-9 in K562 cells. However, we did not observe such an influence at the protein level, in either K562 cells or in human leukocytes. It may suggest that other PTX-triggered molecular events may interfere with PI-9 overexpression in these cells at the further, post-transcriptional levels. According to these results, PTX did not affect resistance of target cells to natural cytotoxicity. Altogether, PTX inhibits natural cytotoxicity affecting mainly effector but not target cells and in case of the effector cells, besides previously reported mechanisms, it can also inhibit granzyme A expression.

Influence of pentoxifylline on perforin expression in human PBMC.

Pentoxifylline (PTX) is a methylxanthine derivative that unspecifically inhibits phosphodiesterase activity and thus, it increases intracellular concentration of cyclic nucleotides. Currently, PTX is commonly and chronically used in peripheral blood vessel diseases. Besides its well-known influence on rheologic properties of blood, PTX has also been found to decrease secretion of some cytokines such as IL-12, TNF and IFN-gamma and thus it could exert immunomodulatory activity. Furthermore, PTX inhibits lymphocyte cytotoxicity affecting the perforin-dependent pathway, both in humans and animals. It has also been shown recently that in some murine models, PTX promotes tumor growth. Such a phenomenon, at least partially, could result from PTX-dependent inhibition of natural cytotoxicity. However, the detailed mechanism of PTX influence on cytotoxic activity in humans has not been established. We hypothesized that PTX-dependent inhibition of natural cytotoxicity could result from decrease in perforin expression. In this study, it was shown that pentoxifylline only moderately inhibits perforin expression at the mRNA level in human peripheral blood mononuclear cells (PBMC), and this effect seems to be independent of intracellular cAMP concentration. On the other hand, PTX did not significantly influence perforin expression at the protein level. These results suggest that in humans, contrary to murine models, inhibition of perforin-dependent natural cytotoxicity through pentoxifylline does not result from changes in perforin production. Presumably, influence of PTX on natural cytotoxicity could be caused by e.g. interference with lymphocyte degranulation. Moreover, also other possibilities, such as PTX influence on conformational changes of perforin molecules and/or its influence on susceptibility of target cells to pore-forming of polyperforins cannot be excluded.

Differential influence of pentoxifylline on murine colon adenocarcinoma- and melanoma-derived metastatic tumor development in lungs.

Pentoxifylline (PTX), a methylxanthine derivative, is commonly and in most cases chronically used in patients suffering from peripheral vascular diseases. It increases erythrocyte flexibility, reduces blood viscosity and improves microcirculatory flow. Moreover, PTX has been found to enhance anticancer activity of some chemotherapeutic agents and ionizing irradiation. On the other hand, PTX has been recently shown to inhibit anticancer response and promote murine tumor growth in liver. In this study we show that PTX facilitates development of murine colon adenocarcinoma- but not melanoma-derived tumors also in lungs. It could suggest that tumor-promoting PTX activity is tissue-independent, although, it might depend on the cancer cell line.

Pentoxifylline promotes development of murine colon adenocarcinoma-derived metastatic tumors in liver.

Pentoxifylline is commonly used in the treatment of peripheral vascular diseases. It improves microcirculatory flow and tissue perfusion. Moreover, pentoxifylline displays some immunomodulatory properties that presumably might affect the anticancer response. Therefore, the aim of the present study was to evaluate the influence of pentoxifylline on tumor development. Balb/c mice were injected with murine colon adenocarcinoma C-26 cells intravenously, into the vena portae, and divided into two groups. Mice from the experimental groups received daily intraperitoneal injections of pentoxifylline (30 mg/kg) while the controls were injected with 0.9% NaCl. Two weeks after C-26 cell inoculation mice were sacrificed and autopsy was performed. It was found that the livers of control animals revealed only several small tumor foci, whereas the livers of pentoxifylline-treated mice displayed numerous cancerous outgrowths. The mean liver weight in pentoxifylline group was 2.21+/-0.62 g as compared to 1.36+/-0.15 g in controls (P=0.004). Moreover, the influence of pentoxifylline on murine and human cell line proliferation in vitro was evaluated. It has been observed that pentoxifylline, at pharmacologically achievable concentrations, stimulated the proliferation of murine (C-26) and human (CaSki, U-937) cell lines. However, it did not stimulate human melanoma (WM-35) cell proliferation. Since there has been no evidence so far that pentoxifylline may promote tumor progression, it is still considered to be a safe drug. Moreover, some beneficial properties of pentoxifylline, which could be useful in cancer treatment, have been reported and a few clinical trials with oncological patients have been performed. Surprisingly, our study revealed that pentoxifylline significantly promoted C-26-derived metastatic tumor growth in liver. Although this model might be unique in its sensitivity to tumor-promoting effects of pentoxifylline, it cannot be excluded that similar effects might occur in some cases of tumors developing in humans. Such effects could be relevant, since the stimulatory influence of pentoxifylline on proliferation does not appear to be species- or tissue-specific.

Ghrelin: a recently discovered gut-brain peptide (review).

In the 70s, several new, both peptidyl and non-peptidyl, derivatives that stimulate and amplify pulsatile growth hormone (GH) secretion, independently from growth hormone releasing hormone (GHRH), were synthesized. The family of these molecules have been named growth hormone secretagogues (GHSs). The subsequent discovery of the natural receptor for GHSs (GHS-R) suggested existence of a new regulatory system, participating in GH secretion control. Three years later a natural ligand for GHS-R was identified and was designated 'ghrelin'. Subsequently, it has been found that ghrelin exerts pleiotropic activity. It influences not only GH release but also food intake, function of gastrointestinal tract and cardiovascular system, sleep patterns as well as cancer proliferation. The current knowledge on ghrelin, its structure and function, is reviewed in this article.