Beyond the gap: moonlighting functions of connexins in cancer

Connexins are classically defined as the structural subunits of gap junction channels that mediate direct intercellular communication. In cancer, their role has long been interpreted through this canonical role, with loss of gap junction intercellular communication considered permissive for tumor initiation. Although genomic alterations in connexin genes have been identified across diverse malignancies, their mechanistic implications remain poorly defined, and current cancer treatment strategies largely target channel activity alone. However, accumulating evidence demonstrates that connexins also exert important channel-independent, non-canonical functions that reshape oncogenic signaling networks. These “moonlighting” activities, defined as mechanistically distinct functions performed by a single protein, reposition connexins as active regulators of tumor cell behavior rather than passive conduits of small signaling molecules and messengers. Connexin moonlighting functions influence multiple hallmarks of cancer, including proliferation, survival, invasion, and metastatic dissemination, in a context- and stage-dependent manner. These functions help explain the long-standing paradox that connexins can exert both tumor-suppressive and pro-tumorigenic effects. In this Review, we synthesize recent insights into the roles of connexins in cancer, focusing on gap junction-independent functions. We highlight key unresolved mechanistic questions and discuss how an expanded understanding of connexin biology may inform more precise therapeutic strategies. Connexins are proteins that form small channels between neighboring cells, allowing them to exchange signals, nutrients, and other small molecules. This direct cell-to-cell communication is essential for normal tissue function. In cancer, connexin activity is frequently disrupted, and their loss has long been associated with uncontrolled tumor growth, leading to the view that restoring connexin function could help suppress cancer. This review challenges that simplified view. Accumulating evidence shows that connexins are not merely passive communication channels. They also perform a range of additional, channel-independent functions inside the cell. These include relocating to the cell nucleus to regulate gene activity, interacting with proteins that control cell division and DNA repair, and influencing how cancer cells respond to treatment. This phenomenon, known as protein ‘moonlighting’, means that a single connexin protein can have very different - and sometimes opposing - effects depending f.i. on its location within the cell and context within a certain tumor type. This helps explain a long-standing paradox: why connexins can act as tumor suppressors in some cancers, yet promote tumor growth, invasion, and spread in others. Connexins also shape how tumors interact with the immune system and whether cancer cells acquire stem cell-like properties that make them harder to eliminate. These insights carry important implications for the clinic. Therapies that target only the channel-forming activity of connexins risk overlooking or inadvertently activating these alternative roles. A more complete understanding of connexin biology, including its dependence on cancer type and disease stage, will be essential for developing effective treatments and for harnessing connexins as reliable markers of tumor behavior and therapy response.