Many intracellular signaling proteins act as switches that toggle from an inactive to an active state. Once activated, these signaling proteins can stimulate—or in some cases inhibit—other proteins in the signaling pathway. An interaction that results in stimulation is often depicted by an arrowhead (→), whereas inhibition is indicated by a crossbar (—I). In which of the hypothetical signaling pathways outlined below would activation of the receptor lead to an increase in gene expression?