Differential expression of collagen types I, III, and IV by fat-storing (Ito) cells in vitro

It has been observed that Ito cells in vitro undergo phenotypical changes ("activation") similar to those noted in vivo during the development of liver fibrosis. Because conflicting data have been published on the amount and different types of collagens synthesized by Ito cells in vitro, collagen biosynthesis was studied at different "activation" stages on both the protein and RNA levels. Immunoprecipitation of endogenously labeled collagen showed that freshly isolated ("resting") Ito cells synthesize mainly collagen type IV. Collagen type I was hardly detectable in the earlier stage of primary culture, but it clearly increased starting 5 days after isolation. Compared with the basal rates measured at day 3 after isolation, collagen types I, III, and IV increased 7.5-, 3.5-, and 1.9-fold, respectively, until day 7 of culture. The relative ratios of newly synthesized collagen types I, III, and IV on day 3 after isolation were approximately 10%, 45%, and 45%, and they changed to 45%, 40%, and 15% on day 7 of primary culture. On the RNA level, freshly isolated Ito cells contained predominantly collagen type IV- and III-specific transcripts. By densitometric analysis, collagen type I, III, and IV messenger RNAs increased 6.2-, 2.5-, and 3.5-fold from day 3 to day 7 of primary culture. These results indicate that "resting" Ito cells synthesize primarily collagen type IV and could be a major cellular source of this basement membrane component in normal liver. "Activated" Ito cells switch to the synthesis of the interstitial collagen types I and III and might be mainly responsible for the accumulation of collagen types I and III in fibrotic liver diseases.