Pathogenetically, uterine leiomyomas (ULs) can be interpreted as the result of a monoclonal abnormal proliferation of myometrial cells. Oncogene-induced senescence (OIS) is a frequent phenomenon in premalignant lesions that leads to a growth arrest mainly by the activation of two potent growth-inhibitory pathways as represented by p16(Ink4a) and p19(Arf). The relevance of OIS for the development of UL has not been addressed, but HMGA2, encoded by a major target gene of recurrent chromosomal abnormalities in UL, has been implicated in the repression of the Ink4a/Arf (CDKN2A) locus. This prompted us to examine if HMGA2 contributes to the growth of leiomyomas by repressing this locus. Contrary to the expectations, we were able to show that generally ULs express significantly higher levels of p19(Arf) mRNA than myometrium and that UL with 12q14 approximately 15 rearrangements showed higher expression levels than UL with other cytogenetic aberrations. Furthermore, the finding of a significant correlation between the expressions of p19(Arf) and CDKN1A shows that p19(Arf) triggers senescence rather than apoptosis in UL. Furthermore, the expression levels of HMGA2, p19(Arf), and CDKN1A were found to be correlated with the size of the tumors, indicating that an enhanced growth potential is counterbalanced by the p19(Arf) pathway. Mechanistically, the UL may thus execute a program already present in their cell of origin, where it is activated to protect the genome, for example, in the case of enhanced proliferation. In summary, the results identify the p19(Arf)-TP53-CDKN1A pathway as a major player in the growth control and genomic stability of uterine fibroids.

HMGA2 is a major regulator of benign tumorigenesis from mesenchyme-derived tissues and stem-cell self-renewal. It has been postulated that HMGA2 mediates its critical function by decreasing p16(Ink4a)/p14(Arf) expression and cellular senescence. To repress the oncogenic activity of HMGA2, the lin-28-let-7 axis is thought to increasingly repress the expression of HMGA2 with age. To understand the HMGA2-p14(Arf) -relationship in benign tumorigenesis, we performed a series of experiments on mesenchymal stem-cells, i.e., the proposed cells of origin of lipomas and uterine leiomyomas. The expression of both genes was inversely correlated during senescence in vitro but contrary to the expectations in adipose tissue derived stem cells (ADSCs) stimulation of HMGA2 by FGF1 increased the expression of p14(Arf) . Based on the assumption that in ADSCs p14(Arf) is repressing HMGA2, siRNA silencing of p14(Arf) was performed resulting in a significant upregulation of HMGA2. To see if p14(Arf) can repress HMGA2 by a TP53-dependent mechanism, nutlin-3, a known MDM2 antagonist, was used which not only increased the activity of the senescence, associated markers p21 and beta-galactosidase, but also decreased the expression of HMGA2, suggesting that p14(Arf) indeed influences HMGA2 by a p53-dependent mechanism because nutlin-3 stabilizes p53. Accordingly, the HMGA2 response triggered by serum was reduced by treatment of ADSCs with nutlin-3. As to the interaction between HMGA2 and p14(Arf) in benign tumorigenesis, we propose a model where akin to MSC self-renewal during tissue repair the simultaneous increase of p14(Arf) with HMGA2 ensures genomic stability, whereas in turn p14(Arf) can repress HMGA2 via TP53.

In previous research, it was shown that recombinant HMGA2 protein enhances the proliferation of porcine chondrocytes grown in vitro, opening up promising applications of this embryonic architectural transcription factor for tissue engineering, such as in cartilage repair. In this paper, we describe the development and analyses of two synthetic fragments comprising the functional AT-hook motifs of the HMGA2 protein, as well as the nuclear transport domain. They can be synthesised up to large scales, while eliminating some of the problems of recombinant protein production, including unwanted modifi cation or contamination by the expression hosts, or of gene therapy approaches such as uncontrolled viral integration and transgene expression even after therapy. Application of one of these peptides onto porcine hyaline cartilage chondrocytes, grown in in vitro monolayer cell culture, showed a growth-promoting effect similar to that of the wild type HMGA2 protein. Furthermore, it also promoted cell growth of adult adipose tissue derived stem cells. Due to its proliferation inducing function and vast availability, this peptide is thus suitable for further application and investigation in various fi elds such as tissue engineering and stem cell research.

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