New drug delivery strategies targeting the GnRH receptor in breast and other cancers.

Cancer is the uncontrolled division of abnormal cells in a specific organ. Globally, about one in six deaths is due to cancer. Despite the plethora of research being undertaken worldwide to find a cure for cancer, it remains a significant challenge. Cancer targeting via agents designed to interfere with some specifically or highly expressed molecules in cancer cells has been a shift in the treatment of various forms of cancers. The development of drug delivery systems, specifically to cancer cells, is a common approach that succeeded in increasing the efficacy and reducing the side effects of different anticancer agents. Gonadotropin-releasing hormone (GnRH) is a naturally occurring hormone with receptors overexpressed in many types of cancers related or unrelated to the reproductive system. Several drug delivery systems were developed using GnRH derivatives as targeting agents. In this review, we first discuss the role of GnRH and its receptors in cancer. Then, we provide a detailed insight into different delivery systems developed using GnRH derivatives as targeting agents in various types of GnRH receptor overexpressing cancers. Some promising findings from these studies indicate that GnRH receptor targeting is a potential strategy to efficiently guide anticancer therapeutics, diagnostic agents, and nucleic acids directly to cancer cells. Lastly, some limitations of the current research and suggestions for more successful outcomes in clinical trials of these delivery systems are highlighted.

Effect of fluoroquinolones on mitochondrial function in pancreatic beta cells.

Hyper- and hypoglycaemias are known side effects of fluoroquinolone antibiotics, resulting in a number of fatalities. Fluoroquinolone-induced hypoglycaemias are due to stimulated insulin release by the inhibition of the KATP channel activity of the beta cell. Recently, it was found that fluoroquinolones were much less effective on metabolically intact beta cells than on open cell preparations. Thus the intracellular effects of gatifloxacin, moxifloxacin and ciprofloxacin were investigated by measuring NAD(P)H- and FAD-autofluorescence, the mitochondrial membrane potential, and the adenine nucleotide content of isolated pancreatic islets and beta cells. 100 µM of moxifloxacin abolished the NAD(P)H increase elicited by 20mM glucose, while gatifloxacin diminished it and ciprofloxacin had no significant effect. This pattern was also seen with islets from SUR1 Ko mice, which have no functional KATP channels. Moxifloxacin also diminished the glucose-induced decrease of FAD-fluorescence, which reflects the intramitochondrial production of reducing equivalents. Moxifloxacin, but not ciprofloxacin or gatifloxacin significantly reduced the effect of 20mM glucose on the ATP/ADP ratio. The mitochondrial hyperpolarization caused by 20mM glucose was partially antagonized by moxifloxacin, but not by ciprofloxacin or gatifloxacin. Ultrastructural analyses after 20 h tissue culture showed that all three compounds (at 10 and 100 µM) diminished the number of insulin secretory granules and that gatifloxacin and ciprofloxacin, but not moxifloxacin induced fission/fusion configurations of the beta cell mitochondria. In conclusion, fluoroquinolones affect the function of the mitochondria in pancreatic beta cells which may diminish the insulinotropic effect of KATP channel closure and contribute to the hyperglycaemic episodes.

Acute metabolic amplification of insulin secretion in mouse islets is mediated by mitochondrial export of metabolites, but not by mitochondrial energy generation.

OBJECTIVE: The ß-cell metabolism of glucose and of some other fuels (e.g. α-ketoisocaproate) generates signals triggering and acutely amplifying insulin secretion. As the pathway coupling metabolism with amplification is largely unknown, we aimed to narrow down the putative amplifying signals. MATERIALS/METHODS: An experimental design was used which previously prevented glucose-induced, but not α-ketoisocaproate-induced insulin secretion. Isolated mouse islets were pretreated for one hour with medium devoid of fuels and containing the sulfonylurea glipizide in high concentration which closed all ATP-sensitive K(+) channels. This concentration was also applied during the subsequent examination of fuel-induced effects. In perifused or incubated islets, insulin secretion and metabolic parameters were measured. RESULTS: The pretreatment decreased the islet ATP/ADP ratio. Whereas glucose and α-ketoisovalerate were ineffective or weakly effective, respectively, when tested separately, their combination strongly enhanced the insulin secretion. Compared with glucose, the strong amplifier α-ketoisocaproate caused less increase in NAD(P)H-fluorescence and less mitochondrial hyperpolarization. Compared with α-ketoisovalerate, α-ketoisocaproate caused greater increase in NAD(P)H-fluorescence and greater mitochondrial hyperpolarization. Neither α-ketoacid anion enhanced the islet ATP/ADP ratio during onset of the insulin secretion. α-Ketoisocaproate induced a higher pyruvate content than glucose, slowly elevated the citrate content which was not changed by glucose and generated a much higher acetoacetate content than other fuels. α-Ketoisovalerate alone or in combination with glucose did not increase the citrate content. CONCLUSIONS: In ß-cells, mitochondrial energy generation does not mediate acute metabolic amplification, but mitochondrial production of acetyl-CoA and supplemental acetoacetate supplies cytosolic metabolites which induce the generation of specific amplifying signals.

Studies of first phase insulin secretion using imposed plasma membrane depolarization.

The first phase of glucose-induced insulin secretion is generally regarded to represent the release of a finite pool of secretion-ready granules, triggered by the depolarization-induced influx of Ca2+ through L-type Ca2+ channels. However, the experimental induction of insulin secretion by imposed plasma membrane depolarization may be more complicated than currently appreciated. A comparison of the effects of high K+ concentrations with those of KATP channel closure, which initiates the electrical activity of the beta cell, suggests that 40 mM K+, which is a popular tool to produce a first phase-like secretion, is of supraphysiological strength, whereas the 20 mV depolarization by 15 mM K+ is nearly inefficient. A major conceptual problem consists in the occurrence of action potentials during KATP channel closure, but not during K+ depolarization, which leaves the K+ channel conductance unchanged. Recent observations suggest that the signal function of the endogenously generated depolarization is not homogeneous, but may rather differ between the component mainly determined by KATP channel closure (slow waves) and that mainly determined by Ca2+ influx (action potentials).

The insulinotropic effect of fluoroquinolones.

Antimicrobial fluoroquinolones induce, with strongly varying frequency, life-threatening hypoglycemias, which is explained by their ability to block K(ATP) channels in pancreatic B-cells and thus to initiate insulin secretion. In apparent contradiction to this, we observed that none of the fluoroquinolones in this study (gatifloxacin, moxifloxacin, ciprofloxacin, and a number of fluorophenyl-substituted compounds) initiated insulin secretion of perifused mouse islets when the glucose concentration was basal (5mM). Only when the glucose concentration was stimulatory by itself (10mM), the fluoroquinolones enhanced secretion. The fluoroquinolones were ineffective on SUR1 Ko islets, which do not have functional K(ATP) channels. All of these fluoroquinolones depolarized the membrane potential of mouse B-cells (patch-clamping in the whole-cell mode). Using metabolically intact B-cells (perforated-patch mode) however, 100microM of gatifloxacin, ciprofloxacin or moxifloxacin were unable to depolarize when the glucose concentration was 5mM, whereas other K(ATP) channel blockers (tolbutamide and efaroxan) remained effective. Only at a very high concentration (500microM) gatifloxacin and moxifloxacin, but not ciprofloxacin induced repetitive depolarizations which could be antagonized by diazoxide. In the presence of 10mM glucose all fluoroquinolones which enhanced secretion markedly elevated cytosolic calcium concentration ([Ca(2+)](i)). In the presence of 5mM glucose gatifloxacin and moxifloxacin at 500microM but not at 100microM elevated [Ca(2+)](i). It is concluded that fluoroquinolones in the clinically relevant concentration range are not initiators, but rather enhancers of glucose-induced insulin secretion. The block of K(ATP) channels appears necessary but not sufficient to explain the hypoglycemic effect of fluoroquinolones.