THE INABILITY OF CELLS TO GROW IN LOW IRON CORRELATES WITH INCREASINGACTIVITY OF THEIR IRON REGULATOR PROTEIN (IRP)

We studied the factors that determine the differing growth requirement s of low-iron-tolerant (LIT) versus high-iron-dependent (HID) cells fo r extracellular nontransferrin iron. The growth of LIT cells HeLa and THP-1, when transferred from transferrin (5 mu g/ml) medium into low-i ron (5 mu M ferric citrate) medium, was not significantly affected whi le HID cells Jiyoye and K562 showed nearly no growth. HeLa and THP-1 c ells, as well as Jiyoye and K562 cells, do not produce transferrin in sufficient amounts to support their growth in low-iron medium. Surpris ingly, similar rates of iron uptake in low-iron medium (0.033 and 0.03 2 nmol Fe/min and 10(6) cells) were found for LIT cells HeLa and HID c ells K562. Furthermore, the intracellular iron level (4.64 nmol/10(6) cells) of HeLa cells grown in low-iron medium was much higher than iro n levels (0.15 or 0.20 nmol/10(6) cells) of HeLa or K562 cells groan i n transferrin medium. We demonstrated that the activity (ratio activat ed/total) of the iron regulatory protein (IRP) in HID cells Jiyoye and K562 increased more than twofold (from 0.32 to 0.79 and from 0.47 to 1.12, respectively) within 48 h after their transfer into low-iron med ium. In the case of LIT cells HeLa and THP-1, IRP activity stayed at s imilar or slightly decreased levels (0.86-0.73 and 0.58-0.55, respecti vely). Addition of iron chelator deferoxamine (50 mu M, i.e., about ha lf-maximal growth-inhibitory dose) resulted in significantly increased activity of IRP also in HeLa and THP-1 cells. We hypothesize that the relatively higher bioavailability of nontransferrin iron in LIT cells , over that in HID cells, determines the differing responses observed under low-iron conditions.