It is possible that the rapid proliferation of the K15- GFP+ cells observed after 6 days of 0.5 mM spermidine administration did not allow enough time for the K15 FG-4592 protein to assemble in the cells, and to reach the level of VE-822 detection. It has also been shown before that polyamines have an effect on the post-translational regulation of proteins, affecting protein degradation by direct or indirect effect on proteases. Therefore, changes in protein expression may not always correlate with the changes observed at the mRNA level. Our study presents the first evidence that spermidine is a novel determinant in human eSCs biology, most notably of K15 and K19 expression by primary human epithelial progenitor cells in situ and in vitro. These findings are in line with the prior demonstration that ODC is expressed in the bulge region of the HF , where it colocalizes with that of K15 and K19 expression. While polyamines are known to affect the keratin composition of wool follicles , it was previously unknown that polyamines actually regulates the expression of human eSC-associated keratins. Moreover, we provide the first available evidence that inhibiting the key enzyme of polyamine synthesis down-regulates K15 expression. Thus K15 expression in situ is profoundly regulated by spermidine, and both polyamines and ODC activity impact on the expression of this HF epithelial progenitor cell marker keratin. Our finding that ODC expression on the gene and protein level underlies a negative, dose-dependent feedback regulation by spermidine underscores the apparent importance of keeping intrafollicular polyamine synthesis in check. That the highest dose of spermidine tested did not reduce ODC expression may suggest that adequate ODC activity remains needed as a part of the biological stress response to excessive spermidine levels. We had hoped to obtain specific leads from our microarray analysis on how spermidine may exert its anagen-prolonging, stem cell-modulatory, and K15/K19-regulatory effects. While these results identified five novel intrafollicular candidate target genes for spermidine-mediated signaling that have not yet been investigated in the spermidine literature, these genes do not sufficiently explain the underlying mechanisms of action. However, the fact that the identified candidate genes are important for vital cell organelles and cell homeostasis fits well to the general concept that spermidine supports HF and eSC vitality. For example, synoviolin is a ubiquitin ligase, which plays an important role in endoplasmic reticulum-associated protein degradation , the NACA gene encodes the nascentpolypeptide- associated complex alpha polypeptide, a part of the protein translation chaperone complex , and SLC25A3 is a mitochondrial phosphate carrier, which is essential for the aerobic synthesis of adenosine triphosphate.