HMGA1 Levels Influence Mitochondrial Function and Mitochondrial DNA Repair Efficiency

Abstract

HMGA chromatin proteins, a family of gene regulatory factors found at only low concentrations in normal cells, are almost universally overexpressed in cancer cells. HMGA proteins are located in the nuclei of normal cells except during the late S/G 2 phases of the cell cycle, when HMGA1, one of the members of the family, reversibly migrates to the mitochondria, where it binds to mitochondrial DNA (mtDNA). In many cancer cells, this controlled shuttling is lost and HMGA1 is found in mitochondria throughout the cell cycle. To investigate the effects of HMGA1 on mitochondria, we employed a genetically engineered line of human MCF-7 cells in which the levels of transgenic HMGA1 protein could be reversibly controlled. "Turn-ON" and "turn-OFF" time course experiments were performed with these cells to either increase or decrease intracellular HMGA1 levels, and various mitochondrial changes were monitored. Results demonstrated that changes in both mtDNA levels and mitochondrial mass inversely paralleled changes in HMGA1 concentrations, strongly implicating HMGA1 in the regulation of these parameters. Additionally, the level of cellular reactive oxygen species (ROS) increased and the efficiency of repair of oxidatively damaged mtDNA decreased as consequences of elevated HMGA1 expression. Increased ROS levels and reduced repair efficiency in HMGA1-overexpressing cells likely contribute to the increased occurrence of mutations in mtDNA frequently observed in cancer cells.Aberrant alterations in mitochondrial function are observed in a wide range of human conditions and diseases (10, 109), including cancer (19,22,51). Because of their prevalence, mitochondrial abnormalities in cancer cells have been extensively investigated (17,39,84,118). Even in the presence of an adequate oxygen supply, the mitochondria of cancer cells are almost universally deficient in their ability to generate ATP via oxidative phosphorylation and therefore rely heavily on glycolytic metabolism for energy production (the so-called Warburg effect) (119, 120). Likewise, cancer cells often exhibit electron transport chain deficiencies (90), generate smaller amounts of ATP and higher levels of reactive oxygen species (ROS) than normal cells (45, 65), display calcium signaling defects (101), and frequently exhibit alterations in their mitochondrial transmembrane potential (56, 57). Many cancers also exhibit reductions in both mitochondrial mass and mitochondrial DNA (mtDNA) levels (31,80,118,125,128), as well as significantly altered levels of mtDNA transcription (36,50,55,66). It is also well documented that mtDNA in cancer cells is more susceptible to somatic mutation than nuclear DNA (nDNA) (8,12,22,93,104), partially as a consequence of the limited repair capabilities of mitochondria compared to the mechanisms responsible for the repair of nDNA (12,13,39,76).The underlying molecular causes for mitochondrial dysfunction in most cancers are unknown, although available evidence suggests that there are many possible contributing factors that interact with ...

High mobility group (HMG) proteins: Modulators of chromatin structure and DNA repair in mammalian cells

“…It has also been demonstrated that over-expression of HMGA1 proteins in cancer cells results in the accumulation of HMGA1 protein in mitochondria [147,148] resulting in the inhibition of BER of oxidative damage in mitochondria DNA (mtDNA) [149]. The mechanisms for repair of lesions in mtDNA are complex and highly dependent on both the type and extent of damage involved but, unlike the nucleus, mitochondria lack the enzymes necessary for NER [150] and thus rely primarily on BER to remove oxidatively damaged bases [151].…”

ATM protein kinase: the linchpin of cellular defenses to stress

“…It is of interest that several ATM substrates demonstrate mitochondrial translocation (CREB, p53) and affect mitochondrial functions (HMGA1). HMGA1 translocates to mitochondria in the S-G2 phases of cell cycle [232] and levels of HMGA1 are correlated with mitochondrial mass, levels of ROS and repair of mtDNA damage. The role of ATM-dependent phosphorylation of HMGA1 in the mitochondrial biogenesis pathway remains to be explored.…”

Quantitative PCR-Based Measurement of Nuclear and Mitochondrial DNA Damage and Repair in Mammalian Cells

“…These include the utilization of the QPCR method to determine the ability of modulating experimental conditions to influence DNA damage levels or repair in mtDNA or nDNA, such as a 2009 study by Jung et [28][29][30][31][32][33][34][35][36][37][38][39][40][41]. QPCR has also been employed to directly compare mtDNA and nDNA damage in aged tissues [42,43] as well as determine the effects of disease conditions [37,[44][45][46] and conditions such as oxidative stress [31,40] on repair in mtDNA and nDNA.…”