Overexpression of PACEsol improves BMP-7 processing in recombinant CHO cells

Enhancing the production of recombinant human TGF-β1 through an understanding of TGF-β1 synthesis, signaling, and endocytosis in CHO cells

To enhance the production of recombinant human transforming growth factor-beta1 (rhTGF-β1) in Chinese hamster ovary (CHO) cells, rhTGF-β1 was first characterized for endocytosis, signaling pathway, and overall maturation process. The mature rhTGF-β1 used for clinical application was internalized into CHO cells and inhibited the growth of CHO cells in a dose-dependent manner. However, mature rhTGF-β1 was mostly produced in the form of latent rhTGF-β1 in cultures of recombinant CHO (rCHO) cells producing rhTGF-β1 (CHO-rhTGF-β1). The concentration of active mature rhTGF-β1 in the culture supernatant of CHO-rhTGF-β1 cells was not high enough to compromise yield. In addition, a significant amount of unprocessed precursors was produced by CHO-rhTGF-β1 cells. Overexpression of PACEsol, a soluble form of furin, in CHO-rhTGF-β1 cells was effective for the proteolytic cleavage of unprocessed precursors. The highest mature rhTGF-β1 concentration (6.4 μg mL-1 ) was obtained with the PACEsol-expressing clone, which was approximately 45% higher than that of the parental clone (P < 0.01). Thus, a comprehensive understanding of the intrinsic properties of rhTGF-β1 with respect to the overall maturation process, signaling pathway, and endocytosis is essential for effectively enhancing the production of mature rhTGF-β1 in CHO cells.

Improving bone morphogenetic protein (BMP) production in CHO cells through understanding of BMP synthesis, signaling and endocytosis

Bone morphogenetic proteins (BMPs) are a group of growth factors with the clinical potential to regulate cartilage and bone formation. Functionally active mature recombinant human BMPs (rhBMPs), produced primarily in Chinese hamster ovary (CHO) cells for clinical applications, are considered difficult to express because they undergo maturation processes, signaling pathways, or endocytosis. Although BMPs are a family of proteins with similar mature domain sequence identities, their individual properties are diverse. Thus, understanding the properties of individual rhBMPs is essential to improve rhBMP production in CHO cells. In this review, we discuss various approaches to improve rhBMP production in CHO cells by understanding the overall maturation process, signaling pathways and endocytosis of individual rhBMPs.

Selective endocytosis of recombinant human BMPs through cell surface heparan sulfate proteoglycans in CHO cells: BMP-2 and BMP-7

Cell surface heparan sulfate proteoglycan (HSPG)-mediated endocytosis results in poor yields of recombinant human bone morphogenetic proteins (rhBMPs) from CHO cell cultures. Upon incubation of rhBMP-2 and rhBMP-7 with CHO cells at 37 °C, both rhBMP-2 and rhBMP-7 bound to the cell surface HSPGs in CHO cells, but only rhBMP-2 was actively internalized into CHO cells. Cell surface HSPGs were found to serve as the main receptor for rhBMP-2 internalization. It was also found that the cell surface HSPG-mediated endocytosis of rhBMP-2 occurred through both the clathrin- and caveolin-dependent pathways. Blockage of rhBMP-2 internalization by the addition of structural analogs of HSPGs such as dextran sulfate (DS) and heparin dramatically increased rhBMP-2 production in recombinant CHO (rCHO) cell cultures. Compared to the control cultures, addition of DS (1.0 g/L) and heparin (0.2 g/L) resulted in a 22.0- and 19.0-fold increase in the maximum rhBMP-2 concentration, respectively. In contrast, the production of rhBMP-7, which was not internalized into the rCHO cells, did not dramatically increase upon addition of DS and heparin. Taken together, rhBMPs have a different fate in terms of HSPG-mediated internalization in CHO cells. HSPG-mediated endocytosis of each rhBMP should be understood individually to increase the rhBMP yield in rCHO cell cultures.

Cell Factory Engineering

Rational approaches to modifying cells to make molecules of interest are of substantial economic and scientific interest. Most of these efforts aim at the production of native metabolites, expression of heterologous biosynthetic pathways, or protein expression. Reviews of these topics have largely focused on individual strategies or cell types, but collectively they fall under the broad umbrella of a growing field known as cell factory engineering. Here we condense >130 reviews and key studies in the art into a meta-review of cell factory engineering. We identified 33 generic strategies in the field, all applicable to multiple types of cells and products, and proven successful in multiple major cell types. These apply to three major categories: production of native metabolites and/or bioactives, heterologous expression of biosynthetic pathways, and protein expression. This meta-review provides general strategy guides for the broad range of applications of rational engineering of cell factories.

Improving the production of recombinant human bone morphogenetic protein-4 in Chinese hamster ovary cell cultures by inhibition of undesirable endocytosis

Endocytic regulation serves a critical role in modulating the extracellular level of signaling molecules, such as bone morphogenetic proteins (BMPs). Unfortunately, endocytosis may result in poor yields of recombinant human BMP-4 (rhBMP-4) from Chinese hamster ovary (CHO) cell cultures. When rhBMP-4 was incubated with CHO cells, rhBMP-4 was actively internalized into cells. Cell surface bound heparan sulfate proteoglycans (HSPGs) served as the major receptors for rhBMP-4 internalization. Removal of cell surface heparan sulfate (HS) by heparinases or reduction of HSPG synthesis by knockdown of xylosyltransferase2 (xylt2) in CHO cells decreased internalization of rhBMP-4. In addition, treatment with endocytosis inhibitors (chlorpromazine, genistein, and dynasore) identified a clathrin- and dynamin-dependent endocytic pathway as the major route for rhBMP-4 internalization. To enhance product yield by minimizing rhBMP-4 internalization in recombinant CHO (rCHO) cell cultures, we have tested various strategies to reduce HSPG synthesis (knockdown of xylt2 and sodium chlorate treatment) or inhibit the binding of rhBMP-4 to cell-surface-bound HSPGs (supplementation with heparin or dextran sulfate [DS]). Among these approaches, DS, which is a linear anionic sulfated polysaccharide with similarity to HS chains, was the most effective in enhancing rhBMP-4 production in rCHO cell cultures. Compared with the control cultures, DS addition to the culture medium (1.0 g/L) resulted in 1.4-fold and 2.3-fold increases in maximum rhBMP-4 concentration in batch and fed-batch cultures, respectively. Taken together, the addition of DS, an effective competitor of HSPGs, improved rhBMP-4 production in rCHO cell cultures through blockage of rhBMP-4 internalization.

Production, Isolation, and Structural Analysis of Ligands and Receptors of the TGF-β Superfamily

The ability to understand the molecular mechanisms by which secreted signaling proteins of the TGF-β superfamily assemble their cell surface receptors into complexes to initiate downstream signaling is dependent upon the ability to determine atomic-resolution structures of the signaling proteins, the ectodomains of the receptors, and the complexes that they form. The structures determined to date have revealed major differences in the overall architecture of the signaling complexes formed by the TGF-βs and BMPs, which has provided insights as to how they have evolved to fulfill their distinct functions. Such studies, have however, only been applied to a few members of the TGF-β superfamily, which is largely due to the difficulty of obtaining milligram-scale quantities of highly homogenous preparations of the disulfide-rich signaling proteins and receptor ectodomains of the superfamily. Here we describe methods used to produce signaling proteins and receptor ectodomains of the TGF-β superfamily using bacterial and mammalian expression systems and procedures to purify them to homogeneity.

Overproduction of recombinant human bone morphogenetic protein-7 in Chinese hamster ovary cells

Bone morphogenetic protein-7 is a multifunctional growth factor involved in various cellular processes such as osteogenesis, kidney and eye development, brown adipogenesis, and bone metastasis, and thus has been considered to have therapeutic potential for treating various diseases. In this study, we established a Chinese hamster ovary (CHO) cell line stably overexpressing recombinant human BMP-7 (rhBMP-7). Over the course of a 14-day fed-batch culture process in a 7.5-l bioreactor (5-l working volume) using chemically defined medium, the established cells could produce over 188 mg/l of rhBMP-7 protein. The rhBMP-7 was purified to homogeneity from the culture supernatant using a two-step chromatographic procedure that resulted in a recovery rate of approximately 55%, with protein purity greater than 95%. The purified rhBMP-7 was further demonstrated to be functionally active by measuring the proliferation of MC3T3-E1 cells, revealing a half-maximal effective concentration of 28.31 ng/ml.