Proliferation of Multiple Cell Types in the Skeletal Muscle Tissue Elicited by Acute p21 Suppression.

Although in the last decades the molecular underpinnings of the cell cycle have been unraveled, the acquired knowledge has been rarely translated into practical applications. Here, we investigate the feasibility and safety of triggering proliferation in vivo by temporary suppression of the cyclin-dependent kinase inhibitor, p21. Adeno-associated virus (AAV)-mediated, acute knockdown of p21 in intact skeletal muscles elicited proliferation of multiple, otherwise quiescent cell types, notably including satellite cells. Compared with controls, p21-suppressed muscles exhibited a striking two- to threefold expansion in cellularity and increased fiber numbers by 10 days post-transduction, with no detectable inflammation. These changes partially persisted for at least 60 days, indicating that the muscles had undergone lasting modifications. Furthermore, morphological hyperplasia was accompanied by 20% increases in maximum strength and resistance to fatigue. To assess the safety of transiently suppressing p21, cells subjected to p21 knockdown in vitro were analyzed for γ-H2AX accumulation, DNA fragmentation, cytogenetic abnormalities, ploidy, and mutations. Moreover, the differentiation competence of p21-suppressed myoblasts was investigated. These assays confirmed that transient suppression of p21 causes no genetic damage and does not impair differentiation. Our results establish the basis for further exploring the manipulation of the cell cycle as a strategy in regenerative medicine.

No metagenomic evidence of tumorigenic viruses in cancers from a selected cohort of immunosuppressed subjects.

The possible existence of yet undiscovered human tumorigenic viruses is still under scrutiny. The development of large-scale sequencing technologies, coupled with bioinformatics techniques for the characterization of metagenomic sequences, have provided an invaluable tool for the detection of unknown, infectious, tumorigenic agents, as demonstrated by several recent studies. However, discoveries of novel viruses possibly associated with tumorigenesis are scarce at best. Here, we apply a rigorous bioinformatics workflow to investigate in depth tumor metagenomes from a small but carefully selected cohort of immunosuppressed patients. While a variegated bacterial microbiome was associated with each tumor, no evidence of the presence of putative oncoviruses was found. These results are consistent with the major findings of several recent papers and suggest that new human tumorigenic viruses are not common even in immunosuppressed populations.

Tumor-educated mesenchymal stem cells promote pro-metastatic phenotype.

Multipotent mesenchymal stem cells (MSCs) are recruited into tumor microenvironment in response to multiple signals produced by cancer cells. Molecules involved in their homing to tumors are the same inflammatory mediators produced by injured tissues: chemokines, cytokines and growth factors. When MSCs arrive into the tumor microenvironment these are "educated" to have pro-metastatic behaviour. Firstly, they promote cancer immunosuppression modulating both innate and adaptive immune systems. Moreover, tumor associated-MSCs trans-differentiating into cancer-associated fibroblasts can induce epithelial-mesenchymal-transition program in tumor cells. This process determinates a more aggressive phenotype of cancer cells by increasing their motility and invasiveness and favoring their dissemination to distant sites. In addition, MSCs are involved in the formation and modelling of pre-metastatic niches creating a supportive environment for colonization of circulating tumor cells. The development of novel therapeutic approaches targeting the different functions of MSCs in promoting tumor progression as well as the mechanisms underlying their activities could enhance the efficacy of conventional and immune anti-cancer therapies. Furthermore, many studies report the use of MSCs engineered to express different genes or as vehicle to specifically deliver novel drugs to tumors exploiting their strong tropism. Importantly, this approach can enhance local therapeutic efficacy and reduce the risk of systemic side effects.

A defective dNTP pool hinders DNA replication in cell cycle-reactivated terminally differentiated muscle cells.

Terminally differentiated cells are defined by their inability to proliferate. When forced to re-enter the cell cycle, they generally cannot undergo long-term replication. Our previous work with myotubes has shown that these cells fail to proliferate because of their intrinsic inability to complete DNA replication. Moreover, we have reported pronounced modifications of deoxynucleotide metabolism during myogenesis. Here we investigate the causes of incomplete DNA duplication in cell cycle-reactivated myotubes (rMt). We find that rMt possess extremely low levels of thymidine triphosphate (dTTP), resulting in very slow replication fork rates. Exogenous administration of thymidine or forced expression of thymidine kinase increases deoxynucleotide availability, allowing extended and faster DNA replication. Inadequate dTTP levels are caused by selective, differentiation-dependent, cell cycle-resistant suppression of genes encoding critical synthetic enzymes, chief among which is thymidine kinase 1. We conclude that lack of dTTP is at least partially responsible for the inability of myotubes to proliferate and speculate that it constitutes an emergency barrier against unwarranted DNA replication in terminally differentiated cells.