Signal Transduction Pathways Involved in Phosphorylation and Activation of p70^S6K Following Exposure to UVA Irradiation

Ultraviolet light A (UVA) plays an important role in the etiology of human skin cancer, and UVA-induced signal transduction has a critical role in UVA-induced skin carcinogenesis. The upstream signaling pathways leading to p70^S6K phosphorylation and activation are not well understood. Here, we observed that UVA induces phosphorylation and activation of p70 ^S6K. Further, UVA-stimulated p70^S6K activity and phosphorylation at Thr³⁸⁹ were blocked by wortmannin, rapamycin, PD98059, SB202190, and dominant negative mutants of phosphatidylinositol (PI) 3-kinase p85 subunit (DNM-Δp85), ERK2 (DNM.ERK2), p38 kinase (DNM-p38), and JNK1 (DNM-JNK1) and were absent in Jnk1-/- or Jnk2-/- knockout cells. The p70^S6K phosphorylation at Ser⁴¹¹ and Thr ⁴²¹/Ser⁴²⁴ was inhibited by rapamycin, PD98059, or DNM-ERK2 but not by wortmannin, SB202190, DNM-Δp85, or DNM-p38. However, Ser⁴¹¹, but not Thr⁴²¹/Ser⁴²⁴ phosphorylation, was suppressed in DNM.JNK1 and abrogated in Jnk1-/- or Jnk2-/- cells. In vitro assays indicated that Ser⁴¹¹ on immunoprecipitated p70^S6K proteins is phosphorylated by active JNKs and ERKs, but not p38 kinase, and Thr⁴²¹/Ser⁴²⁴ is phosphorylated by ERK1, but not ERK2, JNKs, or p38 kinase. Moreover, p70^S6K co-immunoprecipitated with PI 3-kinase and possibly PDK1. The complex possibly possessed a partial basal level of phosphorylation, but not at MAPK sites, which was available for its activation by MAPKs in vitro. Thus, these results suggest that activation of MAPKs, like PI 3-kinase/mTOR, may be involved in UVA-induced phosphorylation and activation of p70^S6K.