TYPE-1 ASTROCYTES INFLUENCE LUTEINIZING - HORMONE-RELEASING HORMONE-RELEASE FROM THE HYPOTHALAMIC CELL-LINE GT1-1 - IS TRANSFORMING GROWTH-FACTOR-BETA THE PRINCIPLE INVOLVED
Rc. Melcangi et al., TYPE-1 ASTROCYTES INFLUENCE LUTEINIZING - HORMONE-RELEASING HORMONE-RELEASE FROM THE HYPOTHALAMIC CELL-LINE GT1-1 - IS TRANSFORMING GROWTH-FACTOR-BETA THE PRINCIPLE INVOLVED, Endocrinology, 136(2), 1995, pp. 679-686
The possible existence of a humoral communication between glial cells
and LHRH-secreting neurons has been studied using the LHRH-secreting G
T1-1 cell line and type 1 astrocytes. Two different designs have been
adopted: 1) GT1-1 cells were coincubated with purified cultures of typ
e 1 rat astrocytes, and 2) GT1-1 cells were exposed to the conditioned
medium (CM) in which type 1 rat astrocytes had been grown for 24 h. L
HRH was measured by RIA in the medium of the GT1-1 cell cultures at di
fferent time intervals. The data show that short periods (1, 3, and 6
h) of either coculture or exposure to previously frozen CM significant
ly increase the release of LHRH from the GT1-1 cells. However, more pr
olonged times of coculture (e.g. 2 and 5 days) or exposure to CM (e.g.
48 h) induce a significant decrease in the amount of LHRH in the medi
um. The stimulatory effect on LHRH release appears to be specific for
type 1 astrocytes (either cortical or hypothalamic), because neither t
he CM, of oligodendrocytes nor the CM of LNCaP cells (a cell line deri
ved from a human prostatic cancer) possess stimulating activities. Hea
ting the type 1 astrocyte-CM to 100 C for 10 min does not eliminate th
e ability of the CM to significantly increase the release of LHRH from
GT1-1 cells at 1, 3, and 6 h. Because of the opposite effects encount
ered in the short and long term experiments, it was hypothesized that
the CM might contain, in addition to LHRH-releasing principle(s), LHRH
-degrading properties. Known amounts of standard LHRH were then added
to type 1 astrocyte-CM, either untreated or submitted to heating at 10
0 C for 10 min. The amount of LHRH added to untreated CM decreases pro
gressively; on the contrary, the amount of LHRH added to heated CM rem
ains unchanged. These results confirm that one or more heat-sensitive
enzymes able to degrade LHRH may be present in the type 1 astrocyte-CM
. As previously mentioned, the experiments reported so far were perfor
med using type 1 astrocyte-CM that had been kept frozen for various pe
riods of time, before being tested for its LHRH-releasing activity. Su
rprisingly, fresh CM proves to be inactive, whereas heated CM is effec
tive; this suggests that the factor involved might be activated by the
two opposite experimental procedures. As it is known that transformin
g growth factor-beta (TGF beta) may be liberated from its supporting p
roteins by freezing or heating, the effects of TGF beta and a TGF beta
-neutralizing antibody were analyzed. The stimulatory effect exerted b
y the frozen CM is completely abolished by 5 mu g of the antibody, whe
reas 5 ng TGF beta 1 proves able to significantly increase LHRH releas
e from GT1-1 cells. In conclusion, the present experiments demonstrate
that type 1 astrocytes of either cortical or hypothalamic origin may
secrete in the medium some factor(s) (probably TGF beta) able to stimu
late the release of LHRH from the hypothalamic LHRH-producing GT1-1 ce
lls; moreover, the data show that one or more enzymes degrading LHRH m
ay be present in type 1 astrocyte-CM. The data provide the first clear
-cut demonstration that glial cells may directly intervene in the cont
rol of LHRH release, integrating the old concept that only neuronal in
fluences may participate in such a phenomenon.