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A sarcoma-derived protein regulates hepatocyte metabolism via autocrine production of tumor necrosis factor-alpha
C P Fischer1, B P Bode, W W Souba
1Department of Surgery, Massachusetts General Hospital, Boston, USA.
Objective:
The effects of conditioned media from the methylcholanthrene (MCA) fibrosarcoma on hepatocyte albumin production and amino acid transport were studied. The authors characterized a factor responsible for the observed effects and investigated the role of tumor necrosis factor-alpha (TNF-alpha) in these events.
Summary Background Data:
Cancer cachexia is mediated in part by TNF-alpha. However, few tumors secrete TNF-alpha, implicating host production of this cytokine in response to as yet uncharacterized tumor-derived factors. Autocrine production of TNF-alpha recently has been described as a potent mechanism for orchestrating hepatic metabolism.
Methods:
Conditioned media from the MCA fibrosarcoma was incubated with isolated primary rat hepatocytes. Albumin production and TNF-alpha production by hepatocytes was measured by enzyme-linked immunosorbent assay and amino acid transport assayed by tritium (3H)-labeled amino acid uptake. Dialysis membranes ranging from 3 kD to 100 kD were used to determine the size of the factor/factors responsible for the observed effects.
Results:
Conditioned media from the MCA fibrosarcoma contained no TNF-alpha, whereas treatment of primary rat hepatocytes with the conditioned media resulted in a 53-fold increase in TNF-alpha production by hepatocytes compared with control. Treatment of hepatocytes with MCA fibrosarcoma-conditioned media resulted in decreases in hepatic albumin production of 46%, 61%, and 42% over 3 days of treatment, and these effects were reversible by the addition of antibody to TNF-alpha. Treatment of hepatocytes with MCA fibrosarcoma conditioned media resulted in increases in hepatocyte amino acid transport via inductions of System N (1.87 fold) and System A (1.93 fold). These effects were partially abrogated by the addition of antibody to TNF-alpha. Dialysis experiments determined the molecular weight of the factor or factors responsible for the observed effects to be greater than 100 kD. The effects of the MCA fibrosarcoma conditioned media were abolished by both trypsin treatment and heat inactivation, indicating the protein nature of the factor being studied.
Conclusions:
A tumor-derived protein has been isolated from the MCA fibrosarcoma. The protein inhibits hepatocyte albumin production and increases amino acid transport in vitro via the autocrine production of TNF-alpha.