Progress towards the Replacement of the Rabbit Blood Sugar Test for the Quantitative Determination of the Biological Activity of Insulins (USP <121>) with an In Vitro Assay

For the quantification of insulin activity, United States Pharmacopeia (USP) general chapter <121> continues to require the rabbit blood sugar test. For new insulin or insulin analogue compounds, those quantitative data are expected for stability or comparability studies. At Sanofi, many rabbits were used to fulfil the authority's requirements to obtain quantitative insulin bioactivity data until the in vivo test was replaced. In order to demonstrate comparability between the in vivo and in vitro test systems, this study was designed to demonstrate equivalency. The measurement of insulin lispro and insulin glargine drug substance and drug product batches, including stress samples (diluted or after temperature stress of 30 min at 80 °C), revealed a clear correlation between the in vitro and in vivo test results. The recovery of quantitative in vitro in-cell Western (ICW) results compared to the in vivo test results was within the predefined acceptance limits of 80% to 125%. Thus, the in vitro ICW cell-based bioassay leads to results that are equivalent to the rabbit blood sugar test per USP <121>, and it is highly suitable for insulin activity quantification. For future development compounds, the in vitro in-cell Western cell-based assay can replace the rabbit blood sugar test required by USP <121>.

SOCS1 function in BCR-ABL mediated myeloproliferative disease is dependent on the cytokine environment

Treatment with tyrosine kinase inhibitors is the standard of care for Philadelphia chromosome positive leukemias. However the eradication of leukemia initiating cells remains a challenge. Circumstantial evidence suggests that the cytokine microenvironment may play a role in BCR-ABL mediated leukemogenesis and in imatinib resistance. Gene expression analyses of BCR-ABL positive ALL long-term cultured cells revealed strong reduction of SOCS mRNA expression after imatinib treatment, thereby demonstrating a strong inhibition of cytokine signaling. In this study we employed SOCS1—a strong inhibitor of cytokine signaling—as a tool to terminate external cytokine signals in BCR-ABL transformed cells in vitro and in vivo. In colony formation assays with primary bone marrow cells, expression of SOCS1 decreased colony numbers under pro-proliferative cytokines, while it conferred growth resistance to anti-proliferative cytokines. Importantly, co-expression of SOCS1 with BCR-ABL led to the development of a MPD phenotype with a prolonged disease latency compared to BCR-ABL alone in a murine bone marrow transplantation model. Interestingly, SOCS1 co-expression protected 20% of mice from MPD development. In summary, we conclude that under pro-proliferative cytokine stimulation at the onset of myeloproliferative diseases SOCS1 acts as a tumor suppressor, while under anti-proliferative conditions it exerts oncogenic function. Therefore SOCS1 can promote opposing functions depending on the cytokine environment.

The Phosphatases STS1 and STS2 Regulate Hematopoietic Stem and Progenitor Cell Fitness

FLT3 and c-KIT are crucial regulators of hematopoietic stem and progenitor cells. We investigated the role of STS1 and STS2 on FLT3 and c-KIT phosphorylation, activity, and function in normal and stress-induced hematopoiesis. STS1/STS2-deficient mice show a profound expansion of multipotent progenitor and lymphoid primed multipotent progenitor cells with elevated colony-forming capacity. Although long-term hematopoietic stem cells are not increased in numbers, lack of STS1 and STS2 significantly promotes long-term repopulation activity, demonstrating a pivotal role of STS1/STS2 in regulating hematopoietic stem and progenitor cell fitness. Biochemical analysis identified STS1/STS2 as direct phosphatases of FLT3 and c-KIT. Loss of STS1/STS2 induces hyperphosphorylation of FLT3, enhances AKT signaling, and confers a strong proliferative advantage. Therefore, our study reveals that STS1 and STS2 may serve as novel pharmaceutical targets to improve hematopoietic recovery after bone marrow transplantation.

Interferon-γ impairs proliferation of hematopoietic stem cells in mice

Balancing the processes of hematopoietic stem cell (HSC) differentiation and self-renewal is critical for maintaining a lifelong supply of blood cells. The bone marrow (BM) produces a stable output of newly generated cells, but immunologic stress conditions inducing leukopenia increase the demand for peripheral blood cell supply. Here we demonstrate that the proinflammatory cytokine interferon-γ (IFN-γ) impairs maintenance of HSCs by directly reducing their proliferative capacity and that IFN-γ impairs restoration of HSC numbers upon viral infection. We show that IFN-γ reduces thrombopoietin (TPO)-mediated phosphorylation of signal transducer and activator of transcription (STAT) 5, an important positive regulator of HSC self-renewal. IFN-γ also induced expression of suppressor of cytokine signaling (SOCS) 1 in HSCs, and we demonstrate that SOCS1 expression is sufficient to inhibit TPO-induced STAT5 phosphorylation. Furthermore, IFN-γ deregulates expression of STAT5-mediated cell-cycle genes cyclin D1 and p57. These findings suggest that IFN-γ is a negative modulator of HSC self-renewal by modifying cytokine responses and expression of genes involved in HSC proliferation. We postulate that the occurrence of BM failure in chronic inflammatory conditions, such as aplastic anemia, HIV, and graft-versus-host disease, is related to a sustained impairment of HSC self-renewal caused by chronic IFN-γ signaling in these disorders.

The lysosomal polypeptide transporter TAPL is stabilized by interaction with LAMP-1 and LAMP-2

TAPL (ABCB9) is a homodimeric polypeptide translocation machinery which transports cytosolic peptides into the lumen of lysosomes for degradation. Since the function of proteins is strongly dependent on the interaction network involved, we investigated the interactome of TAPL. A proteomic approach allowed identification of the lysosome-associated membrane proteins LAMP-1 and LAMP-2B as the most abundant interaction partners. Albeit with low frequency, major histocompatibility complex II subunits were also detected. The interaction interface with LAMP was mapped to the four-transmembrane helices constituting the N-terminal domain of TAPL (TMD0). The LAMP proteins bind independently to TAPL. This interaction has influence on neither subcellular localization nor peptide transport activity. However, in LAMP-deficient cells, the half-life of TAPL is decreased by a factor of five, whereas another lysosomal membrane protein, LIMP-2, is not affected. Reduced stability of TAPL is caused by increased lysosomal degradation, indicating that LAMP proteins retain TAPL on the limiting membrane of endosomes and prevent its sorting to intraluminal vesicles.