630 M. Guzmán
from glioma cell lines (Galve-Roperh et al. 2000; Sánchez et al. 2001a; Massi et al.
2004), and in the case of JWH-133 also from cells of tumour biopsies of patients
with glioblastoma multiforme (Sánchez et al. 2001a).
2.2
Mechanism of Action
Although the downstream events by which cannabinoids exert their antitumoural
action are not completely unravelled, there is certain evidence for the implication
of at least three mechanisms: tumour cell apoptosis, tumour cell growth arrest,
and inhibition of tumour angiogenesis.
2.2.1
Apoptosis
Glioma cells (Sánchez et al. 1998a; Galve-Roperh et al. 2000) and other cancer cells
in culture (Guzmán 2003; Maccarrone and Finazzi-Agro 2003) undergo apopto-
sis upon long-term cannabinoid challenge. Activation of cannabinoid receptors
and accumulation of the proapoptotic sphingolipid ceramide seem necessary for
glioma cell apoptosis (Table 1). Of interest, following cannabinoid receptor acti-
vation, two peaks of ceramide generation are observed in glioma cells that have
different kinetics (minute- versus day-range), magnitude (two- versus fourfold),
mechanistic origin (sphingomyelin hydrolysis versus de novo ceramide synthesis)
and function (metabolic regulation versus induction of apoptosis) (Guzmán et al.
2001). Studies employing two subclones of C6 glioma cells—one sensitive and the
other resistant to THC—indicate that the apoptotic action of cannabinoids relies
on the second peak of ceramide generation (Galve-Roperh et al. 2000). It has been
suggested that cannabinoids enhance ceramide synthesis de novo via induction
of serine palmitoyltransferase, a regulatory enzyme of sphingolipid biosynthesis
(Gómez del Pulgar et al. 2002b). It is worth noting that ceramide content has
been inversely related with malignant progression and poor prognosis of human
astrocytomas. Thus, low-grade astrocytomas have higher ceramide content than
high-grade astrocytomas (Riboni et al. 2002). Pharmacological inhibition of de
novo ceramide synthesis also prevents cannabinoid-induced death of prostate tu-
mour cells (Mimeault et al. 2003).
The increased ceramide levels observed in glioma cells upon cannabinoid chal-
lenge leads to prolonged activation of the Raf-1/MEK/ERK signalling cascade
(Galve-Roperh et al. 2000). It is generally accepted that ERK activation leads to cell
proliferation. However, the relation between ERK activation and cell fate is com-
plex and depends on many factors, one of which is the duration of the stimulus, as
prolonged ERK activation may mediate cell cycle arrest and cell death. Sustained
Akt inhibition (Gómez del Pulgar et al. 2002b) and JNK and p38 MAPK induc-
tion (Galve-Roperh et al. 2000; Sarker et al. 2003) may also contribute to glioma
cell death (Table 1). Nevertheless, further investigation is necessary to clarify the