Prothymosin alpha (ProTalpha) is an abundant acidic nuclear protein that may be involved in cell proliferation. In our search for its cellular partners, we have recently found that ProTalpha binds to linker histone H1. We now provide further evidence for the physiological relevance of this interaction by immunoisolation of a histone H1-ProTalpha complex from NIH 3T3 cell extracts. A detailed analysis of the interaction between the two proteins suggests contacts between the acidic region of ProTalpha and histone H1. In the context of a physiological chromatin reconstitution reaction, the presence of ProTalpha does not affect incorporation of an amount of histone H1 sufficient to increase the nucleosome repeat length by 20 bp, but prevents association of all further H1. Consistent with this finding, a fraction of histone H1 is released when H1-containing chromatin is challenged with ProTalpha. These results imply at least two different interaction modes of H1 with chromatin, which can be distinguished by their sensitivity to ProTalpha. The properties of ProTalpha suggest a role in fine tuning the stoichiometry and/or mode of interaction of H1 with chromatin.

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A number of studies have indicated that the small nuclear acidic protein prothymosin a is associated with cellular-proliferation events. For example, c-myc causes immediate transcriptional activation of prothymosin a, and prothymosin a antisense oligonucleotides inhibit myeloma cell division.To investigate the regulation of prothymosin a, we examined its mRNA and protein levels during the cell cycle of mononuclear cells and fibroblastic cells. We isolated immunoreactive material from cellular extracts and immunolocalized the protein to the nucleus during the cell cycle. We report here that the material present in the cells is prothymosin a rather than the amino-terminal peptide thymosin 01.['HIThymidine-incorporation studies associate maximum accumulation of mRNA and protein with the S/G2 phase of the cell cycle. This induction of prothymosin a mRNA seems to resemble cyclin B expression and is more pronounced in fibroblasts. Moreover, transient-transfection experiments indicate that transcription factor E2F is a strong positive regulator of the prothymosin a gene. Our results are consistent with the hypothesis that prothymosin a is involved in proliferation checkpoints of the cell cycle.

Transcription factors play an essential role in regulating both cell proliferation and programmed cell death. Proliferation and apoptosis-related transcription factor immunoexpression patterns were concomitantly investigated in tissue sections of normal thyroid, goiters, follicular adenomas and well-differentiated papillary and follicular carcinomas using antibodies against prothymosin alpha, E2F-1, p53, Bcl2, and Bax proteins. Proliferation and apoptotic indices were determined by Ki-67 immunoreactivity and the terminal deoxynucleotidyl transferase-mediated deoxy uridine triphosphate nick-end labeling technique, respectively. Prothymosin alpha and E2F-1 immunoexpression levels were found to be significantly elevated in well-differentiated carcinomas compared to adenomas, goiters and normal tissues (P < 0.05). Both proteins were directly correlated with the proliferation index (P < 0.05). E2F-1 was additionally correlated with the apoptotic index (P < 0.05). The majority of cases were negative for p53 staining. Positive Bcl2 immunostaining was detected in all thyroid histotypes. None of the normal tissues showed Bax immunoreactivity, while positive accumulation differed significantly between hyperplastic and neoplastic histotypes. Direct correlations were observed between prothymosin alpha and Bcl2 as well as between E2F-1 and Bax immunoexpression (P < 0.05). These data demonstrate that prothymosin alpha and E2F-1 are strongly involved in the proliferation processes of thyroid neoplasias. Furthermore, prothymosin alpha may promote cell survival through the Bcl2 anti-apoptotic pathway, while E2F-1-induced apoptosis via p53-independent pathways may be associated with transcriptional activation of bax pro-apoptotic gene.

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