A method of forming a continuous monolayer of primary hepatocytes

Abstract: The present invention provides a monolayer sheet structure of primary hepatocytes that could not be formed by conventional cell layer formation technology, along with a simple method of forming such a monolayer sheet structure, and this invention is a monolayer sheet structure formed as a confluent monolayer of primary hepatocytes cultured on a culture substrate, said monolayer sheet structure is formed by treating the hepatocytes with a protein-phosphorylation inhibitor having an indolo [2,3-a] carbazole or other similar structure in the monolayer formation stage.

Description: BACKGROUND OF THE INVENTION 1. Field of the Intention This invention relates to a method of manufacturing a monolayer sheet structure of primary hepatocytes, and more particularly, to a method of forming a confluent monolayer sheet structure of primary hepatocytes so as to allow a dramatic improvement in the adhesion (attachment) and extension (spreading) of primary hepatocytes on the culture substrate. This invention falls into the category of elemental technology for the systems engineering needed to construct artificial internal organs and bioreactors using cells, and is particularly useful as a technique for forming cell aggregates. 2. Description of the Related Art The most widely used conventional technique for forming monolayers of primary hepatocytes is the simple method consisting of suspending isolated primary hepatocytes in animal cell culture medium containing various bioactive hormones (such as dexamethasone) or serum, and spreading them on a culture substrate (such as a plastic laboratory dish coated with collagen, etc.) , after which they are subjected to standing culture for a few hours to 24 hours. However, the monolayer structure of hepatocytes formed by this method is a intermittently linked, patch-like discontinuous monolayer of one to tens of cells, rather than a so-called confluent monolayer sheet structure, which is continuous and without gaps. Even from this state, a confluent state might be achieved through cell proliferation in the case of other cells, but primary hepatocytes do not proliferate easily, making it impossible to subsequently form a confluent sheet structure from the monolayer sheet structure formed during the first 24 hours. Under these circumstances, and reflecting on the state of the art as described above, the present inventors discovered as a result of exhaustive research that the adhesion and extension of primary hepatocytes on a culture substrate could be dramatically enhanced by treatment thereof with a suitable amount of a protein-phosphorylation inhibitor having an indolo [2,3-a] carbazole skeleton (typical example: staurosporine) during formation of the primary hepatocytes monolayers according the above described conventional technique, and achieved this invention by exploiting this effect. SUMMARY OF THE INVENTION The present invention provides a method of manufacturing a monolayer sheet structure by culturing primary hepatocytes on a culture substrate to produce a confluent monolayer sheet of closely adjoining cells, which method comprises the steps of: (1) suspending said primary hepatocytes in an animal cell culture medium; (2) seeding said primary hepatocytes on said culture substrate; (3) subjecting said primary hepatocytes to standing culture for a few to 24 hours, during which time said culture medium may optionally be exchanged with another culture medium, wherein a protein-phosphorylation inhibitor having an indolo [2,3-a] carbazole skeleton is added to said culture medium in step (1) and, if said culture medium is exchanged with another culture medium, said other culture medium also contains a protein-phosphorylation inhibitor having an indolo [2,3-a] carbazole skeleton. The Detailed Description of The Invention This invention is aimed at providing a confluent monolayer sheet structure of primary hepatocytes that could not be formed with conventional cell monolayer formation technology, and also providing a simple method As noted hereinabove, this invention provides a method of manufacturing a monolayer sheet structure of primary hepatocytes using a protein-phosphorylation inhibitor (compound) with an indolo [2,3-a] carbazole skeleton. This invention, which is to solve the problems described above, comprises the following technological means. (1) A method of manufacturing a monolayer sheet structure by culturing primary hepatocytes on a culture substrate to produce a confluent monolayer, which comprises treating the hepatocytes with a protein-prosphorylation inhibitor having an indolo [2,3-a] carbazole skeleton in the step of formation of the monolayer to form the monolayer sheet structure of primary hepatocytes. (2) The method of manufacturing a monolayer sheet structure of primary hepatocytes according to (1) above, wherein the hepatocytes are treated with staurosporine to form the monolayer sheet structure of primary hepatocytes. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a phase contrast microscope image of monolayer of primary cultured hepatocytes of prior art (A) and monolayer of primary cultured hepatocytes produced in accordance with the method of this invention (B); and Fig. 2 shows a schematic explanatory drawing (A) of a dye permeation assay using transwell filters, and (B) of the results of the assay. DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, this invention is explained further in more detail. Conventional techniques for forming monolayers of primary hepatocytes as well as partial variations on such techniques have been described in detail, for example in "Test Methods for Primary Cultured Hepatocytes" by Toshikazu Nakamura, Gakkai Shuppan Center, 1987, and "Isolated hepatocytes preparation, properties and applications" by M. N. Berry, A. M. Edwards and G. J. Barritt, eds. in Elsevior, 1991. A representative method is that of 1) isolating primary hepatocytes from livers or liver sections by a method such as collagenase perfusion method, 2) suspending them in an animal cell culture medium (such as Williams' E medium, Dulbecco's modified eagle's medium and the like) containing various physiological active hormones (such as dexamethasone and the like) or a few percent to about 10% serum, 3) seeding them on a culture substrate (such as a plastic laboratory dish coated with collagen, etc.) and subjecting them to standing culture for a few to 24 hours, and 4) in many cases, inserting a medium exchange operation after several hours of the seeding of the cells in the step of the standing culture. In practising the present invention, these methods can be used for monolayer formation of primary hepatocytes in addition to treatment of the cells with a protein-phosphorylation inhibitor having an indolo [2,3-a] carbazole skeleton. One compound that typifies the protein-phosphorylation inhibitor having an indolo [2,3-a] carbazole skeleton used in the method of this invention is staurosporine, the structural formula of which is given here: Standard nomenclature [9S-(9a,10b,11b,13a)]-9,13-epoxy-2,3,10,11,12,13-hexahydro-10-methoxy-9-methyl-11-(methylamino)-1H,9H-diindolo[1,2,3-gh:3',2',1'-1m]pyrrolo[3,4-j][1,7]benzodiazonin-1-one; chemical formula C 28 H 28 N 4 O 3 , molecular weight 466. Biochemically, it is known to exhibit inhibitory action against the activity of various protein kinases such as protein kinase C by competitive inhibition of ATP binding. Its effects on cells are known to include inducing differentiation, inducing apoptosis and inhibiting platelet aggregation and the like (Biochemical Dictionary, 3rd Ed., Tokyo Kagaku Dojin, 1998; Omura, S. et al., J Antibiot (Tokyo) , 48, 535-548, (1995)). In the method of this invention, preferably the staurosporine described above is used, but it is also possible to use another protein-phosphorylation inhibitor (compound) having an indolo [2,3-a] carbazole skeleton. Examples are the published substances K252a, K252b, KT5720 and KT5823 (these are described, for example, in the Journal of Cellular Physiology, 179:179-192 (1999), No. 180, Page 182) or KT5926, GF109203X, Ro31-8425, UCN-01, UCN-01-Me, RK-286C, CGP41251 (these are described, for example, in The Journal of Antibiotics, Vol. 48, No. 7, p535-548, No. 536, Fig. 1). In this invention, the protein-phosphorylation inhibitor having an indolo [2,3-a] carbazole skeleton is defined as meaning a substance that exhibits protein inhibition activity, and which is a compound having an indolo [2,3-a] carbazole (IC) skeleton, and derivatives of such a compound. In practising the present invention, it is possible to form a confluent monolayer sheet structure of primary hepatocytes within 24 hours by the simple operation of adding a protein-phosphorylation inhibitor (staurosporine) having an indolo [2,3-a] carbazole skeleton to the medium in the appropriate stages of the method of primary hepatocytes monolayer formation which are described herein above as the stages of 2) the stage of suspending primary hepatocytes in a medium in the method of forming a monolayer of primary hepatocytes, 3) the stage of subjecting them to standing culture after seeding the cells on a culture substrate, and 4) the stage of exchanging of the medium. In the embodiment described above, the protein-phosphorylation inhibitor (staurosporine) having an indolo [2,3-a] carbazole or other similar skeleton can be dissolved directly in the medium to be used, or a prepared stock solution of the inhibitor in an organic solvent such as dimethylsulfoxide, and the like, can be diluted in the medium to be used. In the embodiment described above, the protein-phosphorylation inhibitor (staurosporine) having an indolo [2,3-a] carbazole skeleton is usually used at a final concentration of about 10nM-100nM, but is not limited to this range. In the embodiment described above, "primary hepatocytes" refers to hepatocytes or liver parenchymal cells isolated from livers, and while those derived from mammals such as rats or humans can be used, they are not limited to these examples. Examples The present invention is illustrated below through examples, but this invention is not limited in any way by the following examples. Example 1 In order to isolate primary hepatocytes, rat livers were used and primary hepatocytes were isolated from the rat livers by ordinary methods, that is, by collagenase perfusion and four subsequent low-speed centrifuge isolation. The hepatocytes were suspended in an animal cell culture medium (Williams' E medium) containing 10nM Dexamethasone and 10% fetal bovine serum, and seeded at a cell concentration of 10 x 10 4 cell/cm 2 on 6-well cell culture plastic plates coated with type I collagen. For the samples to be treated, staurosporine was added to the medium to a final concentration of 50nM. Next, they were subjected to standing culture for 4 hours at 37 °C in a CO 2 incubator in the presence of 5% (v/v) CO 2 , and the medium was replaced with Williams' E medium containing 10nM insulin, 10nM dexamethasone and 0.7 µg/ml aprotinin. For the samples to be treated, staurosporine was again added to the medium to a final concentration of 50nM. Finally, after being subjected to the standing culture for 20 hours, the condition of the cells was observed through a phase contrast microscope. The results are shown in Fig. 1. In Fig. 1, A shows a phase contrast microscope image (magnification: × 200) of a primary cultured hepatocytes monolayer made by conventional methods (without staurosporine), while B shows an image of the primary cultured hepatocytes monolayer prepared by the method of this invention (with 50nM staurosporine). The results in Fig. 1 show clearly that in the case of the hepatocytes not treated with staurosporine, cell aggregates are formed, which comprises intermittently linked patch-like discontinuous monolayers of one to tens of cells. On the other hand, in the case of the hepatocytes treated with staurosporine, adhesion (attachment) and extension (spreading) thereof is improved, and cell aggregates are formed, which comprise a confluent continuous monolayer sheet of closely adjoining cells. Example 2 In parallel with Example 1 above, primary rat hepatocytes were seeded on transwell filters (3-micron [3µm] pore polytetrafluoroethylene coated with type I collagen) suitable for 24-well cell culture plates, and differences in the tightness of the cell monolayer structure produced by staurosporine treatment were measured by a dye permeation assay. Fig. 2,A shows a schematic explanatory drawing of a dye permeation assay using transwell filters, while Fig. 2,B shows results of the assay. In the dye permeation assay using transwell filters shown in A of Fig. 2, primary hepatocytes were seeded on transwell filters installed in wells on 24-well cell culture plates, and after the monolayer thereof was formed, 20 µ l of dye (0.25% bromphenol-blue dye) was added to the upper liquid layer (medium) on the filter. 50 µ l samples were then taken with the passage of time from the lower liquid layer (medium) on the filter at a constant temperature of 37 °C, and absorbancy thereof (OD 655 nm) was measured. In the results of B of Fig. 2, courses with the passage of time of dye permeation through hepatocytes layers formed by conventional methods on transwell filters are indicated by the white circles ○, while courses of dye permeation through hepatocytes layers formed by the methods of this invention are indicated by black circles . All measurements were repeated three times, and measured values were converted to relative values in which the value of a control sample after 30 minutes was counted as 1, and a mean ± a standard deviation were calculated and plotted on the graph. It is clear from the results in Fig. 2 above that the hepatocytes monolayers treated with staurosporine showed much less permeation of dye than that of the untreated cell layers. This shows that the tightness between cells is greatly enhanced by treatment thereof with staurosporine. The effects achieved by this invention are that 1) a close, confluent continuous monolayer sheet structure of primary hepatocytes is provided, 2) the aforementioned monolayer sheet structure can be formed within 24 hours through simple operations, and 3) the monolayer sheet structure prepared in accordance with the method of this invention can be used preferably for construction of artificial internal organs and bioreactors for which the confluent monolayer sheet structure of hepatocytes is suitable, and for cell polarity tests by using transwells.

Claims: A method of manufacturing a monolayer sheet structure by culturing primary hepatocytes on a culture substrate to produce a confluent monolayer sheet of closely adjoining cells, which method comprises the steps of: (1) suspending said primary hepatocytes in an animal cell culture medium; (2) seeding said primary hepatocytes on said culture substrate; (3) subjecting said primary hepatocytes to standing culture for a few to 24 hours, during which time said culture medium may optionally be exchanged with another culture medium, wherein a protein-phosphorylation inhibitor having an indolo [2,3-a] carbazole skeleton is added to said culture medium in step (1) and, if said culture medium is exchanged with another culture medium, said other culture medium also contains a protein-phosphorylation inhibitor having an indolo [2,3-a] carbazole skeleton. The method of manufacturing a monolayer sheet structure of primary hepatocytes according to Claim 1, wherein said protein-phosphorylation inhibitor is staurosporine.

Inventors: INAGAKI HIDETOSHI, MAEDA SUMIO, SAITO TAKAO, INAGAKI, HIDETOSHI, MAEDA, SUMIO, SAITO, TAKAO
Applicants: NAT INST OF ADVANCED IND SCIEN, NATIONAL INSTITUTE OF ADVANCED INDUSTRIAL SCIENCE AND TECHNOLOGY
Application year: 2000
Main classification: C12N5
IPC codes: A61M1, C12R1, C12N5