Prothymosin alpha antisense oligomers inhibit myeloma cell division.

Abstract

The function of prothymosin a has been investigated by using four different antisense oligodeoxyribonucleotides directed at selected regions of its mRNA. In every case, when synchronized human myeloma cells were released from stationary phase by incubation in fresh medium containing antisense oligomers, cell division was prevented or inhibited; sense oligomers and random antisense oligomers had no effect. A detailed analysis of synchronized cell populations indicated that sense-treated and untreated cells divided m17 hr after growth initiation, whereas cells incubated with antisense oligomer 183, a 16-mer targeted 5 bases downstream of the initiation codon, entered mitosis approximately one cell division late. The failure to divide correlated directly with a deficit in prothymosin a and with the continued presence of intact intracellular antisense oligomers over a period ofat least 24 hr. Because antisense oligomers had no effect either on the timing of the induction of prothymosin a mRNA upon growth stimulation or on mRNA levels seen throughout the cell cycle, we concluded that antisense DNA caused specific hybrid arrest of translation. Our data suggest that prothymosin a is required for cell division. However, there is no evidence that prothymosin a directly regulates mitosis.The function of prothymosin a is debatable. Evidence from this and other laboratories suggests that the protein is neither a precursor of thymosin a,, a putative thymic hormone, nor a secreted thymic hormone itself (1-3). Instead, several observations support a role in cell proliferation: (i) Prothymosin a mRNA and protein are present in virtually all mammalian tissues, and a homologous protein has been detected in yeast (1,(4)(5)(6)(7)(8); these findings are consistent with an activity essential to most cells. (ii) The amounts of prothymosin a and its mRNA are roughly proportional to the proliferative activity of the tissue from which they are isolated (1,4,7). (iii) Prothymosin a mRNA is induced in normal human lymphocytes and in serum-starved NIH 3T3 cells upon growth stimulation with mitogens or serum, respectively (1).Using COS cells transfected with the human prothymosin a gene, we have recently shown that prothymosin a is a nuclear protein. We have also found that chimeric proteins composed of all or part of prothymosin a fused with (3-galactosidase are targeted to the nucleus only when the basic amino acids at the carboxyl terminus of prothymosin a are included (9). The presence of a nuclear localization signal indicates a function carried out, at least in part, in the nucleus; the precise nature of that function is unknown.In the present study, we have directly evaluated the relationship between prothymosin a and cell division. Using antisense oligomers to inhibit accumulation ofprothymosin a in a synchronized population of myeloma cells, we have established that cells deficient in prothymosin a cannot divide, that the inhibition is reversible, and that degradation of intracellular antisense oligomers accompanies resumpti...

A 180-kDa Protein Kinase Seems to Be Responsible for the Phosphorylation of Prothymosin α Observed in Proliferating Cells

“…This is in agreement with the efficiency of antisense oligonucleotides complementary to ProT␣ mRNA for paralyzing cell growth (15). Furthermore, the proposed implication of the c-Myc protein in the transcriptional regulation of the ProT␣ gene (16 -18), although controversial (19), would support this hypothesis.…”

Prothymosin α in Vivo Contains Phosphorylated Glutamic Acid Residues

“…Because the amount of prothymosin ␣ mRNA (and presumably protein) found in a cell is directly proportional to cell growth, the protein is believed to play a role in cell proliferation (1). This idea was reinforced by the observation that synchronized human myeloma cells, in the presence of antisense oligodeoxyribonucleotides directed at prothymosin ␣ mRNA, were unable to divide while detectable amounts of the antisense oligonucleotides remained inside the cell (7). There are now many examples of a link between prothymosin ␣ and growth in systems as diverse as developing mouse embryos (8); normal, mitogenstimulated, and malignant lymphocytes (9,10); and regenerating liver (10).…”

Regulation of prothymosin α by estrogen receptor α: molecular mechanisms and relevance in estrogen-mediated breast cell growth

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