Mutant α-Subunit of the G Protein G12 Activates Proliferation and Inhibits Differentiation of 3T3-F442A Preadipocytes1

We studied the G protein α-subunit Gα12 in various tissues and cell lines. Significant amounts of Gα12 were detected by immunoblots in liver, chromaffin cells, RINm5F cells, 3T3-F442A cells, and preadipocytes, but not in adipocytes, sperm, kidney, NB2A cells, or brain. To study the role of Gα12 in adipose tissue differentiation, the preadipocyte cell line 3T3-F442A was transfected with wild-type Gα12 or a constitutively activated mutant of Gα12. Stable expression of the activated mutant of Gα12 stimulated cell growth and inhibited preadipocyte differentiation. In contrast, wild-type Gα12 overexpression inhibited preadipocyte differentiation, without any effect on cell proliferation. The role of Gα12 on the Raf/MEK/mitogen-activating protein kinase (MAPK) cascade was studied. In confluent preadipocytes, expression of the activated mutant of Gα12 induced an increase in B-Raf expression, but no change in MAPK activity. Differentiation was associated with a decrease in MAPK activity in control 3T3-F442A cells...