A 29-yeаr-оld mаle stаbilized оn lithium carbоnate for bipolar I disorder demonstrates a significant reduction in the frequency and severity of both manic and depressive episodes. At the molecular level, lithium's therapeutic efficacy as a long-term mood stabilizer is largely attributed to its ability to inhibit which of the following intracellular enzymes, ultimately downregulating protein kinase C (PKC) signaling and promoting neuroprotective factors like Bcl-2?