Apoptosis of lymphocytes induced by chromium(VI/V) is through ROS-mediated activation of Src-family kinases and caspase-3

Mechanistic insights into Cr(VI)-induced carcinogenicity and possible implication of Cr(V) species formed by the redox reactions of chromium-bearing species have attracted interest. We have previously demonstrated that when human peripheral blood lymphocytes are exposed to the Cr(V) complexes, viz., sodium bis(2-ethyl-2-hydroxybutyrato)oxochromate(V), Na[CrVO(ehba)2] and sodium bis(2-hydroxy-2-methylbutyrato)oxochromate(V), Na[CrVO(hmba)2], apoptosis and formation of reactive oxygen species (ROS) are observed. The molecular mechanisms involving cellular signaling pathways leading to apoptosis are addressed in the present study. Treatment of lymphocytes with Na[CrVO(ehba)2] and K2Cr2O7 leads to the activation of the Src-family protein tyrosine kinases namely, p56lck, p59fyn, and p56/53lyn, which then activates caspase-3, both of which are under the partial influence of ROS. Inhibition of the Src-family tyrosine kinases activity by PP2 and of caspase-3 by Z-DEVD-FMK reverses apoptosis, thereby suggesting their importance. Antioxidants only partially reverse the apoptosis induced by Cr(VI/V), suggesting that pathways other than those induced by ROS cannot be ruled out. Although the complex, Na[CrVO(ehba)2] is known to be relatively stable in aqueous solutions, previous studies have shown that the Cr(V) complex, Na[CrVO(ehba)2] disproportionates to Cr(VI) and Cr(III) forms at pH 7.4 through complex mechanistic processes. Dynamics studies employing EPR data show that the Cr(V) state in Na[CrVO(ehba)2] is relatively more stable in RPMI-1640 medium containing plasma. Formation of ROS during the reaction of redox partners with Na[CrVO(ehba)2] is an early event and compares favorably in kinetic terms with the reported rate processes for disproportionation. This investigation presents evidence for the direct implication of Cr(V) in Cr(VI)-induced apoptosis of lymphocytes.