Functional roles of HMGA2 in human embryonic stem cells and human tumor cells
Date of Issue2009
School of Biological Sciences
The state of chromatin in human embryonic stem cells (hESCs) is a key factor determining stem cell identity. The non-histone chromatin-associated factor high-mobility group protein A2 (HMGA2) has been studied mostly in the mouse where its function seems critical for embryonic cell growth and adipocytic cell differentiation, leading to a pygmy phenotype with greatly reduced fat tissue in homozygous knock out mice. We showed that among the major HMG proteins, HMGA2 was highly expressed in two hESC lines. Interestingly, expression was further upregulated during early embryoid body formation before it quickly droped to or below the level found in undifferentiated cells. We also showed that HMGA2 was stably associated with inter- and meta-phase hESC chromatin, and that up to 12 HMGA2 protomers stably associated in vitro with a single nucleosome core particle of known atomic structure. These suggested that HMGA2 might interact with nucleosomes in a way that imposed a global effect on the state of hESC chromatin, which might contribute to the establishment of both hESCs identity and the initiation of specific differentiation programs. We then established HMGA2 as a regulator of human genes linked to mesodermal cell differentiation, adipogenesis, and hESCs growth through siRNA technology in combination with quantitative reverse transcriptase polymerase chain reaction, stem cell-specific microarray analyses, and cell proliferation assays. HMGA proteins are also present in high copy numbers in most neoplasias, and correlations between the degree of malignancy, patient prognostic index, and HMGA levels have been firmly established. Intriguingly, HMGA2 is also found in rare tumor-inducing cells, which are resistant to chemotherapy. Here, we demonstrated that HMGA1a/b and HMGA2 possessed intrinsic dRP and AP lyase activities, which most likely resided in their AT-hook DNA binding domains. We also showed that HMGA2 could be covalently trapped at genomic abasic sites in cancer cells. The associated lyase activities promoted cellular resistance against DNA damage, which was targeted by base excision repair (BER) pathways, and this protective effect directly correlated with the level of HMGA2 expression. Furthermore, we demonstrated an interaction between AP endonuclease 1 and HMGA2 in cancer cells, which strongly supported our conclusion that HMGA2 could be incorporated into the cellular BER machinery.