Drug-induced xenogenization of tumors: A possible role in the immune control of malignant cell growth in the brain?

In recent years, immune checkpoint inhibitors (ICpI) have provided the ground to bring tumor immunity back to life thanks to their capacity to afford a real clinical benefit in terms of patient's survival. Essential to ICpI success is the presence of tumor-associated neoantigens generated by non-synonymous mutations, since a direct relationship between mutation load of malignant cells and susceptibility to ICpI has been confidently established. However, it has been also suggested that high intratumor heterogeneity (ITH) associated with subclonal neoantigens could not elicit adequate immune responses. Several years ago we discovered that in vivo treatment of leukemic mice with triazene compounds (TZC) produces a marked increase of leukemia cell immunogenicity [a phenomenon termed Drug-Induced Xenogenization (DIX)] through point mutations able to generate strong tumor neoantigens (Drug-Induced Neoantigens, DIN). Immunogenic mutations are produced by TZC-dependent methylation of O6-guanine of DNA, that is suppressed by the DNA repair protein methyl-guaninemethyltransferase (MGMT). This minireview illustrates preclinical investigations conducted in animal models where DIN-positive murine leukemia cells were inoculated intracerebrally into histocompatible mice. The analysis of the literature indicates that the growth of xenogenized malignant cells is controlled by anti-DIN graft responses and by intra-cerebral or intravenous adoptive transfer of anti-DIN cytotoxic T lymphocytes. This survey reminds also that PARP inhibitors increase substantially the antitumor activity of TZC and can be administered with the intent of suppressing more efficiently tumor load and possibly reducing ITH through downsizing the polyclonality of xenogenized tumor cell population. Finally, the present report illustrates a hypothetical clinical protocol that could be considered as an example of future development of DIXbased tumor immuno-chemotherapy in brain malignancies. The protocol involves oral or intravenous administration of TZC along with loco-regional (i.e. intracerebral "wafer") treatment with agents able to increase tumor cell sensitivity to the cytotoxic and xenogenizing effects of TZC (i.e. MGMT and PARP inhibitors) without enhancing the systemic toxicity of these DNA methylating compounds.

The aging brain, a key target for the future: the protein kinase C involvement.

The brain represents the primary centre for the regulation and control of all our body activities, receiving and interpreting sensory impulses and transmitting information to the periphery. Most importantly, it is also the seat of consciousness, thought, emotion and especially memory, being in fact able to encode, store and recall any information. Memory is really what makes possible so many of our complex cognitive functions, including communication and learning, and surely without memory, life would lose all of its glamour and purpose. Age-associated mental impairment can range in severity from forgetfulness at the border with pathology to dementia, such as in Alzheimer's disease. In recent years, one of the most relevant observations of research on brain aging relates to data indicating that age-related cognitive decline is not only due to neuronal loss, as previously thought; instead, scientists now believe that age-associated functional changes have more to do with the dysfunctions occurring over time. Within this context a prominent role is certainly played by signal transduction cascades which guarantee neuronal cell to elaborate coordinated responses to the multiple signals coming from the outside and to adapt itself to the environmental changes and requests. This review will focus the attention on protein kinase C pathway, with a particular interest on its activation process, and on the role of protein-lipid and protein-protein interactions to selectively localize the cellular responses. Furthermore, information is emerging and will be discussed on the possibility of mRNA stabilization through PKC activation. This review will also approach the issue on how alterations of these molecular cascades may have implications in physiological and pathological brain aging, such as Alzheimer's disease.

The different facets of protein kinases C: old and new players in neuronal signal transduction pathways.

Signal transduction pathways are crucial for cell-to-cell communication. Various molecular cascades allow the translation of distinct stimuli, targeting the cell, into a language that the cell itself is able to understand, thus elaborating specific responses. Within this context, a strategic role is played by protein kinases which catalyze the phosphorylation of specific substrates. The serine/threonine protein kinase C (PKC) enzymes family (at least 10 isoforms) is implicated in the transduction of signals coupled to receptor-mediated hydrolysis of membrane phospholipids. Within this molecular pathway, protein-protein interactions play a critical role in directing the distinct activated PKCs towards selective subcellular compartments, in order to guarantee spatio-temporal and localized cellular responses. A space-specific modulation of biochemical events is particularly important during learning. Among the various mechanisms, the modulation of mRNA decay appears to be an efficient post-transcriptional way of controlling gene expression during learning, allowing changes to take place in selected neuronal regions, in particular at synaptic level. To this regard, recent studies have pointed out that PKC activation is also involved in a novel signalling cascade leading to the stabilization of specific mRNAs. This review will especially focus the attention on the implication of PKC in memory trace formation and how alterations within this molecular cascade may have consequences on physiological and pathological neuronal aging (i.e. Alzheimer's disease).

Protein kinase C signal transduction regulation in physiological and pathological aging.

Calcium/phospholipid-regulated protein kinase C (PKC) signalling is known to be involved in cellular functions relevant to brain health and disease, including ion channel modulation, receptor regulation, neurotransmitter release, synaptic plasticity, and survival. Brain aging is characterized by altered neuronal molecular cascades and interneuronal communication in response to various stimuli. In the last few years we have provided evidence that in rodents, despite no changes in PKC isoform levels (both calcium dependent and calcium independent), the activation/translocation process of the calcium-dependent and -independent kinases and the content of the adaptor protein RACK1 (receptor for activated C kinase-1) are deficient in physiological brain aging. Moreover, human studies have shown that PKC and its adaptor protein RACK1 are also interdependent in pathological brain aging (e.g., Alzheimer's disease); in fact, calcium-dependent PKC translocation and RACK1 levels are both deficient in an area-selective manner. These data point to the notion that, in addition to a well-described lipid environment alteration, changes in protein-protein interactions may impair the mechanisms of PKC activation in aging. It is interesting to note that interventions to counteract the age-related functional loss also restore PKC activation and the adaptor protein machinery expression. A better insight into the factors controlling PKC activation may be important not only to elucidate the molecular basis of signal transmission, but also to identify new strategies to correct or even to prevent age-dependent alterations in cell-to-cell communication.