Biology Reference
In-Depth Information
weight followed by partial hepatectomy. MC-LR doses (1 and 10 µg kg
-1
body weight; 10, 25 and 50
µg kg
-1
body weight; twice weekly) were subsequently administered i.p. for 8 weeks. This resulted
in three-fold increase of positive pre-neoplastic liver loci with a total increase in the area by 11-folds.
MC-LR did not induce such a response when administered alone. N-methyl-N-nitrosourea was
chosen as tumor initiator (at two oral doses of 40 µg kg
-1
body weight) followed by drinking water
(containing MCs equivalent to 1.2 or 4.2 mg kg
-1
body weight per day for 22 weeks). MCs did not
promote either lymphoid or duodenal tumors (Falconer and Humpage, 1996).
Male Fischer 344 rats were injected i.p. with DEN as tumor promoter (200 mg kg
-1
body weight)
and two weeks after initiation it was followed by nodularin injections i.p. (25 µg kg
-1
body weight
twice a week until 12 weeks). The production of four types of nodules with eosinophilic, clear,
mixed and basophilic cells was noted. Two weeks after nodularin administration was stopped, a
staining reaction (GST-P immunostaining, i.e. glutathione S-transferase placental form positive)
revealed dense nodules that stained homogeneously and pale nodules that stained heterogeneously
(Song
et al
., 1999). Thus it was concluded that (i) nodularin has a promoting effect by inducing
hepatocyte proliferation; (ii) proliferation of hepatocytes is a temporary phenomenon and (iii) enzyme
altered hepatocyte dense nodules are progressive and pale nodules are regressive. Since MCs and
nodularins are potent inhibitors of PP1 and PP2A, these are considered to be non-phorbol ester (12-
O-tetradecanoyl phorbol 13-acetate, TPA)-type promoters. Okadaic acid, tautomycin and calyculin
are the other such substances (Fujiki and Suganuma, 1993). Protein phosphatase inhibition leads to
a shift in higher phosphorylation of certain target proteins such as tumor suppressor proteins. This
is considered to be a major post-translational modifi cation resulting in excessive signalling. This
leads to cell proliferation, cell transformation and tumor promotion (Fujiki and Suganuma, 1993;
Ohta
et al
., 1994).
Cell death occurs either by programmed cell death (apoptosis) or necrosis. It is pertinent here to
point out the differences between these two processes. Apoptosis is a normal physiological process
that takes place to maintain cell homeostasis. A number of morphological and physiological changes
occur that involve disintegration of cytoskeleton, cell shrinkage, chromatin condensation, and
activation of specifi c proteases known as capsases (Kerr
et al
., 1972; Wyllie
et al
., 1980; Nicholson and
Thornberry, 1997). In the early events, reversal in position of phosphatidyl serine takes place, i.e.
phosphatidyl serine located on the inner face of plasma membrane becomes exposed on the outer
surface. This acts as a recognition signal for phagocytosis. A number of external stimuli including
receptor ligation and toxins are responsible for apoptosis. On the other hand, necrotic cell death results
due to mechanical or toxic cell injury. A number of morphological changes occur in necrotic cells
such as swelling, plasma membrane rupture, organelle breakdown and ultimately lysis followed by
release of cell contents and an infl ammatory response (Wyllie
et al
., 1980). Characteristic centrilobular
hepatocyte rounding, dissociation, necrosis and breakdown of sinusoidal epithelium leads to severe
interhepatic haemorrhage. Acute toxicity leads to cytoskeletal collapse due to aggregation of actin
fi laments, microtubules and intermediate fi laments within hepatocytes (Hooser
et al
., 1991; Khan
et al
., 1996; Wickstrom
et al
., 1995). Rapid hyperphosphorylation of cytoskeletal proteins such as
cytokeratin seems to be responsible for the pathogenesis secondary to the inhibition of PP1 and
PP2A (Falconer and Yeung, 1992; Ohta
et al
., 1992; Runnegar
et al
., 1993).
Severe inhibition of PP1 and PP2A by MCs and nodularins leads to an imbalance in normal
cellular processes such as cell proliferation and results in cancer production or apoptosis (Boe
et al
.,
1991; Fujiki and Suganuma, 1993; Toivola
et al
., 1994; Fladmark
et al
., 1998; Mankiewicz
et al
., 2001).
Stimulation in cytokinesis and decrease in the rate of apoptosis were observed when MC-LR was
administered as a single dose of 1 nM or multiple doses of MC-LR at pM concentrations (Humpage
