From the evidence accumulated, it is clear that hnRNP are critical regulatory proteins in the cellular response to IR and other stresses. It is also clear from the diversity of responses that have been noted, that biology is only beginning to characterize the role of these multi-faceted proteins in DDR regulation. Advances in molecular biology techniques, especially siRNA and florescence assays, will prove to be critical in furthering our understanding.
The most commonly observed effect of IR on hnRNP was transcript or protein induction. Sixteen hnRNP, A1, A2/B1, A18, AB Type (NM_010448), AB Type (M65028), C1/C2, D, E1, E2, H1, H3, K, L, M, R and U were noted in various studies as being IR inducible mRNA. Most of the observed effects appear to be cell type or growth phase specific and it is difficult to draw broad conclusions from the data. Rather, the induction effects thus far noted may serve as fruitful jumping off points for future research, especially in the cases of hnRNP E, H, R and U which have each been noted in multiple induction studies following IR, but lack followup research to describe the relevant DDR pathways they might connect to.
While hnRNP are characterized primarily as RNA transcript processing agents, there is a surprising lack of evidence of hnRNP relocalization to the cytoplasm or stress granules following IR treatment. HnRNP A1 and B1 relocalization has been noted in response to a variety of stresses, including UV, but not to IR treatment specifically (van der Houven van Oordt et al., 2000
, Guil et al., 2006
). It is likely that the dearth of evidence directly linking IR treatment and hnRNP relocalization is due to a lack of investigation or the technical difficulties associated with IR studies. Further investigations into this response are likely to be fruitful, especially in the examples of hnRNP A1 and B1.
The most important evidence to emerge from the literature reviewed in this paper demonstrates that hnRNP function as upstream and co-dependent regulators of critical DDR proteins. Based on the data found in the literature thus far, it is possible to hypothesize that hnRNP optimize the cell’s DDR response by regulating proteins which participate in HR, NHEJ and general transcriptional arrest.
HnRNP K has been shown to act as a critical co-factor of p53, necessary for p53-dependent cell cycle checkpoints and p53-dependent transcript induction following IR treatment. Further, like p53, hnRNP K is a downstream target of ATR/ATM phosphorylation and HDM2 mediated degradation which coordinates its rapid induction following irradiation (Moumen et al., 2005
). Also, stress induced cytoplasmic relocalization of hnRNP A1 corresponds to a reduction in telomerase activity (of which hnRNP A1 is a constituent) which contributes to cell cycle arrest following IR (Khodarev et al., 2001
). Connection between hnRNP C1/C2 and telomerase activity were also noted and may be involved in cell cycle arrest.
Other hnRNP have been closely associated with HR and NHEJ DSB repair pathways. UV and IR have been shown to produce positive or negative induction of hnRNP B1 (Gamble et al., 2000
, Takao et al., 2002
). Upregulation of hnRNP B1 leads to DNA-PK inhibition (Iwanaga et al., 2005
) and would favor the HR pathway, while downregulation of hnRNP B1 would favor NHEJ. Many general stresses have been shown to upregulate A18 hnRNP, including UV and IR treatment, which in turn upregulates RPA favoring HR repair (Fornace et al., 1988
, Yang and Carrier, 2001
, Fanning et al., 2006
, Barenco et al., 2006
). The interaction of hnRNP C1/C2 with Ku and DNA-PK is dependent on an interaction between hnRNP C1/C2 and mRNA (Zhang et al., 2004
), which are generally downregulated in response to stress because of transcriptional arrest. The dissociation of hnRNP C1/C2 with Ku and DNA-PK almost certainly has implications in the rate of NHEJ, which DNA-PK and Ku regulate, however it is difficult to determine if this would favor or oppose NHEJ without further evidence. Lastly, hnRNP P2 has been shown to be directly involved in HR repair and hnRNP P2-/-
mice are radiosensitive (Calvio et al., 1995