Generation of carboxy-terminally deleted forms of STAT5 during preparation of cell extracts

N-terminal truncation of Stat5a/b circumvents PIAS3-mediated transcriptional inhibition of Stat5 in prostate cancer cells

Transcription factor Stat5a/b is critical for prostate cancer cell survival and for prostate xenograft tumor growth. In addition, the Stat5a/b signaling pathway may contribute to progression of organ-confined prostate cancer to castration-resistant and/or metastatic disease. Expression of nuclear Stat5a/b is clustered to high grade human prostate cancers, and nuclear Stat5a/b in primary prostate cancer predicts early disease recurrence after initial treatment. Here, we show by Western blotting and electromobility shift assay that Stat5a/b protein in human prostate cancer is N-terminally truncated. This short form of Stat5a/b is generated post-translationally in vivo in prostate cancer cells and is the predominant form of Stat5a/b that binds to DNA. We further demonstrate by mutagenesis and co-immunoprecipitations that the N-domain of Stat5a/b is required for binding to PIAS3, and that PIAS3 inhibits transcriptional activity of Stat5a/b in breast cancer cells but not in prostate cancer cells. Thus, the proteolytic cleavage of the N-terminus of Stat5a/b may be a mechanism by which Stat5 evades the transcriptional repression by PIAS3 in prostate cancer cells, and results in increased Stat5-driven gene expression and prostate cancer progression.

STAT5 isoforms: controversies and clarifications

STAT (signal transducer and activator of transcription) family transcription factors are critical regulators of the development and differentiation of many cell types. STAT isoforms are generated by alternative splicing, but have also been suggested to be generated post-transcriptionally. In this issue of the Biochemical Journal, Schuster and colleagues have identified cathepsin G as the protease that cleaves full-length STAT5 (STAT5alpha) to generate a C-terminally truncated form in immature myeloid cells. However, the authors argue that this proteolytically generated isoform does not occur naturally in vivo; rather, it is artificially generated by cathepsin G during the preparation of cell extracts. This new evidence calls into question the physiological significance of this putative isoform and forces the general re-examination of proteolytically generated STAT isoforms.

The G-CSF receptor carboxyl terminus, truncated in AML/SCN, is required for induction of a Stat5 protease activity

Granulocyte colony-stimulating factor (G-CSF) has been shown to stimulate the activation of the signal transducer and activator of transcription 5 (Stat5). We show here that G-CSF-stimulated activation of Stat5 was attenuated when myeloid cells were induced to differentiate with G-CSF. Attenuated activation of Stat5 correlated with reduced Stat5 protein levels, which was associated with upregulation of a Stat5 protease activity. Carboxyl terminal truncation of the G-CSF receptor or expression of leukemogenic proteins Bcr-Abl and Tel-Jak2 abolished the upregulation of the Stat5 protease activity by G-CSF. These data add to our understanding of the roles of G-CSF and Stat5 in normal granulopoiesis and leukemogenesis.

Extracellular matrix regulates alpha s1-casein gene expression in rabbit primary mammary cells and CCAAT enhancer binding protein (C/EBP) binding activity

Previous studies have shown that both the signal transducer and activator of transcription 5 (STAT5) and the CCAAT enhancer binding proteins (C/EBPs) are involved in the regulation of casein gene expression by mammary epithelial cells. Prolactin (Prl) activation of STAT5 is necessary for casein gene expression. The extracellular matrix (ECM) regulates also casein gene expression. Here, we have investigated whether ECM regulates C/EBPs activity in primary rabbit mammary epithelial cells. Isolated primary mammary cells were cultured on plastic or on floating collagen I gel. Prolactin induced alphas 1-casein gene expression when cells were cultured on collagen but not on plastic. It is noteworthy that activated STAT5 was detected in both culture conditions. Several STAT5 isoforms (STAT5a, STAT5b, and other STAT5 related isoforms, some with lower molecular weight than the full-length STAT5a and STAT5b) were detected under the different culture conditions. However, their presence was not related to the expression of alphas 1-casein gene. The binding of nuclear factors to a C/EBP specific binding site and the protein level of C/EBPbeta differed in cells cultured on plastic or on collagen but these parameters were not modified by Prl. This suggests that C/EBP binding activity was regulated by ECM and not by Prl. Interestingly, these modifications were correlated to the expression of the alphas 1-casein gene. Hence, the activation of the alphas 1-casein gene expression depends on two independent signals, one delivered by Prl via the activation of STAT5, the other delivered by ECM via C/EBP.

Stat5a and Stat5b: fraternal twins of signal transduction and transcriptional activation

Stat5a and Stat5b are discretely encoded transcription factors that mediate signals for a broad spectrum of cytokines. Their activation is often an integral component of redundant cytokine signal cascades involving complex cross-talk and pleiotropic gene regulation by Stat5 has been implicated in cellular functions of proliferation, differentiation and apoptosis with relevance to processes of hematopoiesis and immunoregulation, reproduction, and lipid metabolism. Although Stat5a and Stat5b show peptide sequence similarities of > 90%, targeted gene disruptions in mice yield distinctive phenotypes. Prolactin-directed mammary gland maturation fails without functional Stat5a, while disruption of Stat5b in males mitigates growth hormone effects on hepatic function and body mass. The molecular basis for this biologic dichotomy is probably multifaceted. Limited structural dissimilarities between the Stat5a and Stat5b transactivation domains, or subtle differences in the DNA-binding affinities of Stat5 dimer pairs undoubtedly influence gene regulation, but cell-dependent asymmetries in availability of phosphorylated Stat5 can be an underlying factor. Differences in serine phosphorylation(s) of Stat5a and Stat5b, or Stat5 associations with adaptor proteins or co-transcription factors are other potential sources of functional disparity and the signal amplitude, frequency or duration also can be significant. In addition to Stat5 signal attenuation by phosphatase actions or classical feedback inhibition, truncated forms of Stat5 lacking in transactivation capacity may compete upstream for activation and diminish access of full length molecules to DNA binding sites.