eIF5A-Independent Role of DHPS in p21 and Cell Fate Regulation.

Deoxyhypusine synthase (DHPS) catalyzes the first step of hypusination of the elongation translation factor 5A (eIF5A), and these two proteins have an exclusive enzyme-substrate relationship. Here we demonstrate that DHPS has a role independent of eIF5A hypusination in A375 and SK-MEL-28 human melanoma cells, in which the extracellular signal regulated kinase 1/2 (ERK1/2) pathway is deregulated. We found that RNA interference of DHPS induces G0/G1 cell cycle arrest in association with increased p21 expression in these cells whereas eIF5A knockdown induces cell death without increasing p21 expression. Interestingly, p21 knockdown switched DHPS knockdown-induced growth arrest to cell death in these cells, suggesting a specific relation between DHPS and p21 in determining cell fate. Surprisingly, ectopic expression of DHPS-K329R mutant that cannot hypusinate eIF5A abrogated DHPS knockdown-induced p21 expression in these cells, suggesting a non-canonical role of DHPS underlying the contrasting effects of DHPS and eIF5A knockdowns. We also show that DHPS knockdown induces p21 expression in these cells by increasing transcription through TP53 and SP1 in an ERK1/2-dependent manner. These data suggest that DHPS has a role independent of its ability to hypusinate eIF5A in cells, which appears to be important for regulating p21 expression and cell fate.