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How human self-renewal tissues co-ordinate proliferation with differentiation is unclear. Human epidermis undergoes continuous cell growth and differentiation and is permanently exposed to mutagenic hazard. Keratinocytes are thought to arrest cell growth and cell cycle prior to terminal differentiation. However, a growing body of evidence does not satisfy this model. For instance, it does not explain how skin maintains tissue structure in hyperproliferative benign lesions. We have developed and applied novel cell cycle techniques to human skin in situ and determined the dynamics of key cell cycle regulators of DNA replication or mitosis, such as cyclins E, A and B, or members of the anaphase promoting complex pathway: cdc14A, Ndc80/Hec1 and Aurora kinase B. The results show that actively cycling keratinocytes initiate terminal differentiation, arrest in mitosis, continue DNA replication in a special G2/M state, and become polyploid by mitotic slippage. They unambiguously demonstrate that cell cycle progression coexists with terminal differentiation, thus explaining how differentiating cells increase in size. Epidermal differentiating cells arrest in mitosis and a genotoxic-induced mitosis block rapidly pushes epidermal basal cells into differentiation and polyploidy. These observations unravel a novel mitosis-differentiation link that provides new insight into skin homeostasis and cancer. It might constitute a self-defence mechanism against oncogenic alterations such as Myc deregulation.

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Doxorubicin, one of the most effective anticancer drugs currently known, is commonly used against breast cancer. However, its clinical use is restricted by dose-dependent toxicity (myelosuppression and cardiotoxicity), the emergence of multidrug resistance and its low specificity against cancer cells. Nanotechnology is a promising alternative to overcome these limitations in cancer therapy as it has been shown to reduce the systemic side-effects and increase the therapeutic effectiveness of drugs. Indeed, the numerous nanoparticle-based therapeutic systems developed in recent years have shown low toxicity, sustained drug release, molecular targeting, and additional therapeutic and imaging functions. Furthermore, the wide range of nanoparticle systems available may provide a solution to the different problems encountered during doxorubicin-based breast cancer treatment. Thus, a suitable nanoparticle system may transport active drugs to cancer cells using the pathophysiology of tumours, especially their enhanced permeability and retention effects, and the tumour microenvironment. In addition, active targeting strategies may allow doxorubicin to reach cancer cells using ligands or antibodies against selected tumour targets. Similarly, doxorubicin resistance may be overcome, or at least reduced, using nanoparticles that are not recognized by P-glycoprotein, one of the main mediators of multidrug resistance, thereby resulting in an increased intracellular concentration of drugs. This paper provides an overview of doxorubicin nanoplatform-based delivery systems and the principal advances obtained in breast cancer chemotherapy.