non-thermal plasma on mammalian cell activity and apoptosis, including a background and overview, as well as a description, summary and comparison of relevant related studies. These sections are followed by a discussion concerning the gaps that were identified in the existing body of knowledge and the need for further research in determining the effects of non-thermal plasma on mammalian cell activity and apoptosis.

Programmed cell death is markedly different from accidental deaths that can occur for various reasons, including the necrotic processes the result from oxygen deprivation and traumatic injuries (Clark, 2002). In this regard, Clark advises that in sharp contrast to accidental cell death, "Programmed cell death is very different" (2002, p. 25). In support of this observation, Clark notes that the "program" involved in programmed cell death is not a resistance to cellular death but it is rather one of active participation in the death process. According to Clark, "The death program in an individual cell may be initiated in response to external factors -- signals coming from other cells in the body, for example -- but the cell so instructed does not resist this signal" (2002, p. 250).

Indeed, apoptosis occurs when the death program in a particular cell responds to these external signals in a highly cooperative fashion that ultimately leads to its own destruction. In this regard, Clark notes that, "In one of the most common forms of programmed cell death, the cell simply sets in motion the internal cellular mechanisms necessary to complete the program in a process often referred to as cellular 'suicide,' or more properly, apoptosis" (2002, p. 25). It is important, though, to distinguish between programmed cell death per se and apoptosis. Programmed cell death is defined by Clark as being "any genetically regulated program leading to the death of a cell"; by contrast, "apoptosis is one common way of achieving programmed cell death, but it may not be the only means" (2002, p. 25).

In mammalians, Fridman et al. (2007) report that some examples of apoptosis that take place during normal body processes include:

1. The formation of the outer layer of skin;

2. The inner mucosal lining of the intestine; and,

3. The endometrial lining of the uterus, which is sloughed off during menstruation (p. 2).

Moreover, the controlled programmed cell death process in apoptosis takes place without damaging other neighboring cells. According to Fridman et al. (2007), "During apoptosis, cellular macromolecules are digested into smaller fragments in a controlled fashion, and ultimately the cell collapses into smaller intact fragments that can be removed by phagocytosis without damaging the surrounding cells or causing inflammation" (p. 2). Therefore, in some cases, it may be desirable to initiate the apoptosis process in undesirable cells such as cancers, without adversely affecting the surrounding tissues which is where non-thermal plasma appears to represent a valuable addition to the repertoire of treatments currently available and these issues are discussed further below.

Comparison of Recent Studies of Non-Thermal Plasma on Mammalian Cell Activity and Apoptosis.

The self-regulating capability of a cell represents an essential function in mammalians because it facilitates the appropriate growth and development that are required throughout the lifespan, as well as initiating death at the appropriate times in these organisms' lives (Kalghatgi, 2009). For instance, according to Kalghatgi, "Apoptosis, or programmed cell death, is a critical element of this self-regulation. The non-functioning of a tumor-suppressor gene that facilitates apoptosis, or the over expression of an anti-apoptotic protein are both important pathways in cancer development" (2009, para. 3). This process is illustrated in Figure 1 below.

Figure 1. Schematic explaining the process of apoptosis in mammalian cells

Source: Kalghatgi, 2009

A number of therapies have been developed or are actively being researched that seek to modulate these factors in an effort to cure certain types of cancer, including studies concerning how different bioactive agents target various components of the apoptotic pathways; however, a number of these therapeutic interventions remain ineffective for various reasons, including insufficient efficacy as well as inordinately high levels of resistance to drugs (Kalghatgi, 2009). In response to these constraints, the study by Kalghatgi sought to determine the efficacy of an electro-chemical approach to induce apoptosis. In this regard, Kalghatgi notes that, "Non-thermal atmospheric pressure dielectric barrier discharge plasma may provide a novel approach to induction of apoptosis. The purpose of this study was to evaluate the apoptotic effects of non-thermal plasma on human melanoma cells in vitro" (2009, para. 3).

Before examining the findings of the Kalghatgi study in more detail, an understanding of the operation of the supporting technology is important. In general, plasmas are "a gas-like state of matter consisting of positively charged ions, free electrons, and neutral particles. The behavior of most plasma systems is dominated by the electromagnetic interaction between the charged particles" (Cleveland & Morris, 2006, p. 336). More specifically, the definition provided by Pinder and Slack (2004) indicates that non-thermal plasma is "a partially ionized gas comprising a neutral mixture of atoms, molecules, free radicals, ions and electrons" (p. 238). According to Kim, Bahn, Lee, Kim, Jun, Lee and Baek (2010), "Plasma is generated by ionizing neutral gas molecules, resulting in a mixture of energy particles, including electrons and ions. Recent progress in the understanding of non-thermal atmospheric plasma has led to applications in biomedicine. However, the exact molecular mechanisms involved in plasma-induced cell growth arrest are unclear" (p. 530).

Citing a growing body of research concerning the various applications for non-thermal plasma, the hypothesis of the Kalghatgi (2009) study was based on the fact that studies have shown that non-thermal plasma is capable of destroying bacteria or inducing apoptosis in malignant cells (Parkin, Bray, Ferlay & Pisani, 2005). Moreover, non-thermal plasma in non-lethal doses has also been used to good effect in invoking specific biological effects, including gene transfection, cell detachment (Kalghatgi, 2009, wound healing and blood coagulation (Fridman, Shereshevsky, Peddinghaus, Gutsol, Vasilets, Brooks, Balasubramanian, Friedman & Fridman, 2006). In addition, there appear to be some selective attributes of non-thermal plasma that hold significant promise in other applications. For instance, Kalghatgi reports that, "In recent studies of plasma blood coagulation and bacteria deactivation, [non-thermal] plasma did not demonstrate measurable toxicity in the surrounding living tissue" (2009, para. 4).

The foregoing attributes appear to make non-thermal plasma a highly useful tool in the regulation of mammalian cell activity and apoptosis, an assertion that is supported by the findings of the Kalghati (2009) study. For example, Kalghatgi points out that, "Plasma treatment induces apoptosis in melanoma cells through a pathway that appears to be dependent on production of reactive oxygen species by plasma in fluid. Since this plasma effect is non-thermal, this may be a selective way to treat cutaneous malignancies without initiating inflammatory responses" (2009, para. 5). The selective attribute of non-thermal plasma therapies is particularly noteworthy with respect to the treatment of specific types of undesirable cells, including various types of cancer. In this regard, Kalghatgi concludes that, "Non-thermal plasma may be a useful tool to induce directed cell death without inducing necrosis and inflammation" (2009, para. 5).

These potentially valuable mechanisms were also the focus of a recent study by Sensenig, Kalghatgi, Cerchar, Fridman, Shereshevsky, Torabi, Priya, Podolsky, Fridman, Friedman, Azizkhan-Clifford and Brooks (2010). Building on the previous work by Kalghatgi (2009), these authors reports that, "Non-thermal atmospheric pressure dielectric barrier discharge plasma may provide a novel approach to treat malignancies via induction of apoptosis" (Sensenig et al., 2010, para. 1). The purpose of the follow-up study by Sensenig and her associates was to assess the ability of DBD plasma in invoking apoptosis in specifically targeted cells. To this end, these researchers exposed melanoma cells to plasma dosages that were insufficient to invoke necrosis and then evaluated the resulting cell viability and apoptotic activity levels (Sensenig et al., 2010). Among the salient findings that emerged from this study were the following:

1. Trypan blue staining revealed that non-thermal plasma treatment significantly decreased the viability of cells in a dose-dependent manner 3 and 24 h after plasma treatment.

2. Annexin-V/PI staining revealed a significant increase in apoptosis in plasma-treated cells at 24, 48, and 72 h post-treatment (p

3. TUNEL® analysis of plasma-treated cells demonstrated an increase in apoptosis at 48 and 72 h post-treatment (p

4. Pre-treatment with N-acetyl-l-cysteine (NAC), an intracellular reactive oxygen species scavenger, significantly decreased apoptosis in plasma-treated cells at 5 and 15 J/cm2 (Sensenig et al., 2010, para. 3).

Although the precise mechanisms involved remain unclear, the results of the Sensenig et al. study (2010) indicate that non-thermal plasma treatment causes apoptosis in melanoma cells via a pathway that seems to be reliant on the production of intracellular reactive oxygen species (ROS). Based on the their findings, Sensenig and her colleagues observe that, "Dielectric barrier discharge plasma production of intracellular ROS leads to dose-dependent DNA damage in melanoma cells, detected by…