Upstream Stimulatory Factor USF binding to the
Upstream Stimulatory Factor (USF) binding to the −65 E-box is required for the regulation of FAS promoter activity in fasting/feeding , . DNA-PK are identified as a USF-interacting protein . In response to feeding/insulin, the transient double-stranded DNA breaks occurred during the transcriptional activation of FAS promoter, which might recruit DNA-PK to the DSB sites near FAS promoter. Moreover, DNA-PK has recently been shown to be required for USF-1 complex assembly and recruitment of its other interacting proteins. Upon feeding, transient DSB-recruited DNA-PK, which is dephosphorylated/activated by PP1, and the activated DNA-PK mediates feeding-dependent Ser 262 phosphorylation of USF-1, which governs interaction between USF-1 and its partners. USF-1 and SREBP-1 (SRE binding protein), another transcription factor in lipogenic promoters, directly interact for FAS promoter activation , , SREBP-1 interacts more efficiently with the phosphorylated USF-1, which, in turn, enhances the interaction between USF-1 and DNA-PK, leading to USF-1 phosphorylation, an indication of positive feed-forward regulation. In addition, DNA-PK-catalyzed phosphorylation of USF-1 allows P/CAF recruitment and subsequent Lys 237 acetylation of USF-1. As a result, FAS transcription is activated by USF-1 (Fig. 2). Thus, the transcriptional activation of lipogenic genes is impaired in DNA-PK-deficient SCID mice.
Of course, the FAS promoter activation is in a reversible manner in response to nutritional status. USF-1 recruits histone deacetylasse 9 (HDAC9) in the fasted state, which deacetylates USF-1 to repress transcription despite its binding to the E box (Fig. 2). In brief, DNA-PK plays an important role in the USF-1-mediated transcriptional regulation of lipogenic genes during fasting/feeding.
The regulation of DNA-PK activity How DNA-PK is activated or deactivated in response to DNA damage and other cellular stress, is still relatively unknown . Previous studies have shown that two serine/threonine protein phosphatase PP5 and PP6 are involved in DNA-PK activity modulation , . The Wabl group has shown PP5 interacts with DNA-PKcs and dephosphorylates with surprising specificity at least two functional sites, and GSK 2837808A with either hypo- or hyperphosphorylation of DNA-PKcs at these sites show increased radiation sensitivity . Nevertheless, mass spectrometry analysis revealed that DNA-PK interacts with the protein phosphatase-6 (PP6) SAPS subunit PP6R1. PP6 is a heterotrimeric enzyme that consists of a catalytic subunit, plus one of three PP6 SAPS regulatory subunits and one of three ankyrin repeat subunits. Endogenous PP6R1 co-immunoprecipitated DNA-PK, and IR enhanced the amount of complex and promoted its import into the nucleus. In addition, siRNA knockdown of either PP6R1 or PP6 significantly decreased IR activation of DNA-PK, suggesting that PP6 activates DNA-PK by association and dephosphorylation . In addition, Taccioli group has shown that part of DNA-PK complex is associated with cytoplasm membrane, and raft-resident proteins may separately recruit DNA-PK components to the cytoplasm membrane. Furthermore, an irradiation-induced differential protein phosphorylation pattern is dependent upon DNA-PKcs in lipid rafts . Recently, the epidermal growth factor (EGF) receptor (EGFR) was reported to associate with DNA-PK too, and ionizing radiation, but not stimulation with EGF, triggers EGFR import into the nucleus and enhances the DNA-PKcs binding with EGFR. During the EGFR nuclear translocation process, the proteins Ku70/80 and the protein phosphatase 1 are also observed to transport into the nucleus. As a consequence, DNA-PK kinase activity is increased. Blockade of EGFR import by the anti-EGFR monoclonal antibody C225 abolished EGFR import into the nucleus and radiation-induced DNA-PK activation, inhibited DNA repair, and increased radiosensitivity of treated cells , . Moreover, Houslay group also found the GTP exchange factor, exchange protein activated by cAMP (EPAC) coupled to Rap2 in the nucleus, and cAMP regulated the nuclear/cytoplasmic trafficking of DNA-PK through the EPAC and PKA pathways . Intersecting regulatory inputs for cAMP employ EPAC to transduce positive effects, namely the Rap2-dependent nuclear exit and activation of DNA-PK, whereas protein kinase A (PKA) provides the negative input by antagonizing these actions . These observations suggest modulation on DNA-PK nuclear translocation influences the DNA-PK activity too.