Regulation of the Cell Cycle by MicroRNAs: Quiescence Versus Proliferation

Stem cells are crucial for both homeostasis and wound healing in self renewing tissues such as the epidermis. Under normal conditions, it is well accepted that stem cells rarely cycle. Interestingly, following injury, stem cells become activated and begin to cycle, resulting in an increased proliferative rate. Abnormal proliferation is a key characteristic many diseases, including chronic skin ulcers, psoriasis and squamous cell carcinoma, I propose to investigate how the cell cycle is regulated in epidermal keratinocytes. Our preliminary data indicate that a microRNA family (miRs-103/107), which is associated with stem cells, negatively regulates the cell cycle in epidermal keratinocytes. This suggests that miRs-103/107 contribute to maintaining the slow-cycling nature ascribed to stem cells. In support of this idea, we also have data that p90RSK2 and HOXD13, two important regulators of the cell cycle, are downstream targets of miRs-103/107. Collectively, our preliminary data lead us to hypothesize that this novel interaction of miRs-103/107 with HOXD13 and p90RSK2 in epidermal keratinocytes regulates the cell cycle, leading to an on/off switch of proliferation. To test this hypothesis, we will perform gain- and loss-of-function studies for miRs-103/107 as well as their direct targets HOXD13 and p90RSK2 to determine such a miRNA family and their direct target genes regulate the cell cycle in epidermal keratinocytes by Fluorescence-Activated Cell Sorting (FACS) cell cycle analyses, holoclone assays in submerged cultures, label-retaining cell assays in 3D organotypic and progenitor competition assays using mosaic raft cultures. Finally, we will explore whether loss of miRs-103/107 promote proliferation in vivo and consequently enhance wound healing. Maintenance of a proper cell cycle is crucial for normal epidermal homeostasis and wound healing, thus this study will help explore the previously unconsidered roles of miRs-103/107 in these processes. Clarifying how keratinocyte proliferation is affected by such interactions between miRs-103/107 and their target genes HOXD13 and p90RSK2 will form the foundation for the development of innovative therapies, such as ectopic delivery of antagomirs to increase proliferation during the wound-healing process. Given that abnormal proliferation is also a key characteristic in many diseases, development of miRs-103/107 inhibitors and/or HOXD13/p90RSK2-overexpression-based therapies could have potential utility in diseases such as psoriasis and squamous cell carcinoma. Therefore, upon completion of the proposed studies, a logical extension of this proposal will be to investigate how this regulatory axis functions in these diseases of abnormal epidermal proliferation.