Gadd45a and Gadd45b Protect Haematopoetic Cells from Ultraviolet Radiation Induced Apoptosis Via Distinct Signaling Pathways.

Gadd45 expression, which is stress inducible, has been associated with growth arrest, but the exact role of gadd45 family genes in apoptosis still remains unclear. We have found that myeloid progenitor cells from gadd45a and gadd45b-deficient mice are more sensitive to ultra-violet radiation, VP-16 or daunorubicin induced apoptosis. indicating that gadd45a or gadd45b protect haematopoetic cells from DNA damaging agents. To determine, how gadd45a or gadd45b proteins exert their anti-apoptotic function, bone marrow cells from wild-type and gadd45a or gadd45b deficient mice were exposed to ultraviolet radiation (UV) and analyzed for expression of stress responsive kinases, including JNK and p38. It was observed that P38 and JNK were activated in wt bone marrow cells in response to UV but not in bone marrow cells defecient in gadd45a. Also, the transcription factor NF-kB was activated in wt bone marrow cells, but not in gadd45a−/− cells. The pharmacological inhibitor SB203580 specific for p38, increased apoptosis in reponse to UV, indicating that p38 is implicated in signaling myeloid cell survival. SB203580 was observed also to inhibit the expression of certain NF-kB target genes, including cIAP-1, c-IAP-2, bcl-2 and bcl-xl, in gadd45a+/+ cells but not in gadd45a deficient bone marrow cells. Taken together this data provides first evidence for the role gadd45a plays in the control of hematopoietic cell survival in response to UV, via modulation of P38 MAPK and NF-kB signaling pathways. Unlike in gadd45a−/− bone marrow cells, p38 activation appeared not to be impaired in gadd45b−/− cells, indicating that gadd45b is not involved in p38 activation in myeloid cells. However, UV induced JNK activation was sustained in gadd45b−/− myeloid cells compared to wt cells, indicating that gadd45b is a negative modulator of UV induced JNK signaling in myeloid cells. UV induced activation of MKK4 an upstream regulator of JNK also was impaired in gadd45b−/−. NF-kB was also found activated in wt cells, but not in gadd45b−/− cells. This data indicates that in bone marrow cells exposed to UV, NF-kB induced expression of Gadd45b plays a protective role against UV induced apoptosis via inhibition of MKK4 kinase which in turn results in suppression of JNK activity. Taken together this data provides evidence that Gadd45a and Gadd45b protect haematopoetic cells from genotoxic-stress induced apoptosis via distinct signaling pathways.