The retinoblastoma protein (pRb) is a cell cycle regulator inactivated in most human cancers. Loss of pRb function results from mutations in the gene coding for pRb or for any of its upstream regulators. Although pRb is predominantly known as a cell cycle repressor, our data point to additional pRb functions in cell adhesion. Our data show that pRb regulates the expression of a wide repertoire of cell adhesion genes and regulates the assembly of the adherens junctions required for cell adhesion. We conducted our studies in osteoblasts, which depend on both pRb and on cell-to-cell contacts for their differentiation and function. We generated knockout mice in which the RB gene was excised specifically in osteoblasts using the cre-lox P system and found that osteoblasts from pRb knockout mice did not assemble adherens junction at their membranes. pRb depletion in wild type osteoblasts using RNAi also disrupted adherens junctions. Microarrays comparing pRb-expressing and pRb-deficient osteoblasts showed that pRb controls the expression of a number of cell adhesion genes, including cadherins. Furthermore, pRb knockout mice showed bone abnormalities consistent with osteoblast adhesion defects. We also found that pRb controls the function of merlin, a well-known regulator of adherens junction assembly, by repressing Rac1 and its effector Pak1. Using qRT-PCR, immunoblots, co-immunoprecipitation assays, and immunofluorescent labeling, we observed that pRb loss resulted in Rac1 and Pak1 overexpression concomitant with merlin inactivation by Pak1, merlin detachment from the membrane, and adherens junction loss. Our data support a pRb function in cell adhesion while elucidating the mechanism for this function. Our work suggests that in some tumor types pRb inactivation results in both a loss of cell cycle control that promotes initial tumor growth as well as in a loss of cell-to-cell contacts, which contributes to later stages of metastasis.

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pRb is known as a classic cell cycle regulator whose inactivation is an important initiator of tumorigenesis. However, more recently, it has also been linked to tumor progression. This study defines a role for pRb as a suppressor of the progression to metastasis by upregulating integrin α10. Transcription of this integrin subunit is herein found to be pRb dependent in mouse osteoblasts. Classic pRb partners in cell cycle control, E2F1 and E2F3, do not repress transcription of integrin α10 and phosphorylation of pRb is not necessary for activation of the integrin α10 promoter. Promoter deletion revealed a pRb-responsive region between −108 bp to −55 bp upstream of the start of the site of transcription. pRb activation of transcription also leads to increased levels of integrin α10 protein and a greater concentration of the integrin α10 protein at the cell membrane of mouse osteoblasts. These higher levels of integrin α10 correspond to increased binding to collagen substrate. Consistent with our findings in mouse osteoblasts, we found that integrin α10 is significantly underexpressed in multiple solid tumors that have frequent inactivation of the pRb pathway. Bioinformatically, we identified data consistent with an ‘integrin switch' that occurs in multiple solid tumors consisting of underexpression of integrins α7, α8, and α10 with concurrent overexpression of integrin β4. pRb promotes cell adhesion by inducing expression of integrins necessary for cell adhesion to a substrate. We propose that pRb loss in solid tumors exacerbates aggressiveness by debilitating cellular adhesion, which in turn facilitates tumor cell detachment and metastasis.

INTRODUCTION To address the lack of genomic data from Hispanic/Latino (H/L) patients with lung cancer, the Latino Lung Cancer Registry was established to collect patient data and biospecimens from these patients. METHODS This retrospective observational study examined lung cancer tumor samples from 163 H/L patients, and tumor-derived DNA was subjected to targeted-exome sequencing (>1000 genes, including EGFR, KRAS, STK11, and TP53) and ancestry analysis. Mutation frequencies in this H/L cohort were compared with those in a similar cohort of non-Hispanic white (NHW) patients and were correlated with ancestry, sex, smoking status, and tumor histology. RESULTS Among adenocarcinomas (n=120) in the H/L cohort, 31% had EGFR mutations, versus 17% in the NHW control group (p < 0.001). KRAS (20% vs. 38%; p=0.002) and STK11 (8% vs. 16%; p=0.065) mutations occurred at lower frequency, and mutations in TP53 occurred at similar frequency (46% vs. 40%; p=0.355) in H/L and NHW patients, respectively. Within the Hispanic cohort, ancestry influenced the rate of TP53 mutations (p=0.009) and may influence the rate of EGFR, KRAS, and STK11 mutations. CONCLUSIONS Driver mutations in H/L lung adenocarcinoma patients differ in frequency from those in NHWs associated with their Indigenous American ancestry. The spectrum of driver mutations needs to be further assessed in the H/L population.

RB1 was the first tumor suppressor gene discovered. Over four decades of work have revealed that the Rb protein (pRb) is a master regulator of biological pathways influencing virtually every aspect of intrinsic cell fate including cell growth, cell-cycle checkpoints, differentiation, senescence, self-renewal, replication, genomic stability and apoptosis. While these many processes may account for a significant portion of RB1’s potency as a tumor suppressor, a small, but growing stream of evidence suggests that RB1 also significantly influences how a cell interacts with its environment, including cell-to-cell and cell-to-extracellular matrix interactions. This review will highlight pRb’s role in the control of cell adhesion and how alterations in the adhesive properties of tumor cells may drive the deadly process of metastasis.