Signal transduction pathways involved in phosphorylation and activation ofp70(S6K) following exposure to UVA irradiation

Ultraviolet light A (WA) plays an important role in the etiology of human s kin cancer, and WA-induced signal transduction has a critical role in WA-in duced skin carcinogenesis. The upstream signaling pathways leading to p70(S 6K) phosphorylation and activation are not well understood. Here, we observ ed that WA induces phosphorylation and activation of p70(S6K). Further, UVA -stimulated p70(S6K) activity and phosphorylation at Thr(389) were blocked by wortmannin, rapamycin, PD98059, SB202190, and dominant negative mutants of phosphatidylinositol (PI) 3-kinase p85 subunit (DNM-Delta p85), ERK2 (DN M-ERK2), p38 kinase (DNM-p38), and JNK1 (DNM-JNK1) and were absent in Jnk1- /- or Jnk2-/- knockout cells. The p70(S6K) phosphorylation at Ser(411) and Thr(421)/Ser(424) was inhibited by rapamycin, PD98059, or DNM-ERK2 but not by wortmannin, SB202190, DNM-Delta p85, or DNM-p38. However, Ser(411), but not Thr(421)/Ser(424) phosphorylation, was suppressed in DNM-JNK1 and abrog ated in Jnk1-/- or Jnk2-/- cells. In vitro assays indicated that Ser(411) o n immunoprecipitated p70(S6K) proteins is phosphorylated by active JNKs and ERKs, but not p38 kinase, and Thr(421)/Ser(424) is phosphorylated by ERK1, but not ERK2, JNKs, or p38 kinase, Moreover, p70(S6K) co-immunoprecipitate d with PI 3-kinase and possibly PDK1, The complex possibly possessed a part ial basal level of phosphorylation, but not at MAPK sites, which was availa ble for its activation by MAPKs in vitro. Thus, these results suggest that activation of MAPKs, like PI 3-kinase/mTOR, may be involved in WA-induced p hosphorylation and activation of p70(S6K).