AIM: To understand the influence of Kupffer cell (KC) on signal transduction pathways in the liver.

METHODS: To decrease selectively the number and function of KC, Kunming mice were ip injected with a single dose of gadolinium chloride (GdCl₃, 20 mg•kg^-1), the time-effect relationship assessment was performed after 1 d, 3 d and 6 d. sALT, sGST, liver glycogen content, phagocytic index, and expression of CD68 were assessed as the indexes of hepatotoxicity and functions of KC respectively, and morphology of KC was observed with transmission electron microscopy. Furthermore, cAMP, PGE₂ level, nitric oxide (NO) content, and mRNA expression of NFkappaBp65, Erk1, STAT1 were examined.

RESULTS: GdCl₃ could selectively cause apoptosis of KC and obvious reduction of KC’s activity, but no hepatotoxicity was observed. One day after KC blockade, NO, PGE₂, cAMP contents in the liver were reduced 21.0%, 6.94-fold, 8.3%, respectively, and mRNA expression of NFkappaBp65 was decreased 3.0-fold. The change tendency of NO, PGE₂, and cAMP contents and mRNA expression of NFkappaBp65 were concomitant with recovery of the functions of KC. The contents of NO, PEG2, cAMP were increased when the functions of KC was recovered. However, all of the changes could not return to the normal level except NO content after 6 d Gdcl₃ treatment. No obvious changes were found in STAT1 and Erk1 mRNA expression in the present study.

CONCLUSION: Hepatic NO, PGE2, cAMP level and mRNA expression of NFkappaBp65 are closely related with the status of KC. It suggests that KC may play an important role in the cell to cell signal transduction in the liver.

Kupffer cells (KCs) account for a major portion of the tissue macrophages and play an important role in the defense mechanisms of the body[1]. KCs are involved in the pathogenesis of chemically mediated liver injury through release of biologically active mediators that promote the pathogenic process[2]. KCs can synthesize and release a variety of immuno-modulating and inflammatory mediators such as oxygen-derived free radicals, nitric oxide, lipid mediators, and cytokines, etc. There are certain points to be elucidated that KCs involve in the pathophysiologic response of liver injury[3]. And now, many new functions have been found. KCs can reverse liver fibrosis and are critical for the progression of alcoholic injury[3,4]. Abolishment of KCs sensitization could prevent alcoholic liver injury[5]. KCs are major contributors to cytokine production in hepatic ischemia/reperfusion[6] and play a stimulatory role in liver regeneration[7]. Up to now, few studies about the influence of KCs on signal transduction in the liver have been reported. NO, PGE2, cAMP are important second messengers transmitting and magnifying messages to modulate gene expression. NFkappaB, STAT, Erk are important nuclear transcription factors, which are involved in the regulation of cell proliferation and differentiation[8,9]. To understand the effect of KCs on the second messengers and nuclear transcription factors is of great importance in studying the mechanism of liver diseases. Gdcl₃, as an inhibitor of KCs, is often used as a tool for studying the role of KC[10]. Kupffer cell toxicant GdCl3 prevents stellate cell activation and the development of fibrosis[11]. The present study was designed to clarify the effect of KC on signal transduction pathway in the liver following GdCl₃-induced KC blockade.

In the present study, ip injection of a single dose of GdCl₃ could selectively cause apoptosis of KCs, but did not induce hepatotoxicity. Therefore, it can be used as a tool for studying the role of KCs.

KCs are the phagocytic macrophages in the liver. NO, PGE₂, and cAMP could transmit and magnify extracellular messages to cells through a cascade system to regulate gene expression and cell proliferation and differentiation[18]. NO, PGE₂, and cAMP had different functions in hepatoprotection and hepatic injury. NO from KCs could induce membrane barrier dysfunction in liver sinusoid[19]. But according to Abou-Elella et al[20], the exacerbation of hepatocyte death by KCs was not related to NO. Hsu et al[21-23], sproposed that KCs be the major source of induction of inducible NO synthase (iNOS) activity and NO production have a beneficial role in hepatic IR injury and the constitutive isoforms of NOS play a hepatoprotective role in hepatic injury. The protective function of NO against hepatic injury might lie in that it could reduce tissue oxygenation[24]. PGE₂ derived from KCs increased cAMP, which caused triglyceride accumulation in the liver and fatty liver[3]. Increase of cAMP levels in KCs occurred during the late stage of polymicrobial sepsis, and might contribute to the depression of macrophage phagocytic function[25]. The current study showed that NO, PGE₂, and cAMP contents in the liver were reduced following KC blockade, and recovered following the functional recovery of KCs. It suggests that Kupffer cells may mediate signaling of second messengers in the liver. Moreover, in the present experiment, the hepatic function did not change following the alterations of NO, PGE₂, and cAMP content, the reasons remain to be researched.

It has widely been accepted that NFkappaB activation plays an important role in the pathophysiology of inflammatory disorders[26]. NFkappaB is an essential component of TNF proliferative pathway and TNF-induced changes in IL-6 mRNA, STAT3, and c-myc mRNA are dependent on NFkappaB activation[27]. NFkappaB activation may be important in "switching off" the cytokine cascade during acute pancreatitis[28]. The current study showed that mRNA expression of NFkappaB in the liver was down-regulated after KC blockade, suggesting that KCs may play an important role in mediating liver diseases and inflammatory disorders via changes of the signal transduction pathway in the liver.

In this experiment, STAT1 and Erk1 mRNA expressions were not affected by GdCl₃. STAT1 played a harmful role in Con A-mediated hepatitis, whereas STAT3 protected against liver injury[29]. Inhibition of STAT1 activation without reduction of STAT1 protein level might be one of the factors that are involved in the cAMP-dependent stellate cell growth arrest[30]. In this study, we only examined the expression of STAT1 mRNA, the relationship between STAT1 and hepatic injury needs to be further studied.