5. Conclusions
The MA-10 cells are an attractive tumor cell line in that one of
the differentiated functions of the cells, namely steroidogenesis,
can be investigated using multiple signaling pathways: PKA, PKC,
and tyrosine kinase. The present study has demonstrated that
activation of these three pathways by hCG, TPA, and EGF, respectively,
leads to increases in both progesterone biosynthesis and
JUNB and c-FOS expression. Heterologous receptor down-regulation
and cellular desensitization, followed by challenges with hCG,
db-cAMP, TPA, and EGF indicate that, under the experimental
conditions used, there is a close parallel between receptor number
and steroidogenesis in most cases investigated. An exception is
that of preincubation with TPA followed by challenge of the cells
with hCG, in which case progesterone production is greater than
that expected from the loss of receptors alone. Expression of JUNB
and c-FOS often, but not always, paralleled the decreases in receptor
number and steroidogenesis. Thus, low levels of intracellular
second messengers and kinases activated may suffice to
render responses comparable to those achieved in the absence of
preincubation